Chapter 3. Comments on Each Chapter of the Draft Fourth National Climate Assessment

FRONT MATTER: REPORT FINDINGS

The Committee reviewed the twelve report findings with particular attention to whether the findings effectively synthesize the key messages and link information provided throughout the draft NCA4 report in a logical and consistent manner. Additionally, the Committee evaluated whether the report section is communicated effectively and the level of technical detail is appropriate for the intended audience.

The report findings are well written and for the most part, they reflect the topics emphasized in the draft NCA4. However, the linkages made in the findings are not always present across individual chapters; some important topics that receive emphasis in chapters are not adequately reflected in the report findings, and in some instances, information provided in the report findings is not well supported in the draft NCA4 chapters. The effectiveness of communicating the report findings may also be improved if some findings are discussed earlier in the findings list, to draw more fully upon the strengths of the draft NCA4. Specific recommendations for addressing these concerns and strengthening the impact of the report findings are provided in this section.

Comments on Select Report Findings

The Communities and Indigenous Peoples Report Findings (and supporting text) effectively link society to a range of climate change impacts discussed in the draft NCA4, which should help messages resonate with readers. The Committee suggests that the NCA4 authors consider broadening the Indigenous Peoples Finding to also include vulnerable populations, since vulnerable populations (i.e., Indigenous Peoples and other vulnerable populations) are discussed in many chapters and may experience many similar impacts and challenges associated with climate change. The discussion of vulnerable populations is especially effective in draft NCA4 Chapter 24, “Northwest,” which could be used as a model to revise this text. Presenting the Communities and Indigenous Peoples Report Findings side-by-side early in the list of findings should also be considered given their similarity and many overlapping impacts.

The Adaptation and Mitigation Report Finding would benefit from some mention of how much adaptation is being implemented and whether there are examples that demonstrate a measurable reduction in climate change impacts as a result of implemented actions (i.e., successes). As detailed in Chapter 2 of this review report, the Committee found examples of response actions to be highly effective for communicating impacts, and discussing this topic more prominently would benefit the report. It would be appropriate to succinctly convey a similar message for this report finding. Listing this finding earlier in the list of findings could also be considered, to give more prominence to response actions. Finally, the Adaptation and Mitigation Report Finding focuses largely on adaptation with little mention of mitigation. It is suggested that better balance be provided.

The Interconnected Impacts Report Finding generally conveys a message applicable to other findings: impacts are linked to one another in complex ways that cannot be sufficiently understood in isolation of one another. The draft NCA4 Chapter 17, which focused on interdependencies, is a strong chapter and its messages should be more broadly referenced across the draft NCA4. That strength (and broader representation across the report) should be captured in the report findings. Placing this synthetic finding earlier in the report findings list could facilitate this and may provide additional context to readers as they read through the full report findings list. Additionally, the Interconnected Impacts Report Finding should be modified to include social systems, including communities. The interconnected nature of climate risk is an issue across almost every topic identified in this report and the societal component is an essential piece of that linkage.

The Committee found the Economy Report Finding to be appropriate and important to include in the draft NCA4. However, the draft report chapters provide little support for inclusion of this finding. Treatment of economic impacts of climate change is very uneven; this topic is constrained largely to Chapter 29, “Mitigation: Avoiding and Reducing Long-Term Risks,” and is only briefly discussed in other places. This may be understandable because defensible economic impact studies are limited and not available for all topics. Generally, a more coherent strategy for handling this topic is needed. It is recommended that discussion of economic impacts of climate change be better woven throughout the draft report where there is available literature to support it and where information is not available, to explicitly say so. The Economy Report Finding should also include language that constrains the framing to areas where robust research exists so that the finding is not misinterpreted as encompassing the entire U.S. economy.

Recommended Topic Additions and Linkages

• Energy: Expanded discussion of energy in the Economy and Interconnected Impacts Findings would be beneficial, in addition to its current inclusion in the Infrastructure Finding. Energy is a critical system that is discussed in many draft NCA4 chapters. It

• supports economic activity and serves as an integral component of interconnected infrastructure systems that are vulnerable to climate change impacts.

• Wildfire: Wildfire poses large risks relevant to the Ecosystems and Ecosystem Services, Agriculture, and Communities Report Findings and is discussed in many national topic and regional chapters. Inclusion of wildfire in the findings would also likely resonate with NCA4 readers given recent trends in wildfire activity in the United States.
• Biodiversity and Ecosystem Services: The interaction and fundamental underpinning of biodiversity and ecosystem services (draft NCA4 Chapter 7) is covered in many other draft NCA4 topic chapters (e.g., Water, Energy, Land Cover and Land Use Change, Forests, Coastal Effects, Oceans and Marine Resources, etc.) and should be highlighted more fully in the Ecosystems and Ecosystem Services Report Finding.
• Increased Topic Linkages: Other linkages that would strengthen the report findings include mentioning forestry in the Agriculture Finding and including ocean ecosystem adaptation and mitigation actions in the Tourism and Recreation Finding.
• Increased Regional Linkages: The Committee suggests considering the value of incorporating regional context into the report findings. While the report findings cover broad topics, the regional chapters demonstrate that impacts are variable across the nation and that some of the report findings will resonate more strongly in some regions than in others.

Other Report Finding Comments

In a few cases, the statements in report findings are either inconsistent or not well supported by chapter text. For instance, the report findings emphasize Indigenous vulnerability to a greater extent than does the chapter on that topic. For the Agriculture Finding, the statement “economies of agricultural regions at risk” is not well-developed in Chapter 10, “Agriculture and Rural Communities.” Careful review to ensure that report finding content is supported by chapter text and that similar emphasis is placed on the topics covered is needed.

A few national topic chapters of the draft NCA4 were not strongly captured in the report findings and the Committee leaves it to the NCA4 authors to determine whether this is an appropriate omission. These chapters include Chapter 5, “Land Cover and Land Use Change,” and Chapter 16, “Climate Effects on U.S. International Interests.”

Additional revisions to the report findings should also be made as appropriate, if changes to associated key messages occur as a result of other recommendations provided in this review report.

CHAPTER 1: OVERVIEW

Summary

The Committee found the material contained in Chapter 1 to be accurate and well written, with attention given to many of the aspects of climate change that are dominant in the discussion of impacts including sea level rise, temperature, and precipitation. However, the strong emphasis on climate science in this chapter is out of balance with the impacts focus of the draft NCA4 as a

whole. Broadly, the Committee recommends placing greater emphasis on the impacts and response efforts in Chapter 1, including in the selection of figures. Expanded synthesis of the national topic and regional chapter contents, as well as some attention to new science published since the NCA3, is also needed.

Chapter 1 should be the synthesis or bridge from the nationwide topic issues and the regional analysis to the major report findings. While the Committee supports the draft NCA4 report findings, this synthesis across topics and scales is lacking. The Committee recommends revising Chapter 1 using the framework of the twelve report findings, with impacts from the national topic chapters and regional analysis being the focus.

Treatment of Uncertainty and Risk

As noted in Chapter 2 of this review report, the content of the Call Out Box “Why is Risk Framing a Useful Tool for Decision-Makers” is helpful to readers. Importantly, it discusses the use of risk framing as a means to convey information in a way that may inform decisions about responding to impacts in the presence of uncertainty. Using complex systems and interdependencies to explain these concepts is effective. The Call Out Box could also be a good place to point out that risk framing is an appropriate framework to discuss both adaptation and mitigation and to talk more about better (adaptive) risk management and point to those alternative approaches. The draft report generally does well in citing and communicating the recent research on the treatment of risk. However, some recent papers could be added, including Bakker et al. (2017), Oppenheimer et al. (2016), and Wong and Keller (2017). Information provided by the International Organization of Standardizations could also serve as a useful resource for further exploring risk (ISO, 2009).

Chapter 1 would be a good place to elaborate on the different types and drivers of uncertainty and risk and how to manage them. Alternatively, some of the material on how to manage risks could be mentioned in the Call Out Box “Confidence and Uncertainty in Climate Science” or in Section 1.4 or Section 1.6. For example, in the discussion of shortcomings of cost-benefit analysis, better alternatives could be introduced, including robust decision-making (citing, for example, Hall et al., 2012; Herman et al., 2015; Ranger et al., 2013; Lawrence et al., 2018) and risk-risk analysis (Viscusi, 1994). Expanded discussion of economic impacts of climate change in the context of risk for more sectors/chapters would also be welcome, as discussed in the context of the Economy Report Finding in the “Front Matter: Report Findings” section earlier in this review chapter.

Chapter 1 would also be the appropriate place to clearly define the scope of uncertainty and risks evaluated in the draft NCA4, preview different types and sources of uncertainty and risk in climate assessment more generally, and discuss how to manage them. As described in Chapter 2 of this review report, it is important to differentiate between sources of uncertainty and risk because different types of risk call for different types of risk management solutions. While the draft NCA4 report cannot be expected to go into detail on all these aspects of uncertainty and risk, it would be helpful if Chapter 1 could provide some conceptual guidance or advice on performing risk analysis and management in a way that accounts for how impacts interact across sectors and scales (e.g., Haimes, 2009; Ayyub, 2014) as well as a generalized treatment of uncertainty (e.g., Klir, 2005; Ayyub and Klir, 2006).

More specifically, discussion of uncertainties and their effects on evaluating risk should differentiate between the following:

• Negative consequences/impacts imposed by climate change
• Uncertainty introduced in predicting events and outcomes due to climate change (as the future is no longer like the past) and increased climate variability, and sources of uncertainty in these predictions
• Effects of the stochastic nature of the environment
• Compounded events
• Model-based uncertainties
• Uncertainties due to divergent expert opinion
• Effects of knowledge gaps due to non-existing research
• Feedbacks and interaction between different sectoral risks
• Feedbacks and interactions between infrastructure (built systems), ecosystems (natural systems), and social systems (interconnected impacts)
• Feedbacks and interactions between climate risks and non-climate stressors (e.g., aging infrastructure, stressed ecosystems, social inequality), which are mentioned on page 19 of the draft NCA4
• Risk of underutilizing existing climate and climate change forecasts, willfully or due to individual or institutional myopia and status-quo bias
• Risk of not having adequate theory and tools to model and predict complex emergent risks

Specific Comments on Chapter Content

In order to help bolster the impacts discussion in Chapter 1, it is suggested that the treatment of adaptation be expanded in Section 1.6. Considerable development of new adaptation activities since NCA3 has occurred and these actions should generally be discussed more thoroughly in Chapter 1 of the draft report. The Committee recognizes that adaptation often occurs at local to regional scales, so it would be difficult to provide a comprehensive national-scale summary in Chapter 1, but a table or figure that provides some context for recent growth in this area is warranted. This would also contribute to improved distinction between the NCA3 and NCA4.

The Call Out Box “What’s New in NCA4?” lists new data products, report chapters, and framing of risk and economic impacts. While this nicely summarizes the advancements in the processes and information available for NCA4, the title could be interpreted to mean that the box discusses new advancements in scientific understanding or what topics may be new in NCA4 relative to NCA3. The Committee recommends providing a clearer distinction about what is new in the science and response action in the draft NCA4, possibly in this Call Out Box. See also “Highlighting New Developments in Climate Science, Impacts, and Responses” in Chapter 2 of this review report for recommendations on approaches to making this distinction.

For the Call Out Box “Confidence and Uncertainty in Climate Science,” it is suggested that “confidence” be defined so that it is not misinterpreted in this context. Additionally, the meaning of “increasing confidence in climate science” may not be clear and could be misinterpreted—is this asserting that the public has increasing confidence in the climate science

community? The Committee thinks this probably means to say that because observed signals—and, to some extent, projections—have become stronger, there is increasing confidence by scientists in the results that have accumulated since NCA3, which reinforces conclusions and climate change trends that were described in NCA3. The language should be updated to better convey this message, or the message intended by the Chapter 1 authors.

The draft NCA4 would benefit from a unifying graphic (or a few graphics) that distills key messages from the national topic and regional chapters. Chapter 1 is an appropriate location for such a graphic. The Committee suggests identifying a dominant climate change impact in each region and an example of adaptation or mitigation that addresses that impact (ideally something that can be summarized well visually). Overlying this information on a national-scale map with the NCA4 regions identified could serve to focus the draft report material in a new way that also highlights response efforts. An approach to thinking about this unifying graphic would be to ask: “What are the fundamental figures that readers should remember from the NCA4?” The Committee recommends that the unifying graphic be accompanied by a short paragraph in Chapter 1 emphasizing the inter-connections among regions, sectors, and biophysical and social components of ecosystems. The structure of the NCA4 report requires separate treatment of such topics in order to provide necessary detail, but the integrated nature of climate change impacts and strategies to address them should not be understated.

The Committee would like to see the draft NCA4 expand discussion of impacts and risks imposed by climate change on National Parks (see Monahan and Fisichelli, 2014). It is appreciated that the draft NCA4 Chapter 1 mentions the impact of particulates from wildfires on the scenic vistas in National Parks (page 48) and one of the Frequently Asked Questions addresses melting glaciers in Glacier National Park. However, there is relatively little mention in the national topic or regional chapters of the much broader suite of likely climate change impacts. This might be expected to be included in the draft NCA4 Chapter 7, “Ecosystems, Ecosystem Services, and Biodiversity.” Placing greater emphasis on the impacts and risks to national parks, which are popularly described as “America’s Best Idea,” could be an effective outreach to the NCA4 readers, since national parks are iconic and representative of the impacts that will be experienced in state parks and other public recreational areas. As noted in their mission statement, “The National Park Service preserves unimpaired the natural and cultural resources and values of the National Park System for the enjoyment, education, and inspiration of this and future generations” (although recognizing that “preserving unimpaired” must be tempered by the fact that environmental and social change may prohibit preservation or restoration to conditions during pre-European times [Revisiting Leopold: Resource Stewardship in the National Parks, 2012]). This mission statement strongly espouses conservation and education over a sustained period.

It is recommended that the full NCA4 author team carefully review the contents of the draft NCA4 Chapter 1 to ensure that the information included is well justified by the associated national topic and regional chapters. In a few cases, the Committee identified instances where the underlying chapter did not adequately support the information included in Chapter 1. For example, the negative impacts on timber prices and economic well-being of forest landowners (page 36, lines 16-20) are not discussed specifically in the forests chapter (Chapter 6 in the draft report), except by reference to a single paper. Similarly, the effects of ground-level air pollutants (page 44, line 7) is discussed in the context of forests, but this topic is not discussed in Chapter 6

except by a single reference; where mentioned, the influence of ground-level ozone is not well explained (see the review for Chapter 6 for more detail).

In general, the regional chapter impact messages are not described clearly in the draft Chapter 1. Inclusion of a unifying graphic, as mentioned earlier in this section, could be one approach to addressing this shortcoming.

Comments on Graphics

The Committee determined that the figures included in the draft NCA4 Chapter 1 are accurate and well selected for demonstrating the state of climate science. However, as noted previously, it is recommended that the authors add a unifying graphic and place more focus on responses and impacts in order to better balance the chapter content with the draft NCA4 as a whole. Comments here are recommendations for the figures contained in the draft NCA4, which the Committee recommends editing if retained.

Figure 1.1

The quantity of information in Figure 1.1 is overwhelming, often overly complicated for the intended audience, and lacking in explanation of some important details. With so much data contained in a single figure, it is challenging to identify what key message(s) the graphic as a whole is trying to convey, and what take-home messages are intended. The text referencing the figure is spread throughout the draft chapter, making it difficult for the figure to stand alone. However, the content was determined to be generally important, and this section outlines a number of suggestions to simplify the graphic and/or draw out the content more directly.

The content of Figure 1.1 is closely aligned with the physical climate science detailed in the CSSR and summarized in Chapter 2 of the draft NCA4. The information is accurate, but careful attention should be given to which portions of the figure are necessary and effective in conveying the climate change impacts that are the central focus of the draft NCA4. Some impacts may not resonate or be readily understandable to a general audience without more detail (e.g., snowpack).

Nearly every time series presented is for a different set of years. The inconsistency in time periods shown is confusing and makes it difficult to readily compare changes across panels (e.g., panel (h) shows area burned by wildfire from 1983 to 2016, while panel (i), located just below (h), shows the percent of U.S. area experiencing drought from 2000 to 2016). Time periods should be standardized wherever possible to make the figure panels more consistent. Additionally, the same colors are used to mean different things in different panels, which may confuse readers (e.g., in panels (a) and (b) red depicts an increase, and in (d) it represents a decrease).

The caption could be better utilized to explain Figure 1.1 and orient the reader. It is unclear what the arrows located on the left side of individual panels are intended to show. Terms like snowpack should also be defined.

An infographic that complements Figure 1.1 could be an approach to simplify the main messages and make the material more accessible to the intended audience. This infographic could include just the arrows provided on the panels or some other tangible element.

It is unclear whether the Figure 1.1 panels are shown elsewhere in the report. Since this figure is presented in the draft NCA4 Overview Chapter, the Committee expected the data to be more fully explained elsewhere in the report; if this is not the case, it may be misleading to readers.

Given the small size of individual panels in Figure 1.1, they are difficult to read. To address this, the figure could be separated into four figures that individually explain the four primary topic areas: Weather and Climate, Snow and Ice, Land and Water, and Oceans and Coasts.

Figure 1.2

Figure 1.2 accurately summarizes information provided in Chapter 2 of the draft NCA4, “Our Changing Planet.” The figure also nicely illustrates the evidence that supports the conclusion that human factors are the dominant influence on recent climate change. However, the figure is relatively complicated for a general audience. To address this, the Committee suggests expanding the caption to more fully summarize the contents in language appropriate for a broad audience. Cross-reference to Figure 2.1, which shows the separate effect of each natural forcing (solar and volcano), combined human-caused forcing, and natural variability on global mean temperature, could be useful for readers interested in individual causes of temperature change. The Chapter 1 authors could also consider developing a graphic more similar to Figure 2.1 to make the messaging of the report and the graphics consistent across chapters.

Figure 1.3

It is recommended that the legend showing the Representative Concentration Pathways (RCPs) be expanded to also include language indicating relative emissions (e.g., lower scenario, higher scenario, etc.) to be easily understood by the intended audience and consistent with the caption.

The use of different baselines for the projected temperature change and the historical observed temperature change may be confusing for some readers. If a common frame of reference could be applied, it is recommended; otherwise, additional explanation of the difference should be provided. Also, the y-axis title indicates the graphic is showing temperature change relative to a 1986-2015 baseline while the caption lists the baseline as 1986-2005, suggesting that one of these years is a typo and should be corrected. This also applies to where this graphic is included as Figure 2.2 in the draft NCA4. It is also suggested that the graphic be made larger (similar width to Figure 1.4) and that the authors consider listing the legend information to the right of the figure so that the RCP/scenarios information is listed to the right of the y-axis showing temperature change projected for the end of the century.

Figure 1.4

Figure 1.4 conveys too many messages for the intended audience to easily follow. In particular, the relationship between the projection scenarios on the graph and the RCPs shown to the right of the figure may not be clear to a non-technical audience. A figure with this level of detail is appropriate for Chapter 2 of the draft NCA4, where it is also provided as Figure 2.3, but

should be simplified if retained in Chapter 1. This could be achieved by showing fewer scenarios and/or making the graphic more visually consistent with Figure 1.3. It is also suggested that the height of the figure be increased so it is more legible and that the y-axis showing feet be converted to inches so that any differences in sea level rise projected in the near term may be visible. The use of both feet and meters on the y-axis but description in text of only feet may be confusing to some readers. It is also recommended that the time period shown on the x-axis be shortened to focus more on the projections rather than the historical rate of rise, or that more justification for the inclusion of the long time period be provided. The Chapter 1 NCA4 authors could consider adding a graphic of a person of average height next to the right side of the y-axis as a way to make the impact of the amount of projected sea level rise more tangible. Finally, it is suggested that the sea level rise graphic used in this chapter show national estimates of sea level rise instead of global estimates, to help make the impacts more relatable to readers and to improve the linkage with the national scope of the draft report.

Figure 1.5

Figure 1.5 is an effective graphic because it provides a national map that highlights regional distinctions. It is suggested that this type of information be illustrated more often across the draft NCA4. More specifically, this figure provides context and clear take-away messages on actions being taken to address climate change, which ties in more directly to the focus of this report than other figures in the draft Chapter 1. The figure could be made more effective by magnifying the size of the orange dots, even if it creates some overlap. In the caption it would also be useful to list the total number of orange dots in the figure and to indicate what gray shading represents. Updating the figure to provide information (or a clearer distinction) between states that have renewable energy portfolio standards (legally binding) and those that have renewable energy goals (not generally legally binding) would also be beneficial. These suggested edits also apply to the inclusion of this figure in Chapter 29 as Figure 29.1.

CHAPTER 2: OUR CHANGING CLIMATE

Summary

Chapter 2 provides a high-level summary of the CSSR as background on observed and projected changes in the climate system and serves as technical input to other chapters in the draft NCA4 report. Key messages are used to summarize the key findings of the fifteen chapters of the CSSR. These key messages are supplemented by discussions in seven boxes covering topics including natural variability, climate change indicators, greenhouse gas emissions targets, extreme events, the 2017 hurricane season, and climate models. Overall, the key messages in Chapter 2 represent the key findings conveyed in the CSSR very well and provide a description of some of the major elements of global and regional climate change, with an emphasis on how those changes might affect the United States. There are no major concerns about this chapter, but the authors may consider some areas for improvement.

Review Comments Related to the Statement of Task

Comment on Key Messages

The authors of Chapter 2 have arranged the key messages in an order that differs somewhat from the chapter order in the CSSR. However, the order makes sense: the chapter starts with global or large-scale changes (e.g., global mean changes, ocean changes, and sea level rise), followed by national changes (e.g., temperature, precipitation, and floods/droughts), followed by larger-scale changes (e.g., Arctic, atmospheric circulation, and ocean circulation) that influence the United States. Each key message is described and followed by more detailed discussions, which are supported by references to key literature. The contents of the seven boxes is well chosen to supplement the key messages and address commonly raised questions about climate change.

Key Message 6 mentions “shifts to more winter precipitation falling as rain rather than snow in many parts of the central and eastern United States.” Such shifts are also expected in the western U.S., so it is not clear why only the central and eastern United States are highlighted. The Committee suggests revising the messages to provide a more complete explanation of where the changes are expected.

The discussion of flood and drought changes provided for Key Message 6 is somewhat confusing because it lacks discussion of the many natural factors and human activities that influence flood and drought. Precipitation, temperature, and evaporation changes are only a few of the many natural factors that contribute to these events. In addition to emissions of heat-trapping greenhouse gases caused by human activities, land use and land cover change, as well as water use and management, affect flood and drought. As such, it should not be surprising that flood and drought trends have not been consistently detected.

With an emphasis on extremes including floods and droughts in Key Message 6, it may be useful to call upon past climatic anomalies in the instrumental record (e.g., the Dust Bowl, hurricanes in the eastern seaboard, and the 1983 El Niño), and during the pre-instrumental period as inferred from proxy measures (e.g., medieval drought) in the supporting text.

Comments on Graphics

For Figure 2.2 in the draft NCA4, the y-axis title indicates the graphic is showing temperature change relative to a 1986-2015 baseline while the caption lists the baseline as 1986-2005, suggesting that one of these years is a typo and should be corrected. This also applies to the use of this graphic as Figure 1.3 in the draft report.

Comments on Literature Cited

The chapter makes good use of peer-reviewed literature and represents an accurate and well-written summary of the CSSR and the current understanding of global and regional climate change.

Comments on Traceable Accounts

The traceable accounts adequately and accurately explain how the key messages were derived.

Other Recommended Improvements

In the context of this chapter, the term “human activities” seems to refer mainly to those activities that emit greenhouse gases, but it is well known that human activities that emit aerosols change the land surface properties and alter land surface biophysical and hydrological states, which have important effects on temperature, precipitation, flooding, drought, etc. It would be useful to clarify the scope of human activities in this chapter, as the narrow definition of human activities presented seems to perpetuate in other chapters of the report.

This chapter could use more explanation of the mechanisms or reasons for the observed and/or projected changes in order to demonstrate the strong scientific underpinning of global and regional climate change. For example, an explanation of the Clausius-Clapyeron relationship would be useful for the readers to relate precipitation changes with temperature changes. Similarly, an explanation of the potential intensity (the theoretical limit of the maximum intensity that can be achieved [Emanuel, 1999]) would be useful for the readers to understand how tropical cyclone intensity changes can be related to sea surface temperature warming and why there is more uncertainty in projecting tropical cyclone frequency changes than intensity changes. Similarly, citing the conceptual knowledge gleaned about global temperature, sea level, and biosphere response from paleoclimate records and longer time horizons of change would help the readers understand the various changes. For example, Clark et al. (2016) provides a different conceptual view of sea level rise following warming events, which is a useful discussion to cite.

This chapter could also include more discussion of the oceans related to carbon dioxide (acidification), warming (ocean heat content), and changes in large-scale circulation and precipitation (e.g., sea surface warming patterns have an important influence on precipitation changes) that have impacts that are appropriate to discuss in the draft NCA4.

Box 2.1 provides a useful discussion of natural variability as a source of uncertainty in understanding past changes and projecting future changes. However, it would be useful to also discuss uncertainty more broadly, and perhaps call it out in a separate box because climate projection uncertainty also includes model and scenario uncertainty. Alternatively, the draft NCA4 Chapter 1 could be a place to expand this discussion and reference it prominently in Box 2.1.

Box 2.7 should acknowledge recent advances in dynamical downscaling at sub-10 km resolution that more explicitly resolves convection. A good example to reference is Prein et al. (2016, 2017), which generates decade-long regional projections of precipitation changes in the

United States. A relatively large body of research is now available, supporting the transformative capability to model regional climate using convection-permitting modeling. Also, a mention that there are important variables or phenomena for which climate changes are still not well understood or downscaled using dynamical or statistical methods (e.g., wind, cloudiness, sea breeze, Santa Ana winds) would be useful for the readers in order to appreciate why certain local/regional phenomena are not discussed.

Chapter 2 could include some discussion of changes that are more controversial but capture the interest of the public. For example, warming is expected to increase the frequency of heat waves. However, cold extremes may still occur because of possible changes in blocking and extreme snowfall may not decrease by as much as expected based on temperature alone (e.g., O’Gorman, 2014). This information could then be drawn upon in other relevant chapters of the draft NCA4.

Long term global and regional observations of the coupled climate system play a crucial role in tracking and understanding changes. This may warrant a separate discussion or box, including salient examples.

An additional box summarizing the key advances in understanding, modeling, and observations of climate change since NCA3 would be useful for readers who want a cursory look at what’s new in NCA4 relative to NCA3. The CSSR report has a nice summary of “Advances Since NCA3” in Box 1.2 of that report (USGCRP, 2017) that the Chapter 2 authors could draw on.

CHAPTER 3: WATER

Summary

Chapter 3 is generally well written and succinctly covers an impressive amount of material on the complex topic of water. The “Regional Roll-Up” section overviews are a good complement to the general national-level information. The chapter is particularly strong on the topic of climate change implications for existing and planned infrastructure, including some strong examples of proactive management actions. The chapter generally makes appropriate reference to the rich literature on these topics. In terms of areas of improvement, the information on direct biophysical impacts to the water cycle is generally underdeveloped compared to the subsequent discussions of mitigation and management. Overall, this chapter would benefit from discussions on how the state of the science (and/or data or management practices) either has or has not evolved since the NCA3. This addition should highlight recent advances in understanding climate change impacts on hydrology, floods, and drought.

In discussing human influence, there is a need to clarify the role of human activities (e.g., emissions of greenhouse gases) that lead to warming (and subsequent changes in the water cycle) versus human activities such as irrigation and water management that have direct impacts on hydrology. This distinction is relevant to attributing historical hydrological changes, as well as projecting future hydrological changes. For example, warming-induced drought and stream temperature rise could be alleviated by water management actions that increase flow (e.g., Wan et al., 2017).

The chapter could also greatly benefit from more direct and explicit discussions connecting to the climate change projections described in Chapter 2, “Our Changing Climate,” of the draft NCA4 report.

Comments Related to the Statement of Task

Comments on Key Messages

In general, the key messages in this chapter are not all well linked to Chapter 2 of the draft NCA4. For example, groundwater depletion and water use are noted prominently in Chapter 3, but were not mentioned in Chapter 2. Conversely, Chapter 2 has some good discussions of flood and drought, while Chapter 3 only briefly mentions the changes in snowpack and rain-to-snow ratio and speculates how increased extreme precipitation may result in more severe flooding. An explicit reference to the draft NCA4 Chapter 2 in the Water Chapter about how climate affects hydrology, floods, and drought would be useful to improve the connection and provide a more complete picture.

Key Message 1 highlights the changes in water quantity and quality, with a focus on robust changes that are well supported by numerous studies. These include reduced snowpack and increased rain-to-snow ratio with warmer temperature; increased human water use due to warmer temperature and other factors; groundwater depletion (due to agriculture); warmer stream temperature; extreme precipitation mobilizing more transport of sediment and nutrients by rivers; sea level rise and salt water intrusion; and wildfires and other factors that affect water quality. These are all important findings that are relevant to the observed and projected changes in many regions, so this key message is very well connected with many regional chapters. However, the text supporting Key Message 1 could be improved by rebalancing considerations of temperature and rainfall changes. Temperature warming is currently the disproportionate focus in the discussion (e.g., changes in snowpack, rain-to-snow ratio, stream temperature), whereas impacts of projected changes to extreme precipitation events (e.g., Wehner et al., 2013; Westra et al., 2013; Feng et al., 2016) and dry spells (e.g., Peterson et al., 2013) are less developed. A brief discussion about the impacts of saltwater intrusion on drinking water treatment could also be added, as well as information on how water utilities relying on ground and surface waters may be affected (e.g., Kolb et al., 2017).

The paragraph about groundwater depletion (page 133, line 27, to page 134, line 5) needs more care to explain the main point: that reduced groundwater availability exacerbates drought risk. As currently written, some readers may come away with the impression that the recent groundwater depletion trends discussed here are a direct outcome of past climate changes, which they, for the most part, are not. Rather, they are mostly associated with agricultural

intensification and some other sectoral usages. When considering climate change impacts on future groundwater storage, it is important to consider both the demand (pumping) and the supply (recharge); there is substantial literature that considers both sides of the water balance that could be referenced. Regarding importance of groundwater sustenance, the Chapter 3 authors should consider adding a comment about groundwater-dependent ecosystems.

Key Message 2 is well supported by the cited literature and makes good use of examples. A reference to the draft NCA4 Chapter 17, “Sectoral Interdependencies, Multiple Stressors, and Complex Systems,” would be useful, as water is a key component of the water-energy nexus that requires cross sectoral considerations. Additionally, a few references are made to the representation of extreme hydrologic events in the paleo-hydrology literature, including in the text supporting this key message in relation to infrastructure design standards. The Chapter 3 authors should clarify whether they are recommending incorporation of paleo-environmental information into water infrastructure design practices.

Key Message 3 focuses on water management in a changing future and highlights the scientific challenge as well as institutional, political, and legal challenges and the need to manage vulnerabilities for a wide range of uncertain conditions. The text supporting this key message provides robust reference to the literature. Figure 3.3 is an effective illustration of the issues surrounding the balance of water supply and demand and the uncertainty. As noted for Key Message 2, reference to the draft NCA4 Chapter 17 would be useful here, as water must be managed in the context of not only providing water resources but also considering energy, agricultural, and other sectors.

Uncertainty should be included in Key Message 3 and not just in the supporting text that follows the message. The discussion about uncertainty––using the Great Lakes and other places as examples––is strong. This addition could better convey that uncertainty in projecting future changes is a major obstacle to addressing risks that change with time.

Comments on Literature Cited

Generally, the appropriate literature is well cited in Chapter 3, although some specific references to add are noted throughout this chapter review.

Comments on Graphics

Figure 3.2 is misleading because it suggests uniform depletion across the vast Ogallala region. In reality, the northern part of the aquifer system has experienced relatively stable groundwater levels, whereas the southern portion has undergone extensive overdraft and depletion. It is a common point of confusion and the text should be updated to clarify this regional distinction.

Comments on Traceable Accounts

The Chapter 3 authors assert that formal attribution of flood and drought to human-induced climate change has not been established. This statement is somewhat at odds with the authors’ assignment of “medium confidence” to more severe floods in the future. This nuance requires more discussion to avoid confusion. For example, formal attribution of flood and drought to human-induced climate change may not be possible without considering human activities such as irrigation, groundwater use, and water management. In this chapter as well as others, the term “human activities” is often used to attribute or project future changes, but only those changes related to the emission of greenhouse gases such as fossil energy use and land cover and land use change. It is important to clearly distinguish human activities related to emission of greenhouse gases from human activities related to irrigation and water management. The latter have direct impacts on floods and drought (both agricultural and hydrological). For example, water management practices that regulate streamflow for flood protection and low flows could reduce the detection of increasing floods/droughts. This can have broader implications if the reduced detection is combined with aging infrastructure such that protection against flood and drought in the future is lessened, increasing risks.

Other Recommended Changes

The “Chapter Development” section provided in the traceable accounts indicates that Chapter 3 was developed to place more emphasis on vulnerability, risk, and management than in the NCA3 report, since the NCA3 focused largely on climate change impacts on hydrology, flood, and drought. While this is an effective approach to keep the chapter succinct, it is still important to summarize key information in order to provide necessary context for the reader. A brief discussion regarding climate change impacts on hydrology, floods, and drought, as well as highlighting the advances made since NCA3 would strengthen the chapter. For example, since NCA3, many new aridity studies have been published, suggesting increased aridity due to warming (Sherwood and Fu, 2014) and that care should be taken in using offline approaches for projecting aridity changes (Milly and Dunne, 2016). Some studies have investigated the changes in flood (Das et al., 2013; Mallakpour and Villarini, 2015), flood seasonality (Ye et al., 2016), and drought (Cook et al., 2015). Several studies have investigated the relative impacts of climate change and human water use (e.g., water management) and suggested that because water

management could mitigate drought changes in the future, projections of future drought characteristics should consider both climate change and direct human influences on the water cycle (Wanders and Wada 2015; Wan et al., 2017). It is important to convey the importance of human impacts through water use and water management when attributing past changes in flood and drought and when projecting future changes.

There are many major water management “hotspots” in the United States that are anticipated to be climate-sensitive that are not mentioned or well developed in Chapter 3. It is beyond the scope of this chapter to discuss them all, but the Chapter 3 authors should consider some additional comments or case studies about other problem areas, as space allows. Such concrete case examples tend to be very useful for illustrating climate change impacts to the reader (e.g., California Central Valley groundwater depletion, Floridian Aquifer salinization, Chesapeake Bay ecosystems).

Glaciers should be discussed in a national topic chapter because impacts are evident and there is high confidence in glacial changes. The Water Chapter seems like a reasonable chapter in which to include brief discussion of glacial melt, with cross-reference to the Alaska regional chapter as appropriate.

In this chapter, as noted for the draft NCA4 as a whole, increased consistency in the discussion of similar topics and cross-referencing between relevant sections of this chapter and both regional and other topic chapters’ key points regarding water is needed.

CHAPTER 4: ENERGY SUPPLY, DELIVERY, AND DEMAND

Summary

Chapter 4 provides a strong overview of climate change implications on the nation’s energy system and the actions that are underway to protect energy security and promote energy sector resilience. The text details the challenges that industry and government (at all levels) face as they attempt to ensure that energy, which underlies most U.S. economic activity, is not significantly disrupted by extreme weather and climate change. It also discusses actions necessary to escalate the pace, scale, and scope of efforts to ensure the safe and reliable provision of energy now and in the future.

The Committee has some suggestions for improvements. The chapter should include more discussion about distributed generation, including that which is relevant to renewable energy. Specifically, the section about energy sector transformations should address how renewable energy sources could be impacted by climate change and extreme weather, and how these impacts could affect or protect other associated systems. Also, because of the nature of energy ownership, the importance of private-sector actions, public-private partnerships, and new financial models should be further highlighted, potentially as part of a key message. Chapter 4 includes information about interdependencies and should cross-reference Chapter 17 of the draft NCA4, “Sectoral Interdependencies, Multiple Stressors, and Complex Systems.”

Comments Related to the Statement of Task

The chapter’s coverage of the energy sector is appropriate, but further consideration of the interconnected nature of the energy sector as more of an energy system or process spanning from sources, to generation, to consumption, and any feedback impacts in the context of system dynamics might be insightful. For example, the impact of Hurricane Katrina on fuel production and refining could be expanded. Linkages with other climate change impacts could also be explored and relevant chapters in the draft NCA4 cross-referenced. Interconnected topics that may impact the energy sector include climate impacts on water systems, increased salinity and changes in wildfires. Cross-referencing to relevant chapters discussing these issues should be considered.

Comments on Key Messages

The key messages are generally clear, consistent, and at an appropriate technical level for the intended audiences. Key Messages 1 and 2 affirm current understanding of: the critical nature of energy supply and delivery systems and their interconnectedness and interdependencies with other vital systems; the changing nature of energy supply and delivery systems and roles of energy technologies, markets, and policies in affecting the systems’ vulnerabilities to climate change and extreme weather; critical systems that are becoming more interconnected. However, an examination of the subject matter as an energy process might be useful.

Key Message 3 should also highlight that the listed actions are being taken by both the public and private sector as well as public-private partnerships. In addition, the key message should directly call out the use of risk methods to inform policy and decision-making practices for achieving energy and economic efficiencies.

Comments on Figures

Overall the graphics are clear, internally consistent, and communicated well for the intended audiences. Figure 4.1 is interesting and informative and Figure 4.2 illustrates critical infrastructure interdependencies and is an appropriate choice to include in the chapter.

Figure 4.3 is interesting, although it might be too specialized for a non-technical reader. A figure that shows more generally the various impacts of the energy system or the energy process and associated impacts from exploration and generation to distribution and consumption would be useful for the chapter.

Comments on Literature Cited

The chapter accurately reflects the peer-reviewed scientific literature with a particular focus on literature since the NCA3. No critical content areas were left out from the chapter.

However, as recommended earlier, more discussion of the underlying process may be an insightful addition for readers.

Comments on Traceable Accounts

The traceable accounts underlying the key messages for Chapter 4 provide consistent, transparent, and credible documentation of the foundational literature. The key messages accurately reflect supporting evidence and are communicated effectively.

Other Recommended Changes

Because energy supplies are typically privately owned and operated, private sector actions should be discussed in more detail and the importance of public-private partnerships should be highlighted. Potential changes to risk management and adaptation also require new planning and design philosophies that rely on adaptive-type methods. One framework that could be mentioned is “Real Options” (Zhao et al., 2004; Chiara et al., 2007), which provides an economic assessment framework for policy and decision-making.

The energy system includes primary fuel supply, power generation, and transmission and distribution systems. Throughout the energy system, infrastructure is aging and frequently exceeding design lifespans. The chapter could be improved by making reference to related issues and system exacerbations from a changing climate in order to provide a more complete understanding of how climate change impacts interact with other existing challenges in maintaining the energy system. In addition, while the chapter appropriately discusses electricity sector vulnerabilities, it should also mention that increased demand for distributed sources (including renewables electricity) may make management of these multiple facilities more challenging. Renewable energy systems also have their own challenges, including, but not limited to, assuring adequate biomass (especially in times of drought) and system stability for wind and solar energy systems during extreme weather events and changes in regional climate.

CHAPTER 5: LAND COVER AND LAND USE CHANGE

Summary

Chapter 5 does a nice job of linking literature on land cover effects on climate and climate impacts on land cover and land use. This is done especially well through focusing on impacts on disturbance regimes, species distributions, and land use suitabilities. The chapter also provides an overview of how changes in land cover can have both mitigation and adaptation benefits. It is appropriately detailed in its review of both implications on climate change and from climate change.

The key messages cover important interactions between land cover and land use and the climate, but they could be articulated to better reflect the main takeaways. The NCA3 chapter on this topic explicitly called out mitigation and adaptation opportunities associated with land use. These topics are addressed throughout the chapter in the draft NCA4, presumably in the interest of reducing chapter length. Because land cover and land use approaches to adaptation and

mitigation are not consistently handled within other national topic and regional chapters, this represents a missed opportunity. While land management changes have been shown to help with adaptation and mitigation, such as in the agricultural sector, some land use change will likely need to occur as productivities are affected by climate and soil restoration activities are prioritized. This point is not clearly addressed anywhere in the draft report and should at least be articulated well in Chapter 5. Chapter 5 also includes a description of the state of the sector along with land use projections. The land use projections, however, are poorly described and are not well integrated into this chapter or the other relevant chapters in the draft NCA4. In addition, linkages from the draft NCA4 Chapter 5 to other chapters, and linkages among the various sections within Chapter 5 (including confidence statements) are not consistent.

Comments Related to the Statement of Task

Comments on Key Messages

Key Message 1 lacks clarity in two respects. First, the message should be clearer about the distinction between effects on local climate (e.g., through water and energy balance impacts, as highlighted in the message) and effects on greenhouse gas emissions, which are also raised in the text, but not in the message. Second, because feedbacks are by definition bidirectional, the message should better articulate what the feedbacks are “between the climate and land systems” rather than “on the climate system.”

The supporting text for Key Message 1 includes evidence for both impacts on water and on energy balances. These impacts modify climate locally and affect changes in greenhouse gas emissions, which modify climate globally. The key message seems to focus on the former, so the supporting text should as well.

Key Message 2 could be clearer if it started with a simple statement that “climate change affects land use and ecosystems.” While this point is in the message, it comes across as secondary and should be more prominent.

Comments on Graphics

Figure 5.1 is busy and difficult to read. The figure could be simplified by including only one data product for land cover on the figure or by selecting a different graphic. The two land-cover products are not adequately different to justify including both. Given the available time

series, it is suggested that the results of the National Land Cover Database product be presented, and that full reference(s) to all data sources in the figure be provided, which are not included in the draft chapter.

Figures 5.2 and 5.3 are too small and busy to easily decipher. Their size could be increased to improve graphic clarity.

Comments on Cited Literature

Generally, the appropriate literature is cited in Chapter 5, but a few additions are recommended. For albedo implications, Jones and Hawkes (2015) should be referenced. Also, discussion of forecasts of changes in irrigated agriculture should probably consider recent trends and regional differences (see Brown and Perves, 2014).

Comments on Traceable Accounts

Findings are generally consistent and transparent, though some inconsistencies are noted in the line comments for clarification (see Appendix B of this review report).

Other Recommended Improvements

A better description and definition of land cover scenarios in the section on future projections is needed. Sleeter et al. (2017) seem to be doing straight-line projections of trends for a “business-as-usual” (BAU) scenario. BAU might imply some (absent) process model and the scenario should perhaps be renamed as “current trends projection” and be better described. None of the projections are referenced elsewhere in the chapter or used to evaluate possible future effects on climate impacts (e.g., in the coastal zone) or on carbon storage.

Chapter 5 could be enhanced by discussion of the potential positive implications of climate for land-use (agriculture and ecosystems), e.g., increased growing seasons (the benefits of which are reduced by increased populations of pests).

It would be helpful if the topics discussed could be better cross-referenced with other chapters. For instance, increased treatment of the implications of fire (draft NCA4 Chapter 6, “Forests”) and mention of land cover and land use changes that are occurring in the Arctic region (draft NCA4 Chapter 26, “Alaska”) should be included.

Key messages from this chapter are not specifically reflected in the draft NCA4’s overarching report findings. Land cover and land use change are processes that interact with climate change, so it is appropriate that they are not included as report findings, but these interactions need to be acknowledged (in some cases better than they are currently) throughout the report.

CHAPTER 6: FORESTS

Summary

Chapter 6 is well written and clear about the impacts of climate change in recent decades and what some future adaptation options for forests may be. It describes the direct consequences of climate change (regeneration growth and productivity, mortality, and range shifts) as well as the indirect effects (wildfire, pathogens, insects, and carbon storage) in a clear fashion. The chapter considers projections of warmer temperatures, reductions in snowpack, increased drought, and increased extreme climate events in its analysis. It also integrates, evaluates, and interprets findings from recent assessments of forest health and drought resiliency. In these respects, the chapter makes general use of the findings of the CSSR and the draft NCA4 Chapter 2. The traceable accounts show that these climate-induced changes are representative of the literature.

A general concern about the chapter is the need for a statement on forest vulnerability that recognizes that forest types, disturbance regimes, and management objectives are highly variable across the United States and that no single management response or adaptation strategy will achieve resilience in all forest types. Forest differences are referred to in the chapter, but the majority of examples emphasize pine-dominated and commodity forests, with little consideration of issues relevant to urban, deciduous, mesic evergreen, tropical, and high-elevation forests. The Committee recommends an opening paragraph describing the diversity of U.S. forest types, disturbance regimes, ownership (public versus private), and uses (commodity, recreation, urban, wilderness), as well as a statement that explains which forests or regions are the focus of the chapter (e.g., there is currently no discussion of forests in Alaska, Hawai’i, or U.S. territories).

The chapter promotes fuel treatments as an adaptation measure to reduce wildfire risk, but again, this strategy is only appropriate for some, not all, forest types. Several studies suggest that fuel treatments have been unsuccessful or are economically unfeasible at a broad scale in many U.S. forests (e.g., Cochrane et al., 2012; Fulé et al., 2012; Butler et al., 2013; Barnett et al., 2016; Bradley et al., 2016; Kalies et al., 2016). Additionally, fuel treatments are unlikely to succeed in the future in light of more extreme fire weather. The success of fuel reduction efforts should be discussed in conjunction with Key Message 1 (greater disturbance with increased frequency of extreme weather). The line comments for this chapter (in Appendix B of this review report) note places in the draft chapter where more geographic or ecological specificity would be helpful, as well as parts of the text where technical terms should be replaced with nonspecialized language to improve readability for the intended audience.

Comments Related to the Statement of Task

Comments on Key Messages

Generally, the key messages for Chapter 6 appropriately reflect the primary issues related to climate change for U.S. forests, although some edits are recommended.

The chapter and supporting text for Key Message 1 does a good job of describing recent changes in disturbance regimes (more wildfires, insect outbreaks, etc.). The mention of more gradual changes in forest productivity, health, and distribution and abundance in the coming decades to centuries makes sense, but the evidence is less clear. See Appendix B of this review report for additional suggested citations to support this key message.

By citing only tree growth and carbon storage, Key Message 2 does not address other important ecosystem services provided by forests, including protection of native biodiversity (including endangered species), provision of important resources for indigenous communities, places for recreation, and maintenance of clean water. The key message should be revised to include a more representative, concise list of services.

Key Message 3 is somewhat unclear as written. In particular, the phrase “broad range of adaptation options for different resources, including applications in planning” is ambiguous. One suggested revision to address this is: “Forest management activities can increase the resilience of U.S. forests to climate change, and a broad range of management and adaptation options are currently being implemented to protect, maintain and enhance different forest resources. The future pace of adaptation will depend on how effectively….” Key Message 3 should also mention “responses” in addition to resilience, to present a more fully developed conceptual model for thinking about long-term climate impacts in forest ecosystems. Resilience also implies that forests will return to their original state after a short-term disturbance. Large climatic changes however, may cause forests to transition to new states, which will influence adaptation strategies (Millar et al., 2007).

Key Message 3 also does not address risk. It seems important to mention the notion of learning to live with fire in the future in order to better align with Key Message 1. There are several management efforts underway, for example, to reduce wildfire risk for communities in the wildland-urban interface. The examples offered to support Key Message 3 are largely federal programs; it would strengthen the text to mention some of the successful public-private partnerships and state and community efforts that seek to maintain healthy forests and reduce risk. The Committee thinks there is also an overemphasis on fuel treatments and thinning as ways of increasing forest resilience. It should be stated that these management strategies are

appropriate for some forests, when applied in a targeted fashion to reduce fuel loads, especially near structures. These strategies will also be most successful in reducing wildfire risk under non-extreme conditions. Maintaining fuel treatments is not practical on a broad scale, especially given the extreme climate events discussed in Key Message 1. See Appendix B for additional suggested citations to support this key message.

Comments on Graphics

The figures are internally consistent and are clear, but a few edits are suggested.

The caption for Figure 6.4 should explain exactly what is shown. Fire starts or beetle outbreak? Area burned? Insect-related tree mortality?

Figure 6.5 should clearly indicate that it is developed for the Pacific Northwest and the examples are region-specific. The beaver photo (page 237 of the draft NCA4) should indicate the location and also the management program that is implementing reintroduction of beaver communities.

Comments on Literature Cited

The chapter provides many recent citations from the peer-reviewed scientific literature that are very effective. It would be helpful if this new information was used to more clearly identify advancements in research since the NCA3. The Chapter 6 authors could also consider citing additional recent publications, particularly those relevant to the western U.S. forests, wildfires, and risk.

The authors may consider referencing paleoecological literature that demonstrates forest sensitivity to changes in temperature and precipitation in the past, as well as wildfire disturbance. Recent publications related to fire trends and legacy effects, rapid forest change and disturbance synergies (Key Message 1), long-term forest change (Key Message 1), and fire adaptation and risk assessment (Key Message 3) are provided in Appendix B of this review report, in addition to some paleoecological studies.

Comments on Traceable Accounts

Overall, the findings are documented in a transparent and credible way and they reflect supporting evidence. As detailed in the line comments (Appendix B of this review report), some specific information and qualification are needed to accurately portray study-specific evidence and parts of the draft chapter where additional citations might strengthen the information, especially for western U.S. forests, are noted. The assessment of confidence is communicated effectively.

Other Recommended Improvements

In general, some terminology in the chapter should be clarified. Basic terms like “large-scale” versus “small-scale,” “healthy forests,” “fire and other regimes,” “historical range of variability,” and “climate-smart forest management” should be defined for a non-specialist

audience. Similarly, examples that reference published studies should include information about the geographic scope (region, forest type, watershed) and the time span of those investigations. Phrases like “in some forests…,” “in some regions…,” or “recently” are too non-specific.

The formatting for citations in the chapter is inconsistent. Single authored citations for multi-authored papers are sometimes shown and some references are missing. In a few places, the citation does not accurately support the statement or represents a very limited finding.

Inclusion of Chapter 6 Materials in Chapter 1, “Overview”

Negative impacts on timber prices and economic well-being of forest landowners (page 36, lines 16-20) are not discussed specifically in Chapter 6, except by reference to a single paper (page 229, line 35). It would be useful to have more supporting information on these negative impacts in Chapter 6 in order to justify their inclusion in the draft NCA4 Chapter 1. Similarly, the effects of ground-level air pollutants (page 44, line 13), are not discussed in Chapter 6 except by a single reference (page 230, line 24) and the negating influence of ground-level ozone is not well explained—what is the source and what are the consequences?

In general, the Committee was somewhat disappointed that Chapter 1 does not include more reference to forests (relative to agriculture or infrastructure, for example). This omission might be in part because of some of the shortcomings in Chapter 6 described in this review. For example, if the forest chapter had described how climate impacts on forests played out differently in different regions or how adaptation measures (e.g., fuel treatments) must differ among regions, perhaps these points would have appeared in Chapter 1. The draft Chapter 6 contains important general findings that probably characterize many sectors.

CHAPTER 7: ECOSYSTEMS, ECOSYSTEM SERVICES, AND BIODIVERSITY

Summary

The draft Chapter 7 focuses on a subset of topics related to ecosystems, ecosystem services, and biodiversity, with a particularly heavy emphasis on species responses. The key messages in the draft chapter are overly generalized. The chapter would benefit considerably from expanded treatment of ecosystem services and discussion of climate mitigation strategies. Chapter 7 should also be linked more closely to related topics in the draft NCA4. Many strong examples of ecosystem services presented elsewhere in the draft report could be discussed and are noted in this chapter review.

Comments Related to the Statement of Task

Comments on Key Messages

Overall, the key messages reflect understanding about observed and projected impacts, but the evidence is presented with too many generalities. Providing some specific examples would help to support the information in the chapter’s key messages.

For Key Message 1, the links to climate change are made in a general way, with few examples that will stick with a reader. This shortcoming could be improved by including in the message the key elements of the “ecosystem function and services” that are likely to be affected. This key message is also quite long and may be more effective if split into two messages with one focused on systems, communities, and ecosystem responses, and a second focused on ecosystem services.

For Key Message 2, some specific examples (e.g., key species, habitat types, and ecosystem services needing extra attention or showing promising responses to management) would help support the information, which is otherwise vague. Successful management strategies and programs provided as support for Key Message 2 are simply a list of mostly federal programs with no detail. A table providing expanded information would be helpful, and it would be good to list other efforts by states, public-private partnerships, and nongovernment organizations.

It is also unclear why mitigation strategies are not mentioned in Key Message 2. Changes in biodiversity and ecosystem services can have a large impact on carbon sequestration and storage, as is cited in other chapter key messages. Furthermore, it is surprising that climate resilience is not included, given that climate refugia and habitat connectivity are mentioned under adaptation strategies, mostly highlighting invasive species threats. Resilience is referred to throughout the text, but never defined or illustrated.

Comments on Comments on Graphics

The chapter is under-illustrated and Figure 7.1, the only figure, provides anecdotal information about biotic and abiotic changes related to climate change around the country. The examples provided should be discussed in the text with citations to the original publications.

The Committee suggests that the Chapter 7 authors review and consider including updated versions of some figure(s) including those in Staudinger et al. (2012), which served as a technical input to the NCA3. Figures to review could include 2.1, 3.2, 3.7, and 4.2.

Using boxes or other examples to make the general concepts presented in the chapter more understandable and accessible would go far to improve this chapter.

Comments on Literature Cited

The effects of temperature, lengthened growing season, drought, extreme events, and carbon dioxide enrichment have clear influences on ecosystems, yet there is little direct reference to relevant literature, or to the draft NCA4 Chapter 2, “Our Changing Planet.” It is recommended that these relationships be highlighted in the chapter by providing examples of management strategies that target specific ecosystems and ecosystem services (and appropriate citations). More detailed recommendations are provided later in the section.

References for other ecosystem services, in addition to pollination services, should be included to bolster general statements. Also, as stated in comments for Key Message 2, there is no mention of the extensive literature on climate resilience. Some references are also missing and a few are miscited, warranting careful review by the Chapter 7 authors. The paleo references are also over-attributed.

There are too many generalities and references to meta-analyses, rather than the primary literature. Specific examples carry significant weight in convincing stakeholders about the particular topics that are the focus of this chapter. A number of examples that the Chapter 7 NCA4 authors could cite are listed in the “Other Recommended Changes” section of this chapter review and in the line comments (see Appendix B of this review report).

Comments on Traceable Accounts

In general, the traceable accounts need better documentation to support the key messages. There is little specific information about how climate change has impacted species or ecosystems compared to other non-climate drivers of change. Key Message 1 reads like a primer of potential future changes. Key Message 2 is very general and focuses on federal agency response without considering the work of state agencies and non-governmental organizations. Some examples are needed.

Also, the description of evidence for Key Message 2 (pages 273-274) does not align with the discussion in the main text of the chapter (pages 266-267) and needs some case studies, perhaps including examples of resource management actions from regional chapters discussed in a broader context. The evidence also does not describe adaptation efforts to preserve ecosystem services and biodiversity or to maintain resilience. The uncertainties listed on page 275 seem quite accurate, but rather than focus on uncertain species’ responses with respect to management practices, it would be better to discuss the need to protect the provision of ecosystem services.

Other Recommended Changes

The Committee encourages the chapter authors to look at the structure of other related chapters, such as the draft NCA4 Chapter 9, “Oceans and Marine Resources,” as an example of a well-developed and organized chapter.

Chapter 7 would be an ideal place to document links between climate, ecosystems, and equity/access issues for individuals and their communities. Describing loss of ecosystem services is one of the best ways to demonstrate that particular communities will be impacted by climate change. It can also highlight compelling examples of social, cultural, health, and economic disparities that result from changed ecosystem function.

The rationale for framing the chapter around the selected subset of topics (range shifts, phenology, adaptive capacity, invasive species, and emergent properties) is generally unclear. These topics are apples and oranges in terms of issues and the list omits several important topics that are well documented in the literature (see citations provided in the draft NCA4 that are quoted later in this chapter review).

Chapter 7 is missing treatment of specific climate science findings summarized in Chapter 1 of the draft NCA4. In particular:

• It is unclear why there is no treatment of climate mitigation strategies in Chapter 7, only adaptation. Biodiversity and ecosystem services can have significant impact on carbon sequestration and storage. Because the draft NCA4 covers adaptation and resiliency (Chapter 28) and mitigation (Chapter 29), it would be appropriate to discuss and link discussion of these topics in Chapter 7.
• Stressors—only invasive species are mentioned, with no mention of other stressor issues, which are summarized in the draft NCA4 Chapter 2 and in the CSSR. Other areas with a need for increased resilience include disease, increased frequency of extreme events such as fire, drought, riverine and coastal flooding, sea level rise, and ocean acidification.
• The interaction and fundamental underpinning of biodiversity and ecosystem services for topics covered in many other draft NCA4 chapters (Water, Energy, Land Cover and Land Use Change, Forests, Coastal Effects, Oceans and Marine Resources, etc.) should be highlighted in the report findings. Relevant chapters should also be cross-referenced throughout this chapter.

The draft Chapter 7 places most emphasis on species responses, especially invasive species, but does not spend much time evaluating changes in ecosystem services and biodiversity and their consequences. For example, the chapter is missing discussion and documentation of effects of climate-induced changes to biodiversity and other ecosystem services with respect to change in agriculture, coastal protection, recreation, fisheries, timber harvest, carbon sequestration and storage (climate regulation), flood control, water retention, water provision, water quality regulation, hydropower/tidal energy production, disease regulation, temperature regulation, cultural services, etc. While expanded discussion of all these topics would make the chapter unreasonably long, careful consideration of additional topics, including those that tie in well to other impacts discussed in this chapter, should be included to provide better chapter balance.

Specificity would help increase the significance of several statements: over what time span has change been noted in ranges (pages 259-260)? What are the range changes? What organisms are involved? References to examples discussed in the regional chapters would make an important connection. See Appendix B for more specific recommendations. In addition, the synergistic effects of land use change, pollution, and other human activities are not considered in

assessing changes in species distributions, behaviors or interactions. Expansion of the chapter to address these suggestions would provide a more complete evaluation of the chapter’s topics.

The “Regional Roll-Up” section comes across as a mish-mash of examples explained in a low level of detail. More information is needed to support the examples as evidence of climate change. For example, the statement that climate change is threatening salmonid populations in the Southwest and Northwest, without discussing species, land use, fire, and other factors, is incomplete. The reference to bear populations and reduced salmon mortality rates in Alaska is also unclear. This revision could be done as parenthetical additions to the sentences or a more logical breakdown by regions. Chapter 7 should also acknowledge the lack of regional and nation-wide modeling efforts linking changes in climate to changes in biodiversity and ecosystem services, if that is the case.

The chapter should acknowledge uncertainties and “emerging issues” and explain what is well understood, what is more speculative, and what needs more research to be fully understood.

Overall, the draft chapter is written at a level appropriate for non-specialists, although terms like “holistic ecosystem-based approaches,” “harmful impacts of current and future resource management challenges,” “tragedy of the commons,” “seed sourcing,” “novel sectors and livelihoods,” and “assisted migration” should be defined.

Select Detailed Comments

A few line comments are highlighted here; see Appendix B for additional suggestions.

• Page 257 and page 259, lines 21-31 of the draft NCA4. By focusing on the “state” or “stocks” of biodiversity and ecosystems, there is a narrow focus on impacts to species and communities. Virtually no mention of which ecosystem services are likely to be affected is provided. Flows from the “stock” (biodiversity and natural ecosystem components) to affected people is the definition of ecosystem services and should be addressed throughout the draft chapter, including here in the framing. This is the only chapter that links changes in biodiversity and ecosystems to the flows of benefits from ecosystems to people. Other chapters talk about livelihood and economic impacts without citing the flows from biodiversity and ecosystem changes to ecosystem services. In this part of Chapter 7, the mechanistic connection is missing. For example, as noted earlier, the chapter is missing discussion and documentation of effects of climate-induced biodiversity and ecosystem changes on agriculture, coastal protection, recreation, fisheries, timber harvest, and carbon sequestration and storage.
• There is virtually no information provided on trends in ecosystem services, with the exception of those given on page 265 and a few other locations. Some of these topics should be brought forward as part of key messages. The discussion of pollination with the necessary documentation is a rare but useful exception.

A straightforward approach to addressing the gaps noted in this review would be to cite or cross-reference a number of useful studies and findings provided in other draft NCA4 chapters. A few examples are provided here.

Examples from draft NCA4 Chapter 9, “Oceans and Marine Resources”:

• Page 339, line 24 to page 340, line 11. “This means that fishing communities in Hawai’i and the Pacific Islands, the Caribbean, and the Gulf of Mexico are particularly vulnerable to climate-driven changes in fish populations. Declines of 10%–47% in fish catch potential in these warm regions, as compared to the 1950–1969 level, are expected with a 6.3°F (3.5°C) increase in global atmospheric surface temperature relative to preindustrial levels (reached by 2085 under RCP8.5) (Cheung, Reygondeau, et al. 2016). In contrast, total fish catch potential in the Gulf of Alaska is projected to increase by approximately 10%, while Bering Sea catch potential may increase by 46% (Cheung, Frolicher, et al. 2016). However, species-specific work suggests that catches of Bering Sea pollock, one of the largest fisheries in the U.S., are expected to decline (Ianelli et al. 2016), although price increases may mitigate some of the economic impacts (Seung & Ianelli 2016). Ocean acidification is expected to reduce harvest of U.S. shellfish; while future work will better refine impacts, cumulative consumer losses of $230 million are anticipated under the higher scenario (RCP8.5) (EPA 2017). The implications of the projected changes in fisheries dynamics on revenue (Lam et al. 2016; Seung & Ianelli 2016) and small-scale indigenous fisheries remain uncertain (Weatherdon et al. 2016). Native Americans depend on salmon and other fishery resources for both food and cultural value and there will be significant challenges to some place-based communities (for example, Krueger and Zimmerman 2009; Ch. 24: Northwest).”
• Page 343, lines 5-10. “Coastal communities are especially susceptible to changes in the marine environment (Colburn et al. 2016; Himes-Cornell & Kasperski 2017), and the interaction between people and the ecosystem can amplify the impacts, increasing the potential for surprises. In the Gulf of Maine in 2012, warm temperatures caused lobster catches to peak 3-4 weeks earlier than usual. The supply chain was not prepared for the early influx of lobsters, leading to a severe drop in price (Mills et al. 2013).”

Examples from draft NCA4 Chapter 25, “Southwest”:

• Page 1088, lines 21-25. “The irrigation-dependent agriculture of the Southwest provides half of the fruits, vegetables, and nuts of the entire country (USDA, 2015) and most of the national production of wine grapes, strawberries, and lettuce (Starrs and Goin, 2010). Agricultural irrigation accounts for 70% of regional water use (Cooley et al., 2016; Maupin et al., 2014). Consequently, drought and competing water demands pose a major risk for agriculture.”
• Page 1088, lines 37-39. “California has the most valuable ocean-based economy in the country, employing over half a million people and generating $20 billion in wages and $42 billion in economic production in 2014 (NOAA, 2017a).”
• Page 1089, lines 18-23. “Over the last five centuries, many Indigenous peoples in the Southwest have been forcibly relocated onto lands with limited water and resources (Bauer, 2016; Denetdale, 2009; Iverson, 2002). This historical legacy exacerbates the impacts of climate change because Indigenous peoples are restricted to increasingly drier areas. Furthermore, many Indigenous peoples depend on natural resources for cultural and subsistence needs, so climate change can negatively affect material and spiritual health (Norton-Smith et al. 2016).”

• Page 1094, lines 11-17. “Wildfire can pose a threat to people, particularly as building expands in fire-prone areas. Wildfires around Los Angeles from 1990 to 2009 caused $3.1 billion in damages (Jin et al., 2015). Respiratory illnesses and life disruptions from one wildfire north of Los Angeles cost an estimated $84 per person per day (Richardson et al. 2012). In addition, wildfires degraded drinking water upstream of Albuquerque with sediment, acidity, and nitrates (Dahm et al., 2015; Sherson et al., 2015) and in Colorado with precursors of cancer-causing trihalomethane (Hohner et al. 2016).”
• Page 1097, lines 18-21. “Currently, 200,000 people in California live in areas 3 feet (0.9 m) or less above sea level, so much of this population is at risk of losing their homes to inundation by 2100 (Hauer et al. 2016). Storm surges and high tides on top of sea level rise would exacerbate flooding (Griggs et al. 2017).”
• Page 1098, lines 17-19. “Shifts in the timing of Dungeness and rock crab fishery into whale migration season in 2016 contributed to increases in whale entanglements in fishing gear (Chavez et al. 2017).”
• Page 1106, lines 16-18. “The California drought led to losses of more than 10,000 jobs and the fallowing of 540,000 acres (220 000 hectares), at a cost of $900 million in gross crop revenue in 2015 (Howitt et al. 2015).”
• Page 1111, line 22-28. “Access to healthcare, social isolation, housing quality, and neighborhood poverty are also key risk factors for heat- related health impacts (Reid et al. 2012). Urban design strategies to address these risk factors include increasing walkability or bicycle safety and adding trees and other vegetation. These strategies can achieve multiple health benefits, including increasing physical activity, thereby helping residents to maintain a healthy weight, reducing the urban heat island effect, and reducing exposure to harmful air pollutants from vehicles.”

CHAPTER 8: COASTAL EFFECTS

Summary

This chapter starts strong with a description of the economic value of the U.S. coastline and a sampling of adaptation efforts from around the country. The information would be strengthened by explaining how the examples were chosen and why. For instance, were they the only ones highlighted in the regional chapters? Though the synthesis of material is a nice approach, the examples selected are not largely representative of the scale and scope of current efforts. Also, fact-checking with the source communities is needed, given some small inaccuracies noted in this section. The chapter includes strong statements on flooding that are justified by the science. The key messages are generally positive and are oriented toward informing response actions. In particular, Key Messages 2 and 3 are effective because they highlight the problem as well as a solution path (adaptation). As the chapter progresses, the tone becomes a bit more opinionated and speculative, which should be carefully reviewed by the chapter authors.

Comments Related to the Statement of Task

Comments on Key Messages

Key Message 2 covers so much information that it is difficult to identify the most important points, lessening the impact of the message. It is suggested that it be broken down and the information be further prioritized in order to make the takeaways much more concise. Deleting the first sentence and the “However” start to the second sentence is suggested, with that information then conveyed solely in the supporting text.

Key Message 3 introduces the social equity issue, but does not offer strategies to address this vulnerability, which would make the message more effective. Generally, social equity should be introduced report-wide and include examples for how social equity and climate change threats to vulnerable communities can be addressed.

Comments on Graphics

Figure 8.2 is taken from the National Oceanic and Atmospheric Administration (NOAA) and shows selected coastal effects of climate change in each U.S. region. The Miami Beach mention is not quite correct. It states, “Miami Beach, FL has invested $500 million into raising public roads and seawalls, and improving stormwater systems.” Miami Beach is in the midst of this multi-year $500 million program. Only $100 million has been spent to date in improved stormwater drainage, raised roads, and seawalls. The work is ongoing, but the reference makes it appear to have been completed. The adaptation efforts cited should be verified to the direct source, not just NOAA, to confirm program and status. It is also recommended that the efforts in Puerto Rico be placed in time context (before or after the 2017 hurricane season).

Notably absent in the Figure 8.2 description of the Southeast is the Southeast Florida Regional Climate Change Compact.¹ This is one of the original local intergovernmental models of collaboration in climate change adaptation and mitigation planning, beginning in 2009. Recent developments include the creation of the 2017 updated Regional Climate Action Plan.² Figure 8.2 is also hard to read given the extensive amount of small text and it is suggested that this be formatted more appropriately.

It is unclear that the multi-page table (pages 298-302) is associated with Figure 8.2 because no title, description, or linkage to the figure is provided. The table does not seem to be comprehensive, as the Committee identified many missing examples for at least the Southwest region/California. The Chapter 8 authors should consider revising the table to either be more comprehensive or not including it at all. If the table is retained, it should include the examples mentioned in the regional chapters at a minimum, and be explicit in stating the rationale for the inclusion of presented examples.

Figure 8.3 should reference the City of Miami Beach, not the City of Miami. These are two separate municipalities and the incorrect one is noted.

The discussion on equality versus equity presented in Figure 8.4 is important; however, it seems out of place in this chapter, particularly when compared to the other charts, graphs and maps. The discussion would be better placed in the front sections of the draft NCA4, and interwoven throughout chapters as appropriate.

Comments on Literature Cited

It is recommended that reference to news publications (e.g., The New York Times and Scientific American) be avoided to ensure accuracy of the information. A better source of information is the city itself in order to avoid misinterpretation or speculation.

Authors should be specific when discussing the NCA3, the NCA4, and other related assessment products (e.g., on page 314, line 11, clarify what is meant by NCA).

The chapter should include discussion of compound flooding (see Moftakhari et al., 2017).

Comments on Traceable Accounts

The traceable accounts are supported by appropriate confidence levels. However, the key messages could be more effective if conveyed in a more positive tone to balance risk with adaptation and mitigation efforts.

Other Recommended Changes

The economic and social value of coastline properties is presented, but a further dive into real estate, insurance, and banking would be beneficial for the audience of this assessment report,

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¹ See http://www.southeastfloridaclimatecompact.org.

² See http://www.southeastfloridaclimatecompact.org/regional-climate-action-plan.

especially for decision-makers. This could also include port infrastructure and associated economic impacts.

For the topic of coastal inundation, inclusion of more near-term projections could provide valuable information to decision-makers and other stakeholders.

Cross-referencing other draft NCA4 chapters, including Chapter 9, “Oceans and Marine Resources,” and Chapter 11, “Built Environment, Urban Systems, and Cities,” as appropriate would also improve the chapter.

CHAPTER 9: OCEAN AND MARINE RESOURCES

Summary

Chapter 9 is centered around three useful themes that underpin the key messages: projected impacts, opportunities for reducing risks, and emerging issues and research gaps. For the most part, these key messages are well selected and the discussion and findings are reported in a credible, transparent manner. The chapter is well organized and effectively orients the reader around the key conclusions, although there is a somewhat abrupt shift between the tone of writing in the introduction, which seems more appropriate for a general audience, and the more technical tone used in the rest of the chapter.

The chapter could benefit from a more comprehensive treatment of the climate impacts on livelihoods by additional attention to other marine-resources based livelihoods (e.g., aquaculture, transportation, recreation, energy) and better linkage to other relevant draft NCA4 chapters.

Comments Related to the Statement of Task

Comments on Key Messages

The three key messages are important areas of focus that concentrate the discussion on iconic species and ecosystems (coral reefs and sea ice ecosystems), impacts on fisheries and fishing communities, and the interaction of long-term climate change and short-term variation creating conditions for disruptive extreme events in the marine environment. The organization of the information and the literature cited provide appropriate support for the messages and reflect current understanding about observed and projected impacts to the United States.

The points being made in Key Message 2 could be strengthened by integrating a more comprehensive treatment of climate impacts on livelihoods associated with fisheries, including aquaculture and recreational endeavors, in the supporting narrative.

The content of Key Message 3 and supporting text could include stronger discussion of the linkages between extreme events and coastal systems. Cross-reference to the draft NCA4 Chapter 8, “Coastal Effects,” is also suggested.

Comments on Graphics

Chapter figures effectively demonstrate important aspects of the key messages in a clear and consistent manner.

Comments on Literature Cited

The citation of current literature and links to the CSSR provide robust documentation for the discussion and key messages. The examples used are linked to points being made and are effective in the discussion. In general, the messages, discussion, and findings are reported in a credible, transparent manner, despite the somewhat abrupt shift between the tone of writing in the introduction, which is more appropriate for a general audience, and the more technical tone in the rest of the chapter.

Comments on Traceable Accounts

The reported process for gathering data and deciding on key messages and supporting documentation for the chapter is inclusive and contains opportunities for meeting the requirements of the traceable accounts. The information and literature citations in the traceable accounts are linked to the key messages, but the Chapter 9 authors should be consistent in how the reported confidence and likelihoods are ordered within the key messages.

Other Recommended Changes

Risk and adaptation measures are generally addressed in the discussion of the key messages, but examples of approaches that could lead to resilience are not apparent. Chapter 9 should include examples, such as:

• Restoration or protection of natural marine ecosystems and ecosystem impacts on coastal flood protection and improvement in fishery habitat.

• Filtration of land-based runoff that can exacerbate temperature and ocean acidification stresses on coastal species is a widely cited strategy for reducing risk of marine systems to climate change.
• Enhancing life history diversity of species through aquaculture, fishery management, and habitat improvements, which can increase resilience to climate change.
• Investments in local ocean-based communities for more diversified livelihoods (cross-reference with other relevant chapters in the draft NCA4).

Chapter 9 could provide more comprehensive treatment of ocean and marine resources, which would make the chapter content more applicable to a wider audience. This could be accomplished in part by addressing the following concerns:

• Reduce the bias towards treatment of fisheries-related issues and add discussion of climate change impacts on other major coastal resource-based economies, including aquaculture practices, tourism, and energy development.
• Include discussion of mitigation in marine systems, which are substantial due to the carbon sequestration and storage capacity of these systems, by at least providing the reader an entry into that literature or referring to the draft NCA4 Chapter 29, “Mitigation: Avoiding and Reducing Long-Term Risks.”
• Bring the integrated nature of ocean systems into the discussion through linkages among habitats, species/foodwebs, and humans interacting with marine systems through physically inter-connected ocean currents and tides. Climate change impacts on one of those inter-connected system components can have important cascading effects on other components that are important to note. This approach would also add more focus on people in the chapter.
• Provide change estimates for other marine/coastal systems, besides coral reefs and sea-ice systems, if possible.

CHAPTER 10: AGRICULTURE AND RURAL COMMUNITIES

Summary

Overall, Chapter 10 appropriately highlights the high climate sensitivity of the U.S. agricultural sector and rural communities. It also illustrates some of the complex interactions between physical climate changes, terrestrial processes, and management practices that affect the impacts of climate on agriculture. The traceable accounts section provides ample evidence for the validity of the chapter’s main conclusions on how climate changes over the coming decades will introduce new risks to the sector and that it will likely lead to reduced agricultural productivity in the absence of robust adaptation measures.

The chapter currently suffers from readability issues that make the state-of-knowledge difficult to ascertain in several instances. It could be made more effective through substantial improvements to message clarity. The most beneficial improvements would include: reorganizing key messages around impacts rather than forcings in order to avoid oversimplification of impact pathways and unnecessary redundancy in the text; strengthening discussion of recent trends and knowledge advancements since the NCA3; organizing the progression of paragraphs and sections into a tighter logical flow; and explicating baselines in

the agricultural sector against which impacts may be measured (while recognizing that some of these will be moving baselines).

The chapter would also benefit from added or augmented background and science content in some places, as noted in this section of the review report. Related to this, figures and case studies should be more tightly coupled to key messages.

Comments Related to the Statement of Task

Comments on Key Messages

The key messages generally include references to the most important pathways associated with climate change impacts on the agricultural sector and rural communities. However, they currently lead to some oversimplification and redundancy, where significant improvements can be made.

Key messages are currently organized around climate change attributes or forcings: drought (Key Message 1), temperature extremes (Key Message 2), and rainfall intensity (Key Message 3). In reality, these attributes are anticipated to combine to jointly affect agricultural productivity, sometimes in complex ways. For example, yields will be concurrently affected by changes in temperature extremes (heat stress), rainfall intensities (leaching, waterlogging, denitrification), and droughts (water stress). This set of complicated interactions is one of the central challenges of accurately forecasting overall climate change impacts on agriculture, and this point should be reflected in the chapter. As currently written, for example, Key Message 1 intimates that increased drought frequency and duration will be the sole or primary driver of yield impacts. This does not appear to be the chapter authors’ intent, given that yield impacts are referenced in multiple sections, but the text could be interpreted this way.

To address these concerns, the Chapter 10 authors should consider reorganizing the key messages around categories of impacts. For example, yield impacts as the subject of a key message statement discussing the most likely net effects of changes to temperature, rainfall, and drought. The advantages would be that messages could be framed in a way that is clear about the projected impacts, but does not oversimplify the pathways and interactions, which would clear up some redundancy in the text (e.g., the weather impact on yield is currently mentioned multiple times in the chapter).

The key messages are reasonably well-reflected in the report findings. The Agriculture Report Finding also includes the following sentence, which is broadly in line with the recommendation for rewrites to the key messages: “While some regions may see conditions

conducive to expanded (or alternative crop) productivity, overall, yields from major U.S. crops are expected to decline as a consequence of increases in temperatures, and possibly changes in water availability, diseases, and pest infestations” (page 21, lines 22-25).

Comments on Graphics

Figures are mostly focused on industry background information and some climate-driven trends. It is suggested that the Chapter 10 authors consider rebalancing this set of materials to reflect projected impacts emphasized in the key messages.

There may be some missed opportunities for reinforcing key messages with tables, figures and case study boxes. For example, the current sole case study (and Figure 10.3) is about groundwater level trends of the Ogallala Aquifer, which is tangential and potentially misleading; most readers might reasonably assume that the sole case study in the chapter would highlight a well-documented climate change impact, but the southern Ogallala water declines are mostly associated with the juxtaposition of intensive agriculture and low groundwater recharge.

Comments on Literature Cited

The text generally does not provide enough continuity with the NCA3. Key messages between the NCA3 agriculture chapter and the draft NCA4 are rather different. In some instances, this is clearly appropriate, especially where the scope is expanding to impacts on rural communities. In other instances, it is not clear whether the differences reflect new science since the NCA3, intentional differences in highlighted impacts with approximately the same state of knowledge, and/or an artifact of different author group composition and expertise. In accordance with the goal of sustained assessment, effort should be made to comment on the differences between the NCA3 and NCA4 and on the predominant considerations around deviations between them.

The text also does not sufficiently point out the knowledge advances in the scientific literature in the time period following publication of the NCA3 (approximately 2013 to present). On a positive note, the chapter includes a number of relevant journal references published since the NCA3, but there is not much literature synthesis on how data trends, industry trends, model simulation studies, etc. have advanced the state of the science in recent years. Despite the space limitations of the chapter, this is an important addition worth at least one paragraph.

Comments on Traceable Accounts

The traceable account section provides useful information on the chapter development process and contains a good set of references.

Other Recommended Changes

Given the chapter title and scope, it would be valuable to touch on what makes rural communities different from other populations with respect to their sensitivity to climate change. The portions of the chapter text on this topic essentially focus on heightened prevalence of

poverty, along with poverty of historically vulnerable populations. However, there are other distinguishing features for many rural and farm communities that may be helpful to consider vis a vis climate change impacts, such as the tight coupling of personal incomes and local tax receipts (funding to schools, etc.) to commodity prices and farm profitability; the reliance on migrant labor (at least in some regions); and the close proximity to air and water quality issues that arise from intensive farming operations. A case study box on rural communities (or one rural community) would be a welcome addition.

Beyond the key messages, the text suffers from some disorganization that diminishes its readability. For instance, the text does not do enough to separate these inter-related but unique considerations:

• Direct biophysical impacts on crops from changing weather variables (e.g., heat stress effect on yield)
• Indirect effects (e.g., accelerated weed pressures, nutrient stress from increased losses)
• Downstream or cascading effects (e.g., food prices, nutrient leaching, water consumption)
• Adaptation methods and their ability to mitigate/buffer/offset the worst impacts (e.g., changing crop rotations)
• The agricultural sector’s role in greenhouse gas emissions and carbon sequestration and storage

All of these considerations are touched on to some degree, but as written, it is up to the reader to separate them out into logical categories in order to keep from conflating them, and any given paragraph in the draft chapter may comment on several of these categories.

The chapter would benefit from greater explication of how baselines for assessing impacts are defined. Many of the claims about observed changes (e.g., “losses” on page 381, lines 1 and 4; “production declines” on line 23; and “sea level rise” referenced on page 383, lines 12-13) lack reference to baseline date(s), making the actual magnitude of changes difficult to discern. Then, when measuring projected changes from baselines, there is some unevenness in how economic estimates are provided, and in general it is not clear from the current text what the authors think are the most important/risk-prone/expensive impact pathways among the many that are listed in the chapter. Is it possible to compare projected risks or impacts across the different key messages on economic or other quantitative terms? Could current best estimates be tabulated for quick comparison, even at just an order-of-magnitude precision? If so, this would be a helpful reference for the reader. If not, the NCA4 authors should consider adding a sentence stating that current projection skill does not allow for force ranking in that way.

The “State of the Agriculture and Rural Communities Sector” section of Chapter 10 seems like a good place to add context for readers who are not already intimately familiar with the baselines. Consider adding to this section: (1) quantitative comments in the first paragraph on food security importance in addition to economy and jobs; (2) a big-picture overview of the key products and regions of U.S. agriculture; (3) comments on some key industry trends; and (4) a description of the most important takeaways relevant to agriculture from the draft NCA4 Chapter 2, “Our Changing Planet.”

The chapter would benefit from a more logical organization from section to section and paragraph to paragraph. It would be helpful to use section breaks and paragraph separation to

clearly separate discussions of different considerations, such as physical climate changes, direct impacts, indirect impacts, downstream impacts, and adaptation measures. Similarly, separation of comments on past (observed) versus future (forecasted) patterns is needed when evidence is referenced.

There are many parts of this chapter that would benefit from a re-write. The first sentence of each paragraph should be a topic sentence that states the main point of the paragraph. As an example, the paragraph beginning on page 374, line 21, starts with “Observed climatic changes are consistent with those predicted by global climate models.” The next few sentences might be expected to focus on supporting evidence and details about this point, but instead, the next two sentences are about how agriculture is highly climate-sensitive. Then, the final sentence is about adaption happening up to the present date. These three strands do not flow well together and none of them are well-developed with supporting details.

Related to this organizational suggestion, the NCA4 authors should reconsider the order of topics presented. For example, there is a long discussion of adaption methodologies for industry prior to description of the climate changes themselves and the impact potentials that are motivating the adaptation. This order seems backwards and potentially confusing for the reader.

Some additional specific areas of improvement that are addressed in line-by-line comments (see Appendix B of this review report) include:

• Further development of discussion of land use change as an adaptation mechanism, which is acknowledged in the text but not addressed with much detail or any examples
• Impacts of climate change and carbon dioxide accumulation on nutritional quality of crops, which may be an emerging research area to note
• Impacts on pathways of off-farm environmental impacts beyond runoff and erosion
• Some synthesis comments of major modeling and experimental efforts since the NCA3, either in the body text or traceable accounts section

References to consider including in this chapter, along with other detailed comments, are provided in Appendix B.

CHAPTER 11: BUILT ENVIRONMENT, URBAN SYSTEMS, AND CITIES

Summary

The United States builds slightly more than $1 trillion (2017) worth of buildings and infrastructure per year (U.S. Census³) that are largely not designed for a changing climate. Chapter 11 is the home chapter for infrastructure in the draft NCA4 and while it appropriately covers many of the impacts of climate change likely to exacerbate existing challenges in the built environment, urban systems, and cities, there are important areas where mention or expanded treatment would strengthen the chapter and inform readers of the challenges that this broad sector is experiencing. Some areas where expanded treatment is needed include discussion of the non-stationarity in hazards for their use in planning and design practices, consideration of

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³ See https://www.census.gov/construction/c30/c30index.html.

multiple climate hazards (or multi-hazards), and risk-informed frameworks of adaptive management. The impact of urban heat islands should also be more comprehensive and perhaps included in a key message. Chapter 11 could also benefit from cross-referencing to the draft NCA4 Chapter 17, “Sectoral Interdependencies, Multiple Stressors, and Complex Systems,” among others.

Comments Related to the Statement of Task

Comments on Key Messages

The selected key messages are appropriate, generally clear, consistent, and communicated appropriately for the intended audience.

Key Message 2 is a logical outcome from materials covered in the chapter; however, it should mention the lack of current building standards to account for non-stationary hazards in planning and design practices.

Key Message 3 should include interdependencies among the hazards that could exacerbate the impacts. Multi-hazard treatments are necessary to examine such effects, such as the dependence of stagnant air with heat waves, that might lead to persisting poor air quality (e.g., Lombardo and Ayyub, 2015).

Key Message 4 should directly call out the use of risk methods to inform policy and decision-making practices for achieving energy and economic efficiencies in solutions or actions. The key message calls out the benefits of multi-city networks, yet fails to showcase any successful models, such as the Southeast Florida Regional Climate Change Compact,⁴ which has matured since its mention in the NCA3. Also noteworthy is the 100 Resilient Cities Network,⁵

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⁴ See http://www.southeastfloridaclimatecompact.org.

⁵ See http://www.100resilientcities.org.

which was created specifically to address urbanization, globalization, and climate change. The use of these examples in the text supporting this key message would strengthen its impact.

Comments on Graphics

Overall the graphics are clear, internally consistent, and communicated well for the intended audience, with some exceptions noted here.

Figures 11.1, “Current and Projected U.S. Population,” and 11.2, “Projected Changes in the Number of Very Hot Days,” are appropriate to include, but Figure 11.2 should be shown prior to Figure 11.1. Figure 11.1 would be improved by having the years listed on the figure instead of in the caption. The “Number of People” legends should also be revised to provide consistent binned values and the same colors associated with each binned value in all panels.

The message that Figure 11.3 intends to communicate is unclear. The figure might be improved if the caption were enhanced to provide details that correspond to the three illustrations shown in the figure.

Figure 11.4 is generally effective, but the schematics are unclear. Providing details in the caption that correspond to the items shown in the graphic could help address this.

The chapter would benefit from an additional figure that is similar in format to Figure 11.2, but illustrates projected change in precipitation in urban areas.

The photograph on page 425 should be numbered and the caption enhanced to provide greater rationale for its inclusion in the chapter.

Comments on Literature Cited

The chapter reflects the peer-reviewed scientific literature, with a particular focus on recent literature since publication of the NCA3. No critical content areas are left out of the chapter, although the following items should be included.

Applicable engineering standards require updating so that they provide guidance on computing design extremes of hazards based on non-stationary stochastic processes. Additionally, there is also a need to use new planning and design philosophies, such as adaptive design, observational methods, adaptive risk management, etc. (Wright et al., 2013; Ayyub and Wright, 2016). These efforts are being expanded further by the American Society of Civil Engineers Committee in the ongoing development of a manual of practice on adaptive design and risk management on the adaptation to a changing climate, with a report titled, “Climate Resilient Infrastructure.”

Increases in salinity, temperature, and humidity due to a changing climate could result in an increase in corrosion and degradation rates, reducing life expectancy of the built environment and jeopardizing integrity, efficiency, and safety. These effects are not fully explored in the chapter.

Comments on Traceable Accounts

The findings for Chapter 11 are documented in a consistent, transparent, and credible way. They reflect supporting evidence and assessment of confidence levels, although the chapter does not include an assessment of likelihoods, which should be added where appropriate.

Other Recommended Changes

The chapter could be improved by increased treatment of some topics and inclusion of a few additional ones. The Committee recognizes that there are space constraints and that extensive expansion is infeasible, but recommends prioritizing the inclusion of topics that the Chapter 11 authors deem to be effective additions. In revising Chapters 11 and 12, there may be opportunities for the author teams to expand on issues in one chapter or the other and cross-reference. Some issues to consider:

• Expanding the coverage of social and socioeconomic systems and common causal factors across sectors and regions, e.g., consumption behavior, culture, etc.
• Expanded attention to urban transportation systems and underground structures, as they are vulnerable to local flooding, sea level rise, and storm surges.
• Inclusion of impacts of localized flooding due to extreme precipitation.
• More in-depth discussion of urban heat islands is needed, including their magnitude, with reference to the substantial body of literature on this subject. Given the strong effects of heat islands in the built environment, the Chapter 11 authors should consider whether inclusion of this topic in a key message is warranted. Reference to the description of urban heat islands in the draft NCA4 Chapter 5, “Land Cover and Land Use Change,” is also recommended.
• Inclusion of monitoring and control needs resulting from the complexity associated with a system of systems, and non-stationarity of interdependencies.
• Expanded discussion of risk, adaptive design, and “Real Options” (e.g., Woodward et al., 2013) for the purpose of allocating resources effectively to achieve economic efficiency.
• More examples and expanded discussion of the integration of natural ecosystems into city planning and design, such as the value of dunes, wetlands, and mangroves.

Finally, improved linkage to the content of other chapters should be considered. This includes cross-reference to the draft NCA4 Chapter 17, “Sectoral Interdependencies, Multiple Stressors, and Complex Systems,” as well as Chapter 8, “Coastal Effects,” and others as is appropriate.

CHAPTER 12: TRANSPORTATION

Summary

Chapter 12 is well written and appropriately covers the transportation sector as a backbone of economic activity for mobility and connecting key elements of the economy. The chapter is effective in conveying the message that the ability of the transportation sector to

perform reliably, safely, and efficiently is undermined by a changing climate due to hazards such as heavy precipitation, coastal flooding, heat, and changes in average precipitation and temperature. These factors impact individual assets across all modes of transportation, affecting the performance of associated transportation networks and imparting critical ramifications to the economy and society at large.

The chapter addresses key elements associated with climate change impacts on the transportation sector, but could be enhanced by including or expanding the discussion of some key issues. These include mention of potential impacts of disruptive or transformative technologies, such as automated vehicles or autonomous aerial vehicles, and the inability of building standards to account for non-stationarity in hazards in planning and design practices. The chapter would benefit from increased discussion of social and socioeconomic systems as they relate to climate change impacts on transportation, and relationships to human behavior, culture, and other factors.

Comments Related to the Statement of Task

Increased coverage of social and socioeconomic systems, and common causal factors across sectors and regions, such as consumption behavior of people, culture, etc. is needed. It is essential to examine the connections among transportation, social, and other systems, and their interdependencies in order to enhance the management of societal responses to a changing climate. These interdependencies are discussed in the draft NCA4 Chapter 17 and cross-referencing might be appropriate, as these systems are also subject to other stressors, such as population growth, economic demands, and technological changes and their potential compounding effects.

Comments on Key Messages

Key Message 1 should be appropriately qualified to account for potential impacts of disruptive or transformative technologies or behavioral changes of users, such as the use of autonomous vehicles or alternative fuel vehicles. These factors could change the use allocation of energy sources with impacts on the environment and related attributes that may then affect hazards and extremes. In the first sentence of Key Message 1, “system” is plural in the “Executive Summary” section and singular in other sections of the chapter. Text should be updated so that identical language is used in all locations where this key message appears.

Key Message 2 is a logical outcome from materials discussed in the chapter. However, it should mention the deficiency in building standards to account for non-stationarity in hazards in planning and design practices.

Key Message 3 should call out for the use of risk methods to inform policy and decision-making practices for achieving economic efficiencies in solutions or actions. Although additional research is needed to support broad conclusions and system-wide risk assessments, the value of this type of information may be appropriate to indicate.

Comments on Graphics

Figure 12.1 is likely to be confusing to a general audience. The year 2010 has already passed, so the three maps corresponding to 2010 should be based on the data, i.e., all three maps for 2010 should look the same. The inclusion of 2010 as a projection and why it differs among intermediate-low, intermediate, and extreme scenarios should be explained.

The chapter would benefit from figure(s) showing the broader impacts of multi-hazards on the transportation sector. A good example is Figure 5.1 or 5.3 in the transportation chapter of the NCA3 report (Schwartz et al., 2014).

Comments on Literature Cited

The coverage of specific aspects of the transportation sector is lacking and is noted in this section of the review. Many of these suggestions apply more broadly to the built environment and are also emphasized in the review of Chapter 11. In revising Chapters 11 and 12, there may be opportunities for the chapter author teams to expand on these issues in one chapter or the other and cross-reference.

The Chapter 12 authors should consider including discussion of the challenges posed by the use of engineering standards that do not account for climate change and therefore do not provide guidance on computing design extremes of hazards based on non-stationary stochastic processes. The transportation sector constitutes a significant portion of U.S. investment in infrastructure and, like noted in the review of the draft NCA4 Chapter 11, this infrastructure is largely not designed to standards reflective of the changing climate. Applicable engineering standards require updating so that they provide guidance on computing design extremes of hazards based on non-stationary stochastic processes.

Other areas of recommended expansion of discussion include vulnerabilities of metrorail transit systems to local flooding, sea level rise, and storm surges. Additionally, increases in

salinity, temperature, and humidity results in an increase in corrosion and degradation rates that reduce life expectancy of infrastructure and lead to other integrity concerns. As can be adequately supported by available literature, coverage of impacts of future transportation-related technologies, such as alternate fuels as a result of electric cars and autonomous systems, as well as changes in transportation modes and behavior of users and traffic would strengthen this chapter. For Chapter 12 in particular, the importance of taking into consideration changes in human behavior and lifestyle are important, but largely absent in the draft text.

Finally, this chapter would benefit from providing conceptual guidance on performing risk analysis and risk management in order to account for how impacts interact across sectors and scales. Risk analysis and management informs decisions for allocating resources effectively to achieve economic efficiency and could be explored (e.g., Ayyub, 2014).

Comments on Traceable Accounts

The chapter findings are documented in consistent, transparent, and credible terms. They reflect supporting evidence and assessment of confidence levels, although they do not include an assessment of likelihoods, which should be included, when appropriate.

While the chapter cites scientific literature on climate change, it is sparse on scientific literature on climate change impacts affecting transportation. This is acknowledged in the traceable accounts section for Key Message 3, but most references in traceable accounts for all key messages in this chapter are to news reports or gray literature. This may reflect the state of the science on transportation impacts; if peer-reviewed literature on the topic is limited, it could be noted explicitly. The citations are quite useful for establishing previous impacts, although they do not establish deviations from previous conditions or provide insight on attributions, such as separating climate effects from other interdependencies relating to non-climate stressors like aging infrastructure, population, and land-use change. The NCA4 authors should consider whether such an attribution analysis would lead to assigned confidences being overstated.

CHAPTER 13: AIR QUALITY

Summary

Chapter 13 is a new chapter in this NCA and provides useful information on the impacts of air quality on climate change. It is well written and appropriately evaluates scientific evidence on this topic. The technical level is appropriate for a broad audience and effectively conveys the key messages. Recommended improvements include framing climate-related air quality concerns in the broader context of air quality and focusing more strongly on air quality consequences for human health.

Comments Related to the Statement of Task

Comments on Key Messages

For the most part, the key messages are clear, consistent, appropriate, and reflect current understanding regarding the observed and projected impacts of climate change on air quality. However, some adjustments and clarifications may be useful.

Key Message 1 notes that adverse impacts of climate change will compromise ongoing efforts to improve air quality by controlling air pollutant emissions from human activities alone. It may be useful to note that many millions of U.S. persons already live in areas exceeding the health-based standards for air quality. This is mentioned later in the chapter, but, if included in Key Message 1, would provide important context and links to non-climate stressor interaction with climate impacts.

For Key Message 2, the links between wildfire smoke and health are understated because the key message relates to outdoor activities. The chapter does note that “wildfire smoke increases the risk of respiratory and cardiovascular disease and poses a substantial health burden” (page 296, lines 20-22) and cites the appropriate references. Given this scientific evidence, the key message for wildfires’ impact on air quality should be about the consequences for human health, not for recreational activities.

Key Message 4 is appropriate, but it could be strengthened and clarified by specifically stating that the human health risks in the short term could be reduced by improved air quality in the short term from reduced greenhouse gas emissions. This issue, often referred to as “co-impacts” (or “co-benefits”), has been discussed in the scientific literature and in the draft NCA4 Chapter 14. Some examples are West et al. (2013) and Gao et al. (2018).

Comments on Graphics

The graphics are clear, consistent, appropriate, and reflect the supporting evidence.

Comments on Literature Cited

Overall, the report accurately reflects the peer-reviewed scientific literature. The text on temperature-ozone associations—noting that the effect of short-term exposure of ozone varies by temperature—may be a bit overstated, as it focuses only on one national and one regional study.

The scientific literature on air conditioning is about the prevalence of air conditioning, not the use of air conditioning. This does not change the overall meaning of the text, but should be corrected to be accurate.

Comments on Traceable Accounts

Overall, the findings in Chapter 13 are documented in a consistent, transparent, and credible manner.

In the “Major Uncertainties” section of the traceable accounts for Key Message 1, the text states, “The model simulations that project net increases in future ozone levels over the United States with warmer global climate scenarios have variability in the magnitude of the signal as well as the potential regional differences of the climate impacts on ozone across the United States” (page 500, lines 9-11). This section could note that it is the magnitude and spatial patterns that have uncertainty, not the direction of the signal. It would also be worthwhile to be specific that this refers to tropospheric ozone.

Other Recommended Changes

The chapter notes many adverse health outcomes from poorer air quality, but neglects some other health outcomes that are noted to have associations with air quality, perhaps with growing evidence compared to other health outcomes for which there exists strong evidence. An example is birth outcomes (see Green et al., 2005; Warren et al., 2017). These could be added, as well as text to note other health outcomes could be affected, to give a better indication of the overall health effects of changes in air quality from climate change.

CHAPTER 14: HUMAN HEALTH

Summary

Chapter 14 appropriately evaluates scientific evidence on the human health consequences of climate change and is written at a technical level that is appropriate for the intended audience. The key messages are generally effective and highlight major impacts of climate change on human health. Primary recommendations to improve the chapter include placing greater emphasis on the extent of the impact that climate change is expected to have on human health and expanding discussion of the types of health outcomes that are expected to be impacted by climate change. The inclusion of economic impacts in the key messages should also be

reconsidered in the context of the treatment of this topic in other chapters. Most chapters do not report economic information while this chapter does. Generally, the Committee suggests that the NCA4 authors better integrate economic estimates throughout the draft report (see the “Front Matter: Report Findings” section in Chapter 3 of this review report). The NCA4 authors should revise the economics discussion in Chapter 14 to be consistent with treatment of this issue in the report as a whole.

Review Comments Related to the Statement of Task

Comments on Key Messages

Key messages are generally clear and consistent, but some small changes would strengthen their impact. The chapter authors should also review the Health Report Finding provided in the draft NCA4, which the Committee found to be particularly effective in conveying a strong, succinct message about health impacts.

Key Message 1 has two points that may be clearer if divided. The second and third sentences convey that climate change is anticipated to have major and substantial impacts on human health. This is the main subject of the chapter and could be the first key message. The first sentence currently provides a separate message—that some populations are more vulnerable than others. This should either be a separate key message, or listed after the first message of health impacts in general. Within the main text, there should be references provided for the health outcomes listed, including mental health.

While Key Message 3 makes a valuable and relevant point about the economic impact of the health-related consequences of climate change, it seems a bit uneven that economic estimates are provided in the human health chapter and not in other chapters of the draft NCA4. Furthermore, it is unclear how this value was estimated because the method and data for this estimate are not included.

Comments on Graphics

The authors should consider graphics that align better with the key messages. The figure on the locations of hospitals in flood map regions (Figure 14.2) is not the most compelling figure

to convey the consequences of human health in the context of climate change. A figure with a clearer connection to health outcomes would likely be easier to interpret for many readers.

Comments on Literature Cited

Chapter 14 accurately reflects the peer-reviewed literature. More information on other health outcomes would be useful. See “Other Recommended Changes” section.

In the text discussing vulnerable communities, the chapter notes that climate change’s effects on health will not be felt equally. However, the chapter neglects to mention that these effects are not felt equally in the present day either.

Comments on Traceable Accounts

The text discussing confidence and uncertainty would be more helpful if it specified which aspect of the key message has uncertainty (e.g., is the magnitude uncertain but the direction of change certain?).

Comments on Data and Analyses

The data and analyses are handled in a consistent manner with the possible exception of the economic results for which the underlying analysis and methods are not presented or well cited.

Other Recommended Changes

The chapter focuses on several of the key health outcomes that are important for climate change, but there are other health outcomes that are likely to be affected. The chapter should mention these as well, even if the level of certainty differs by health outcome (e.g., birth outcomes and lost school days from changes in air quality). As appropriate, this information should be cross-referenced with other chapters, such as the draft NCA4 Chapter 14, “Air Quality.”

More explicit information on co-impacts (often referred to as co-benefits) could be included, perhaps as a key message of its own or in relation to Key Message 2, which discusses adaptation policies.

CHAPTER 15: TRIBAL AND INDIGENOUS COMMUNITIES

Summary

It is an enormously challenging task to cover climate change on tribal lands due to the inherent diversity and complexity of Indigenous peoples, their relationships to the environments where they reside, and their legal and political positions within the American system of governance. This challenge is exacerbated when “western science” does not acknowledge the contribution of Indigenous holistic worldviews. Despite this challenge, the chapter does an

admirable job of addressing many of the key climate change impacts on Indigenous Peoples based on the available literature. It provides a strong Indigenous voice for climate change science by emphasizing issues that affect Indigenous people and by describing the unique knowledge and perspectives that Indigenous Peoples bring to the issue of climate change.

The Committee’s main concern is that Key Message 3 might be misinterpreted by some readers to mean that Indigenous Peoples have such high adaptive capacity, resilience, and experience with climate impacts that they will not be strongly affected by climate change; that climate change represents only an obstacle to their ongoing adaptation. As presently stated, Key Message 3 raises the possibility that Indigenous peoples might actually be less vulnerable to climate change than are other segments of society because of their effective experience in dealing with climate variability. The Committee doubts that the authors mean to imply this. Alternatively, if that is their intention, then they should explain this more clearly. The bottom line is that it is important that the meaning and intent of Key Message 3 is clear. The draft NCA4 Report Finding 10 about Indigenous Peoples in the report front matter is a good example of one way that the issues might be rephrased.

Review Comments Related to the Statement of Task

Comments on Key Messages

The key messages reflect current understanding, but some specific changes are suggested.

Although Key Message 1 is clearly written, it emphasizes general statements about climate-change impacts rather than providing examples (and associated references) of the major types of vulnerabilities experienced by Indigenous Peoples. In what specific ways are Indigenous Peoples particularly vulnerable?

The Committee would encourage greater specificity in identifying how Indigenous livelihoods and economies are adversely impacted by climate change. Climate impacts on general sectors, resources, and services mentioned in the key message would affect non-Indigenous as well as Indigenous Peoples. Are there ways in which Indigenous Peoples use resources and services that would cause them to be differentially affected? Although the chapter’s lack of specificity is most problematic with respect to Key Message 1, the Chapter 15 authors should consider this same issue with respect to the entire chapter in both the main text and traceable accounts.

The supporting text for Key Message 2 might mention (perhaps on page 556 in the paragraph beginning with “Imagery and reports…”) that indigenous respect for lands and waters

causes climate change impacts on the natural world to have direct psychological impact on indigenous peoples.

Key Message 3 is ambiguous as to whether Indigenous Peoples are so experienced and resilient in addressing climate impacts that they may (or may not) be particularly vulnerable. Perhaps this ambiguity could be minimized by beginning the message with a statement about obstacles to adaptation, and then pointing out that Indigenous Peoples have a history and unique knowledge that will be valuable in addressing this vulnerability, if the obstacles can be removed. The commitment to self-determination seems important in this context. The Committee suggests that this key message be rephrased to first explain why Indigenous communities are vulnerable to climate change (limited capacity to plan and implement adaptation strategies, limited access to traditional territory and resources, and limitations of existing policies, programs, collaborations, and funding mechanisms—as stated in the key message). Then, the message should state that successful climate adaptation will require full engagement and self-determination of Indigenous Peoples and they must draw upon Indigenous knowledge, culture, and experience in addressing the climate changes that affect them.

Boxes for Key Messages 1 and 3 or cross-references to case studies described in other chapters are two possible ways of increasing these details.

“Key Message 4” is mentioned repeatedly in the chapter but is not included among the key messages, text, or traceable accounts. This discrepancy should be corrected.

Comments on Graphics

Figures would be more effective if they mapped closely to key messages. Figure 15.1 addresses Key Message 3 and Figure 15.2 addresses infrastructure elements of Key Message 2. It would be helpful if Figure 15.2 could be broadened to address the health and cultural issues emphasized in the key message more generally. A figure that illustrates Key Message 1 would also be a welcome addition. A figure analogous to Figure 24.2 in the draft NCA4 Chapter 24, “Northwest” could possibly be used to show how multiple climate impacts affect Indigenous economies and livelihoods (and perhaps also the social and cultural and health issues in Key Message 2).

Comments on Literature Cited

The chapter accurately reflects the recent peer-reviewed literature. No major content areas are missing from the report, but multiple citations are often used to support a single generic statement, without providing details. It would be good to include enough specific details to show how impacts on Indigenous Peoples differ from those on society in general. In some cases, it

may be possible to cross-reference examples described in other draft NCA4 chapters, without greatly lengthening the text. The web links in Figure 15.1 do not provide information on tribal adaptation activities, as is claimed in the text. Adaptation activities are described for only three case studies in the third web link provided.

Statements regarding Indigenous Peoples’ adaptation to climate change would be more compelling if specific examples were described in the chapter rather than only in cited references. In the “State of the Sector” section, the text states that “the chapter provides evidence that Indigenous people are taking active steps to adapt to climate change” (page 550, lines 16-18). This is repeated on lines 21-22. On page 556, lines 30-31, the text cites two references that document Indigenous People adapting to and coping with climate change, but does not describe the types of adaptation that is occurring. The traceable accounts state that there is robust documentation of ongoing Indigenous adaptation to climate variability and change. Seven references are cited, but no examples are given. There are, however, many examples given throughout the chapter of Indigenous adaptation planning (e.g., page 559, lines 17-23).

The chapter might also mention the importance of outlets other than peer-reviewed literature (e.g., indigenous websites, where they adhere to NCA4 information quality standards) that document insights and information from Indigenous Peoples and give examples.

Comments on Traceable Accounts

The traceable accounts address all of the issues of the key messages, with appropriate citations, but as noted in the “Comments on Literature Cited” section, they are often without specific examples. The uncertainty analysis emphasizes that it is often difficult to project what will happen to Indigenous Peoples with regard to climate change, because studies have not been done to analyze differences in vulnerabilities between Indigenous and non-indigenous peoples. Perhaps this explains why relatively few specific examples of vulnerabilities are presented in the text. Currently, the uncertainty statements in the traceable accounts note few studies on the impact of climate change on Indigenous People. This is inconsistent with the main text, which states that there is abundant evidence that Indigenous People are adapting to climate change.

Other Recommended Changes

The chapter treats many of the key issues about climate change and Indigenous Peoples, but several opportunities are missed that would strengthen the chapter.

Chapter 15 should mention the growing recognition of the value of traditional knowledge in informing place-based impacts of climate change and adaptation, mitigation, and preparedness measures.

Increased awareness among Indigenous People of climate-change impacts and the participation of Indigenous People in landscape-scale initiatives could be highlighted. The fragmented nature of many Indigenous lands suggests the value of partnership of Indigenous People and neighboring landholders. This would provide opportunities to bring traditional knowledge to bear on broad problems.

The chapter should note that there is increased awareness since the NCA3 of the importance of providing local communities with ready access to information, access, and tools

(e.g., Indian Energy Program on Requesting Technical Assistance) to inform tribal leadership of alternative choices. See also the recent Sustainable Science Special Feature: Applying Cultural Evolution to Sustainability Challenges (Brooks et al., 2017) and Rodriguez et al. (2017). It would be helpful to specify the progress that has been made since the NCA3 in addressing climate-change effects on Indigenous Peoples.

The potential role of tribal lands and resources to contribute to the development of energy independence and sustainable production of clean energy could also be mentioned (Meisen, 2009; Kronk-Warner, 2013).

CHAPTER 16: CLIMATE EFFECTS ON U.S. INTERNATIONAL INTERESTS

Summary

Chapter 16 explores climate change in the context of international interests in a standalone chapter, which is a new and welcome addition to the national climate assessment. The key message themes covered in the chapter—impacts on economy, disasters, conflict, shared resources in border regions—are well-chosen and literature cited is appropriate and current. The analysis of impacts and implications is a bit scattered and not very quantitative, at least in part because data may be few or hard to obtain. Cross-border issues with Mexico could receive more robust treatment.

As a new chapter in the national climate assessment development, it is evident that the scope of this type of chapter is likely still evolving. As written, it is very succinct and could be made more comprehensive and integrative. For instance, it should cross-reference relevant content in the draft NCA4 regional chapters and draw on a broader range of international examples. The international examples currently included in the draft chapter, drawn largely from U.S. Agency for International Development (USAID), are effective. In addition, this chapter could link back to the draft NCA4 Chapter 2, “Our Changing Climate,” and appropriate regional chapters. Impacts on agriculture, which should be multi-fold, are only briefly described and would benefit from increased treatment. The discussion of military planning for climate change could also be expanded to strengthen it. For instance, the Coast Guard, with a multi-faceted mission space and many international roles, could be included.

Comments Related to the Statement of Task

Comments on Key Messages

The second sentence in Key Message 2 has awkward wording that could be revised to make it more focused. A suggested revision is: “The United States plays an international role in supporting developing countries to better anticipate and address the impacts of climate change and natural disasters.” It might also be appropriate to mention and highlight the many agencies in the U.S. government that contribute in this arena. For example, in addition to USAID efforts, there are NOAA and other agency projects under international climate change adaptation and disaster resilience.

In Key Message 3, the focus on Department of Defense could be expanded to include the Coast Guard (Department of Homeland Security) and their presence internationally across multiple mission spaces (humanitarian, stabilization, fisheries, search and rescue, etc.) that impact national security. Increased ship traffic resulting from the reduction in Arctic sea ice will also increase security and emergency response responsibilities for the Coast Guard and Navy.

Comments on Graphics

The graphics included in Chapter 16 are clear, consistent, and are communicated appropriately for the intended audience.

Comments on Literature Cited

To the extent it is available, the chapter reflects the peer-reviewed literature. There have been climate assessments for other nations and a selection of these (including Canada) are surveyed in the draft NCA4 Appendix 4. It would be a useful to reference this appendix in Chapter 16 and provide cross-reference to that information, as appropriate.

Comments on Traceable Accounts

Findings are documented in a consistent, transparent, and credible way which reflects supporting evidence.

The treatment of Syria in the traceable accounts could be shortened and improved by mentioning climate extremes (drought/water availability) combined with agricultural practices as factors in the instability there (e.g., Gleick, 2014).

Other Recommended Changes

The chapter could place more emphasis on climate change projections of drying to the South of the United States in Mexico (thought to be “likely”) and wetting to the North in Canada (also “likely”) and associated impacts. The chapter 16 authors should consider whether more direct links to the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report

and relevant information provided in the CSSR (USGRCP, 2017) could be made in some segments of this chapter.

National security discussions appear to focus on “conflict,” but other issues could be emphasized. For example, displaced populations, famine, water supplies, global transportation networks, and probably others, could be addressed. Climate change assessments by U.S. military and global security organization(s), such as the Center for Climate and Security, could also receive greater emphasis.

The last few years have seen an increase in the volume of displaced populations, mostly due to regional conflicts. The added challenge of changed or anomalous climate on these populations seems to be a “very likely” stress multiplier, which could be explored (Gleick, 2014).

“Success stories” could receive stronger labeling as such. For example, international weather and observation networks and data sharing via the World Meteorological Organization, international space agencies, and IPCC Assessments. Additionally, the Under2 Coalition––created in 2015 with twelve founding signatories and now has more than two hundred––brings together states and regions internationally willing to make a number of key commitments toward greenhouse gas emissions reduction. The U.S. Climate Alliance, established in 2017, has similar concerns and goals.

CHAPTER 17: SECTORAL INTERDEPENDENCIES, MULTIPLE STRESSORS, AND COMPLEX SYSTEMS

Summary

Chapter 17 provides an overview of the connections among natural, built, and social systems and their interdependencies to enhance the management of societal responses to a changing climate. These systems are subject to a variety of non-climate stressors, such as population growth, changes in economic activity, technological changes, and others, that will have compounding effects. The chapter describes how climate change impacts the stressor dynamics of these systems, their underlying processes, and their interdependencies, and highlights this complexity from a regional and sectoral perspective. Understanding these complexities is critical for effectively and efficiently managing climate risks. Throughout the draft NCA4, the significance of these connections, and interdependencies are evident and this chapter explains and makes more prominent these complexities. It is an important and appreciated addition to this draft fourth assessment.

The chapter adequately provides an overview of the interactions and stressors and is clear, consistent, and communicated appropriately for the intended audience. However, the Committee has some suggestions for chapter improvement. Notably, the text barely mentions the connection to an important system—society. The text should enhance the coverage of social and socioeconomic systems, and common causal factors across sectors and regions. Although it is stressed in Key Message 1, the coverage of societal implications in the chapter is lacking. Examples could also better highlight system-related and climate-related risks and vulnerabilities. For instance, following Hurricane Katrina the power outages meant residents could not get money from ATMs, credit cards did not work, and people could not get paid. People needed

money to get their houses repaired and buy supplies, but bank records were often lost and loans could not be secured.

The economic implications for major cross-sector climate change impacts should also be discussed. While “cost” is briefly mentioned in the chapter, the issues associated with economic damages are not. The economic implications can be large and multifaceted and should be included in the text (as highlighted in the Hurricane Katrina example on page 112 of the draft NCA4).

Comments Related to the Statement of Task

Comments on Key Messages

The chapter’s key messages are generally clear, consistent and communicated appropriately for the intended audiences.

Key Message 1 affirms the understanding that it is impossible to evaluate the full extent of climate-related impacts on the United States without considering interactions, connectedness and interdependencies among systems. Although stressed in Key Message 1, the supporting text should be enhanced to cover social and socioeconomic systems, and common factors across sectors and regions that impact individual behavior of people, including their socioeconomic situation and culture (e.g., Hansen et al., 2013). As written, the section only describes the interaction among “sectors,” yet the draft NCA4 as whole places strong emphasis on communities, listing the topic as the draft NCA4 Report Finding 1.

Supporting text for Key Message 1 should also include some discussion of the connection of the impact on communities in the larger system. An example could be drawn from Hurricane Katrina where, because of socioeconomic conditions, adaptive capacity in some communities was limited and evacuation in some areas of the city was hindered. Electricity and communication outages exacerbated outreach and recovery (Zoraster, 2009; Walton, 2015⁶). People were dislocated and infrastructure was damaged, resulting in a decreased customer base for the local power provider Entergy New Orleans, which lost half of its natural gas customers. This eventually resulted in a declaration of bankruptcy by the company.

___________________

⁶ See Electric Light and Power, http://www.elp.com/articles/2015/08/ten-years-after-how-entergy-new-orleans-survived-hurricane-katrina.html.

Key Message 2 is an appropriate and logical outcome based on materials covered in the chapter. Discussion in the section, however, should also mention the need to factor in societal risks in risk assessments. For example, climate change can increase energy demand for cooling, potentially increase air pollution, and increase health impacts. It may be appropriate to cross-reference to other draft NCA4 chapters in the supporting text to convey this information. Framing around social and socioeconomic systems and common causal factors across sectors and regions, such as consumption behavior of people, and culture is also suggested.

Key Message 3 should directly call out the use of risk methods to inform policy and decision-making practices for achieving economic efficiencies in solutions or actions. The example in the key message should also illustrate a how jointly managing a system can help address climate-related risks. The current example in Key Message 3, while clearly related to climate issues, is not directly about addressing climate risk.

Comments on Graphics

Overall the graphics are clear, internally consistent, and communicated appropriately for the intended audiences.

Figure 17.1 shows changes in water storage in the Southwest, 2011-2013. Although the figure is appropriate for the purposes of Chapter 17, having a figure that demonstrates the complexity of interacting sectors over several regions would better fit with the national-scale topical emphasis of this chapter. Figure 4.2 in the draft NCA4 could be a possible model to adapt for Chapter 17. The Chapter 17 authors could also consider adapting Figure 1 in Steininger et al. (2016).

The chapter uses six boxes to illustrate system complexity, which is more boxes than other chapters. This may be imbalanced, depending on guidelines provided to the NCA4 authors, and the content of the boxes should be carefully considered. Boxes are a useful tool to highlight examples. Flagging key messages in Box 17.1 is also effective. However, all boxes should relate to the chapter topic. Box 17.3 is about wolves. While this may be interesting in some contexts, it is unrelated to climate risk and does not further the readers’ understanding of climate interactions. Examples that might be explored to highlight system complexities could focus on the impact of autonomous vehicles on greenhouse gas emissions, vehicle miles traveled and social impacts, or even the implications of a “smarter” grid to improve efficiency. Finally, the boxes illustrate system interdependencies and complexities, but none deal with options and methods that can be used to identify and understand interconnected risks. Expansion to better incorporate risk is encouraged.

Comments on Literature Cited

For the most part, the chapter accurately reflects the peer-reviewed scientific literature, with a particular focus on literature since the NCA3 was published, i.e., since approximately 2013. However, as previously stated, the content on the connection of system impacts on society is not included in this chapter. The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment (USGRP, 2016) could be a useful reference for this topic. In addition, covering this subject matter in terms of an underlying process might be insightful. Specifically, the chapter does not provide conceptual guidance on performing risk analysis and management to account for how impacts interact across sectors and scales for the purpose of allocating resources effectively to achieve economic efficiency (e.g., see Ayyub, 2014).

Comments on Traceable Accounts

The findings are documented in the report in a consistent, transparent and credible manner. They reflect supporting evidence, include an assessment of likelihood, and are effectively communicated.

Other Recommended Changes

The following items should also be mentioned or discussed:

• The economic implications of climate change that causes disruptions across interdependent sectors. These implications can be large and multifaceted and should be included in the text more specifically than as mention of “costs.”
• Societal impacts, such as implications of extreme weather events on homeowner/flood insurance and losses or effects of income level on ability to avoid events, as experienced during Hurricane Katrina (IOM, 2007) and Hurricane Harvey.
• Highlight the need for additional monitoring and control technologies due to the complexity associated with a system of systems.
• Discussion of the non-stationarity of interdependencies. This might fit well in the section about unknown implications and impacts.
• The impact of climate change on air quality is especially relevant in the box about wildfires, and should be cross-referenced with the discussion in the draft NCA4 Chapter 13, “Air Quality.”
• Although consistent with other chapters in the draft NCA4, using the section heading “State of the Sector” (page 613) is not appropriate because the chapter does not deal with a sector of the economy.
• Guidance or references to practical analytical frameworks should be expanded. While the chapter mentions risk management, no specific tools that a practitioner could use were identified. Some that could be mentioned include:
  • U.S. Climate Resilience Toolkit: https://toolkit.climate.gov
  • International efforts and some are risk specific: https://www.climatelinks.org/resources/climate-risk-screening-management-tool
  • Coastal risks: https://coast.noaa.gov/digitalcoast/tools/flood-exposure
  • Flooding: https://msc.fema.gov/portal
  • Water risks more generally: http://www.wri.org/our-work/project/aqueduct/

CHAPTER 18: NORTHEAST

Summary

The Chapter 18 authors have produced an exemplary chapter. The key messages parallel those in other regional chapters by starting with information on significance of climate change impacts, followed by observations, and then projections of what the future might hold. The inclusion of a key message devoted specifically to adaptation (Key Message 5) is a strong component of this chapter. This is appropriate here because of progress made since the publication of the NCA3. In the NCA3 there were adaptation plans, but little action could be reported across the region. Now, there is action in most states; this evolution should be noted explicitly. Improvements to the chapter could be made by placing more emphasis on urban heat islands and extreme events, among other suggestions provided in this chapter review.

Comments Related to the Statement of Task

Comments on Key Messages

In all cases, including diversity across the region (north to south; large urban areas to rural areas, etc.), the key messages reflect current understanding about observed and projected impacts. The parallel structure of key messages adds to their communicative strength. A few small changes, however, could improve them.

Key Message 2 seems to contradict Key Message 5 in the statement, “Adaptive capacity of coastal businesses and infrastructure is limited, posing risks to people, species, and economies.” This inconsistency should be remedied by changing the language or adding appropriate qualifiers.

In Key Message 4, the sentence beginning with “Disruptions” is oddly composed/worded and some focus on historic sites seems out of place. Some clarification or revision to the sentence is recommended.

Key Message 5 represents an important advancement in responding to the impacts of climate change. This key message is different from the corresponding message in the NCA3 because it reports new actions undertaken in the last few years. This difference between the NCA3 and NCA4 should be strongly emphasized.

Comments on Graphics

Figure 18.7 should either be replaced with a simpler version—perhaps just the second part—or explained more thoroughly. Two versions of the same map with shaded regions that are not explained is not appropriate for the intended audience.

Comments on Literature Cited

The majority of references have been published since 2013, and their assessment reflects progress in the science since the NCA3. The coverage, being organized by differentiating urban from rural, clearly shows how new knowledge confirms and strengthens conclusions from the NCA3. The new structure of the NCA4 that allows expanded treatment of regions has been well used by the Chapter 18 author team.

Comments on Traceable Accounts

The traceable accounts are strong and could be used as a model for other chapters where improvements to the traceable accounts are suggested. They are clearly articulated and indicate confidence clause by clause in some cases, which is appropriate. The treatment of uncertainties, confidence, and likelihood is also very good.

Other Recommended Changes

More emphasis should be placed on urban heat islands and extreme events that impact the people where they live. There is a box on this topic covering Rhode Island (under Key Message 3 on rural economies), but it is a dominant, more widespread issue that deserves to be covered earlier in the chapter instead of being diluted and covered partially across multiple sections of the chapter.

As noted for the draft NCA4 as a whole, Chapter 18 could be improved by taking a little more care in differentiating what is new from the shorter Northeast chapter in the NCA3. While the draft NCA4 Chapter 18 does a better job than many others overall in the inclusion of recent information, emphasizing the advancement since the NCA3 could be stronger.

CHAPTER 19: SOUTHEAST

Summary

Overall, Chapter 19 does a nice job of conveying key messages for the Southeast region and emphasizing appropriate materials. Much of the chapter focuses on natural landscapes, which are an important component of climate change in the Southeast, but the chapter would benefit from an expanded perspective on urban systems and their adaptation responses. In particular, urban heat islands and their interaction with extreme heat events could be discussed at the regional level. This should cross-reference the draft NCA4 Chapter 5, “Land Cover and Land Use Change,” where this topic is also discussed, and Chapter 11, “Built Environment, Urban Systems, and Cities,” if the authors follow the recommendation in this review report to increase attention to land cover and land-use change in that chapter.

Increased citation of existing local and city action plans would strengthen this chapter and highlight steps being taken to respond to climate change. Also, port infrastructure at medium-size ports, such as Charleston, South Carolina, is expanding to attract Panamax traffic. This is a major investment in infrastructure that makes these cities more vulnerable to sea level rise and discussion of this could fit in the Charleston case study that is already included in the draft chapter.

Comments Related to the Statement of Task

Comments on Key Messages

Overall, the selected materials and treatment of key messages is appropriate. However, the key messages should be revisited to balance the vulnerability being highlighted with the adaptation effort to address the risk.

Key Message 1 would be more clear if the second sentence began with, “In the absence of adaptation.”

Many of the impacts listed in Key Message 4 are not unique to rural communities and could also logically be included in Key Message 1. The Committee recommends restructuring the key messages to better reflect similar versus distinct impacts in these different settings.

Comments on Graphics

The graphics and maps in Chapter 19 are generally easy to follow for the intended audiences. In particular, Figure 19.10, showing the October 2015 Extreme Rainfall Event, is a very effective case study that is specific and focused. Table 19.1 is also effective.

Figure 19.14, “Projected Changes in Hours Worked,” has large economic impacts. It is a national map and should also be emphasized in either the draft NCA4 Overview Chapter or in a national topic chapter.

The y-axis in Figure 19.8, “Highest Daily Water Levels,” is confusing and should be better explained in the caption.

Comments on Literature Cited

This chapter is heavy on literature, yet light on local action plans. It is recommended that the Chapter 19 authors draw more on existing plans to highlight new activities, particularly to show expanded efforts since the NCA3.

Comments on Traceable Accounts

The traceable accounts are thorough and provide appropriate confidence and likelihood designations, except for Key Message 4. Further explanation of the research supporting the statement, “By the end of the century, over one-half billion labor hours per year could be lost from extreme heat-related impacts (medium confidence)” in Key Message 4 is needed.

Other Recommended Changes

Port infrastructure at medium-size ports in the Southeast, such as Charleston, is expanding. This is a major investment to port facilities and with this infrastructure expansion comes more vulnerability to sea level rise; discussion of this could fit in the Charleston case study.

The Southeast Florida Regional Climate Change Compact⁷ is only mentioned in passing on page 754 and it is recommended that this example be expanded. This is one of the original intergovernmental models of collaboration in climate change adaptation and mitigation planning since 2009 and could be further emphasized in Chapter 19. Recent developments include the creation of the 2017 updated Regional Climate Action Plan,⁸ which could be mentioned or discussed.

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⁷ See http://www.southeastfloridaclimatecompact.org.

⁸ See http://www.southeastfloridaclimatecompact.org/regional-climate-action-plan.

The mention of Miami Beach on page 734 is not quite correct. It states, “Miami Beach, FL has invested $500 million into raising public roads and seawalls, and improving stormwater systems.” Miami Beach is in the midst of this multi-year $500 million program. Only $100 million has been spent to date in improved stormwater drainage, raised roads, and seawalls. The work is ongoing, yet the text makes it appear to be completed. Adaptation efforts cited should be verified to the direct source, in this case the city, to confirm program and status. This specific topic, and the chapter more generally, should also be linked to the similar discussion in the draft NCA4 Chapter 8, “Coastal Effects.”

The Chapter 19 authors should consider elaborating on vector-borne disease as related to climate change (page 725), as this is a growing challenge for local leaders in the Southeast. As appropriate, this should also be linked to relevant discussion in the draft NCA4 Chapter 14, “Human Health.”

While the U.S. Caribbean now has its own chapter and voice (draft NCA4 Chapter 20), the authors should consider pointing out common risks and efforts between the Caribbean and the Southeast, particularly southeast Florida, which can have more in common with the islands than with states in the northern part of the region. The two chapters should also be cross-referenced, as appropriate.

CHAPTER 20: U.S. CARIBBEAN

Summary

The U.S. Caribbean Chapter of the draft NCA4 represents a welcome addition to the national climate assessment. Previously, this region was incorporated into the Southeast regional chapter, which made it more challenging to emphasize unique climate impacts and responses occurring in the Caribbean Region. The Committee commends the NCA4 authors on inclusion of this new chapter and provides suggestions for further capturing important attributes needed to understand climate change on these U.S. islands.

The chapter is well written and at an appropriate level of detail for the intended audience. Findings are presented and documented in a consistent manner. The key messages are consistent with current understanding about observed and projected impacts from the perspective of small islands in this region and are clear despite their multidimensional nature. The chapter could do a better job of explaining and balancing information on the two sets of islands in the U.S. Caribbean—Puerto Rico and the U.S. Virgin Islands. These set of islands differ considerably in size and population, and in some cases, they have differing vulnerabilities and risks associated with climate change. The availability of data to inform this report and current actions being taken to address climate change also varies, with Puerto Rico having a much richer set of information to draw from, as is reflected in the chapter. However, where possible, the discussion should be better balanced to present a more complete understanding of climate change effects in the region. It is also suggested that where appropriate, greater cross-referencing to other chapters in the draft NCA4 be made, including to Chapter 27, “Hawai’i and U.S. Affiliated Pacific Islands.” Despite differences in status (state versus various U.S. territory designations) and some socioeconomic parameters, these island regions are similar in that there are limitations due to their small size and the tight connections between natural resources, culture, and economic activities. Knowledge

gained through evaluation of climate change impacts in one region may therefore help to inform understanding of climate impacts and response actions in the other.

Comments Related to the Statement of Task

Comments on Key Messages

The six key messages focus on critical challenges, emerging issues, opportunities, and success stories for addressing risks related to climate change that are being experienced by the islands and people of the U.S. Caribbean. Despite the paucity of data on the U.S. Virgin Islands, the approach taken paints a complex, but reasonably accurate picture of climate impacts through a multidimensional lens and concludes with a section on the value of adaptive capacity and building resilience.

The key messages are in line with the current understanding about observed and projected impacts to the United States from the perspective and position of small islands in the Caribbean Sea and Atlantic Ocean. They are clear and the supporting text is closely linked to the points being made. The narrative and supporting evidence of the key messages includes good indications of the levels of risk.

Key Message 6 has been used as a means of addressing the underlying conditions that influence the success of U.S. Caribbean communities undertaking adaptation and mitigation initiatives in response to a changing climate. This acknowledgement is an important one, but associated challenges in sustaining this effort could be mentioned. Key Message 6 would benefit from the inclusion of an example or two to improve understandability for readers.

Comments on Graphics

In general, the graphics provide strong support for the messages and the narrative.

For Figure 20.1, the caption should say “seven” inhabited islands, not six. Note there are four inhabited U.S. Virgin Islands, including Water Island. The population size difference (3.4 M versus 106K) should also be noted.

In the caption for Figure 20.5, the described positioning of the graphs is different than the layout.

Figure 20.7 has an important message, but it looks more like life in the Pacific than the Caribbean and therefore may not resonate well with the regional audience.

The two photographs on page 809 of the draft chapter have no figure numbers and they contradict or bring into question the statements in the paragraph above (lines 4-8) that state that the U.S. Caribbean has not been hit by a major hurricane in recent years. This text should be

updated to reflect the 2017 hurricane season and the discussion of this topic in other chapters in the draft NCA4.

Comments on Literature Cited

The chapter utilizes peer-reviewed literature and includes a noticeable number of studies published post 2013 and the NCA3. Much of the data and information are derived from U.S. government technical agencies’ data and reports developed by regional intergovernmental entities and regional technical research groups. The chapter information includes work utilizing climate downscaling to address the resolution challenges small islands have with global and even regional models. This is an important contribution to the understanding of the climate-related data and information associated with the U.S. Caribbean. The expansion of this effort to an even finer resolution that would provide information of greater direct relevance to the Virgin Islands would allow for the picture presented to be more complete (e.g., see Figure 20.3). More generally, expanded discussion of recent science findings from the Virgin Islands is warranted.

The chapter acknowledges that the CSSR does not include data specific to the U.S. Caribbean and tackles the task from a number of information and data sources, including IPCC reports and linking relevant information contained in the CSSR, which support the contentions of the messages.

Comments on Traceable Accounts

Generally, the key messages are presented and documented in a consistent manner and make understandable links between the literature and the likelihood and confidence provided in Key Messages 2-5, although likelihood and confidence statements are not provided in the key messages, as they are in other chapters. The challenge of presenting information for the two sets of islands with very different information bases and conditions makes transparency difficult. There are a number of places in the chapter where it either appears that only Puerto Rico has the noted impact or situation, or that both sets of islands are experiencing the same, which is sometimes not the case. Part of the challenge is that there is often a lack of readily available information from the Virgin Islands. Puerto Rico is 26 times the land size and more than 33 times the population of the U.S. Virgin Islands. Puerto Rico is also significantly more advanced in their efforts to address climate change adaptation as a society than the Virgin Islands, where the level of response to the challenges is notable, but not necessarily well documented at this time.

Key Messages 1 and 6 are not linked to the evidence in a clear manner. In Key Message 1, the focus is on water resources and the negative changes in freshwater availability that are projected. The evidence base and the uncertainties relate to the message, but the confidence statements are linked to a possible impact. It is suggested that more about the impacts be discussed earlier in the text, or a more direct connection be made in the confidence statement paragraph. In Key Message 6, the evidence base and the major uncertainties sections briefly speak to increased adaptive capacity linked to collaboration, joint projects, and shared knowledge. Examples could strengthen these sections. The confidence statements could offer medium confidence for small island states in the U.S. Caribbean being able to develop useful and

needed mitigation and adaptation plans because of cooperation, shared data, and collaborating expert resources.

Other Recommended Changes

The chapter authors should consider using the background section to provide greater context and detail about the region, including information that may be unique relative to other U.S. regions, such as the island population sizes, research and expert resource pools, and the absence of constantly running water sources (in the U.S. Virgin Islands). The sentence, “The Caribbean is expected to warm faster than the global average and to experience greater sea level rise than global estimates from the Intergovernmental Panel on Climate Change (IPCC 2012)” (page 812, lines 2-4) would be an appropriate statement for the introduction or background.

The introduction in Chapter 20 should inform the reader about the imbalance of information from Puerto Rico and the U.S. Virgin Islands. This should include identification of unique attributes and commonalities of the island sets that informs discussion of vulnerabilities, risks, and impacts. Some information that could be noted is that the U.S. Virgin Islands have infrastructure and historical buildings in the inundation zone for sea level rise, including the power plants on both St. Thomas and St. Croix, schools, housing communities, the towns of Charlotte Amalie and Christiansted, Frederiksted, and pipelines for water and sewage.

The narrative of the chapter makes it clear that islands concentrate hazards and exposure in a changing climate. This point could be drawn out more fully in the text to differentiate this region from the others, and emphasize commonalities with Chapter 27, “Hawai’i and Pacific Islands.”

It is recommended that the inclusion of emissions scenarios (Special Report on Emissions Scenarios [SRES] versus RCPs) be explained, or greater consistency adopted. The introduction to the draft NCA4 informs readers of the RCP scenarios that will be used in the document, but a number of citations in this chapter use SRES (see Figure 20.3) and no explanation is included.

Special care should be taken to identify linkages between this chapter and other relevant draft NCA4 chapters. This includes cross-reference with Chapter 27, “Hawai’i and U.S. Affiliated Pacific Islands,” with Chapter 8, “Coastal Effects,” including discussion of coral reefs, and the Chapter 29, “Mitigation: Avoiding and Reducing Long-Term Risks,” section on “Challenges, Opportunities, and Success Stories for Reducing Risk.”

CHAPTER 21: MIDWEST

Summary

The Committee found Chapter 21 to be strong overall. The key messages address main impacts, vulnerabilities, and adaptations in major sectors of concern within the region. Key message structure and language is clear and internally consistent. The chapter is written at an appropriate technical level for the intended audience, striking a reasonable balance between accuracy and scientific content and is generally accessible. The chapter also draws well on current understanding and recent peer-reviewed literature and to the findings reported in the

CSSR. While some minor issues were raised in the review, no major concerns are expressed or substantial improvements recommended.

Comments Related to the Statement of Task

Comments on Key Messages

The key messages are well developed. Each takes the form of presenting the context for the concern within each sector, the impacts of relevant changes in climatic conditions, how these impacts might produce changes of importance to stakeholders in the region, and steps that have been or can be taken to adapt to these changes. This structure makes for relatively long key messages, but provides reasonably complete picture for readers of the changes and their impacts. When comparing the key messages to the chapter’s “Summary Overview” section, it is evident that the third paragraph (page 844, lines 24-34) makes significant mention of the importance of and impacts on the Great Lakes in the region, but this is not mentioned in the key messages. The Chapter 21 authors should consider adding a key message on the Great Lakes, or incorporating Great Lakes impacts into Key Messages 3 and 5 (at least).

Key Message 1 makes no mention of adaptation, whereas the others do. Adaptation in agriculture will be critical and a summary of the categories of adaptations available within this sector could be useful here. This is true for Key Message 1 itself, and in the “Summary Overview” section of the chapter (page 844, lines 8-15).

Key Message 3 and its supporting text could do a better job articulating interactions of climate impacts with land use changes, particularly fragmentation and urbanization effects.

Comments on Graphics

The graphics are generally useful. Figure 21.2 is quite effective.

Figure 21.1 provides a nice non-technical introduction to the idea of vapor pressure deficit, but it is questionable whether the included maps are appropriate to include. The Chapter

21 authors should consider whether the same idea (plant stress) could be conveyed with maps of increased actual evapotranspiration (AET) or potential evapotranspiration (PET) or PET minus AET, which may not require the ecophysiology lesson to accompany it.

Comments on Literature Cited

The discussion of impacts of increased moisture in agricultural systems might benefit from further contextualization and discussion of the potential for future drying. The traceable accounts section cites the CSSR: “future higher temperatures will likely lead to greater frequencies and magnitudes of agricultural droughts throughout the continental United States as the resulting increases in evapotranspiration outpace projected precipitation increases.” Additional literature supports this possible impact based on the Coupled Model Intercomparison Project, including Douville and Piazotta (2017) and Berg et al. (2017). The authors should consider drawing greater attention to this recent literature suggesting future drying as context, at least for the existing discussion of observed and expected increases in moisture.

The section on adaptation in agriculture (page 851, lines 1-10) is missing reference to two key sets of literature. First, there is some work to begin mapping dynamics in irrigation (Brown and Perez, 2014). Irrigation is a possible adaptation, and it is possible to begin to see and comment on the geographic variation in irrigation as an adaptation strategy. Anecdotally, irrigation is being increasingly used on droughty soils in Michigan as a back up in dry years. Second, the chapter makes no mention of land-use change as an adaptation strategy. Agricultural adaptation is not limited in theory to changes in management practices. Abandonment in some areas and (re) initiation in others is also possible. There is some evidence in the econometric literature for this already happening. For instance, Burke and Emerick (2016) provide evidence of the limits to adaptive response to extreme heat in U.S. agriculture, and cannot rule out abandonment of cropping as a recent adaptation. Feng et al. (2013) show that outmigration from rural areas is partly related to declines in yields.

Comments on Traceable Accounts

Between the main text and the traceable accounts, the chapter does an excellent job of supporting the claims in a consistent, transparent, credible way. The traceable accounts related to human health are slim. It would be useful to ensure that each claim in the human health section is supported with traceable accounts.

CHAPTER 22: NORTHERN GREAT PLAINS

Summary

The Committee is pleased that the Great Plains region has been broken into two separate chapters for the NCA4 instead of continuing with the NCA3 model of having a single chapter to discuss the region. Chapter 22 focuses on the consequences of climate change in the Northern Great Plains, with particular attention on water resources, agriculture, ecosystems and recreation, energy development, and Indigenous People. These are appropriate topics for this geographically

diverse, low-population region. The chapter is well written and clear about the impacts of climate change in recent decades and in describing some future adaptation options. It makes good use of the published literature on some topics but not all, probably reflecting the inevitable imbalances of expertise of the author group across this diverse geography and set of topics. The chapter provides a useful baseline context on the physical and economic geography of the region, although some improvements can be made, as noted in this chapter review.

The chapter links to the draft NCA4 Chapter 2, “Our Changing Climate,” and provides a helpful and succinct paragraph on the key aspects of climate change projections specific to the region, highlighting warmer average temperatures, reduced average streamflow and snowpack, variable changes to average precipitation, and increasing precipitation intermittency. In this way, the content of the chapter incorporates findings from the CSSR as it applies to regional concerns. Given that there are strong geographic gradients in some of the climate projections within the Northern Great Plains, a follow-on paragraph describing the sub-regional patterns and an associated figure showing regional projections for basic climate variables (e.g., annual and seasonal temperature and precipitation) would be helpful.

The main concerns for this chapter center around: (1) the lack of discussion of the geographic variability of climate and climate projections across the region to recognize the great range of climate conditions; (2) the rather thin discussion of which agricultural impacts related to climate change (as opposed to other factors) influence decision making; (3) the lack of inclusion of national parks and monuments in the discussion of ecosystems and recreation, including the economic challenges to local communities; and (4) the speculative nature of some of the takeaways concerning climate change impacts to energy production. The phrasing of some of the key messages seems ambiguous and phrases like “parts of the region” (where exactly?) or “unprecedented variability” (over what time span?) require more specificity to be meaningful. The chapter makes no explicit linkages to the NCA3, which would help identify updates in information and coverage. It is also noted that none of the authors are from Montana, North Dakota, South Dakota, or Wyoming (four of the five states included in the region), and the largely extra-regional authorship could raise concerns for some stakeholders in these states.

Comments Related to the Statement of Task

Comments on Key Messages

For the most part, the topics included in the key messages reflect current understanding about observed and projected impacts in the region, and they identify widely recognized issues with respect to climate change. However, the wording of some of the key messages was found to be unclear and open to multiple interpretations. Increased specificity is recommended, as detailed in this section.

Key Message 1 basically suggests that water management is already tricky in the region and any future climate changes will make it even trickier, even if the projected changes in precipitation are relatively modest. The Committee questions whether this is correct. On the point about small climate changes having large impacts on “downstream” effects, it is unclear whether the Chapter 22 authors mean terrestrial hydrology, or industries that depend on them, or both. The message mentions extreme rainfall events, but not drought. Given that this key message focuses on the fact that climate change will make water management more difficult, the text falls short of suggesting that more flood or drought events will cause economic damage and it is unclear whether that a fair interpretation of the message. It is suggested that this key message be rewritten to avoid ambiguity.

For the text supporting Key Message 1, projected changes in precipitation at the national scale (Figure 2.6 in the draft NCA4) show considerable geographic and seasonal variation in precipitation across the Northern Great Plains. Western Montana and Wyoming are projected to become drier whereas the Dakotas and Nebraska will become wetter. In addition, much of the region will experience less precipitation in summer, leading to heightened drought during the growing season. The Committee recommends that Key Message 1 and other statements about projected climate change be specific about the subregion and season being described.

The text supporting the Key Message 1 discussion that climate change will exacerbate existing water management challenges in the region is defensible and in line with the current scientific understanding, and the emphasis in the text on water storage (both as ground and surface water) as a buffer against extremes is appropriate. However, it should be noted that groundwater recharge rates are highly variable, ranging from rapidly recharged floodplain aquifers supported by irrigation (western Montana), to aquifers that replenish on much longer time scales (Madison Limestone Aquifer, Ogallalah/High Plains Aquifer). For the groundwater-irrigated parts of the region, there has been some work on modeling changes to recharge and withdrawals under future climate change that can help assess the most likely impacts on water for food production. These should be referenced. It would be helpful to add case studies from those watersheds that are likely to become the most problematic in this light. See also the “Comments on Traceable Accounts” section of this chapter review.

Key Message 2 states that climate change will improve the region’s agricultural industry on average in the coming decades, but should make clear that rising temperatures and extreme events will offset most of the benefits in the long term. The primary challenges to agriculture will come from drought in the western subregion and higher precipitation and flooding on the eastern side, and this should be noted. Seasonality of precipitation is particularly critical for agriculture. Reference to geographical migration of agricultural practices and enterprises is not well discussed in the text and is unclear in the key message. Overall, it is recommended that Key Message 2 be reworded for greater clarity.

The supporting text for Key Message 2 appropriately points out the complex and countervailing trends that will likely determine the net effect of climate change on agriculture in this region. On the one hand, warming in some of the colder parts of the region may lengthen the crop growing season and accelerate crop phenological development. Those short-term changes can potentially benefit productivity of existing agricultural land, as well as open up new land for cultivation potential (although Key Message 3 argues for a cautionary approach to agriculture expansion). On the other hand, rising temperatures, increasing variability in rainfall, and the likelihood of more extreme events are generally detrimental for agricultural productivity. These points should be strengthened in the text.

There are a few other potential mechanisms relevant to Key Message 2 that are mentioned in the draft NCA4 Chapter 10, “Agriculture and Rural Communities,” and could be included here. A general suggestion would be to better leverage some of the science referenced in Chapter 10 in order to make a more detailed assessment of the individual potential impacts and how they are likely to come together. Specifically, the conclusion that climate change will benefit agriculture at least in the near term is plausible but not well supported by the limited evidence provided. The Committee suggests that either a stronger scientific case made for this if it exists, or a rephrasing of the text to indicate that there is not enough known to have confidence in whether the net impact on the region will be positive or negative. The Committee agrees with the comments on adaptive management, but a substantially open question is whether farmers can adapt their practices fast enough as future changes accelerate; see the NCA3 agriculture chapter for a good discussion on this topic (Hatfield et al., 2014). Also, the supporting text for Key Message 2 describes some important trends, especially more land being broken out for row crops, but does not explain how much attribution belongs with climate change.

A clearer discussion about the multiple factors that drive producer decision-making with respect to agriculture would provide useful context. This analysis is discussed to some degree in draft NCA4 Chapter 10 and should be cross-referenced in this chapter. At the farm or ranch level, local climate variability is one component among several in the decision- making process (e.g., government policies, insurance, global and local prices, contracts, expected price received, production inputs, pests). Contrary to statements in the text, not all land-use change is a result of climate change and the response of farmers and ranchers will be based on a number of local and global factors.

In general, the agricultural references in Chapter 22 emphasize eastern region crops, with less consideration of dryland winter/spring wheat, hay production, small grain, pulse crops, livestock, and legume/oil seed rotational crops in Montana and North Dakota (see the “Comments on Literature Cited” section of this chapter review for some suggested references). The Matador Range example is a good one for grazing operations in the western subregion, but how that program relates to the Collaborative Adaptive Rangeland Management experiment is unclear.

Key Message 3 concerns the ecological and economic consequences of rising temperatures. The three pathways by which climate change affects recreation are clear, but winter recreation is not included in the list. The data supporting the economic importance of these pathways are weak and citations should be specific to this region. The paragraph on whitebark pine (page 930, line 37 to page 931, line 5) has no associated economic analysis and seems out of place with the other examples. It could be moved to the draft NCA4 Chapter 7, “Ecosystems, Ecosystem Services, and Biodiversity,” or at least referenced in that chapter. In the case of low-elevation areas with a matrix of natural and agricultural lands, at least a note should be added stating that it is difficult to attribute loss of natural habitat to climate change, if no studies have quantified them. Attribution of climate change is important given the emphasis of the key message to use past trends as evidence of how the communities and industries will or will not adapt to future climate changes.

Key Message 3 does not consider the iconic national parks (e.g., Yellowstone, Glacier, Badlands, Grand Teton) and monuments (Charles Russell, Teddy Roosevelt, Devils Tower, Mount Rushmore) in the region and the importance of these protected regions for tourism and recreation. Likewise, the impact of climate change on managed and protected ecosystems (endangered species, fire, invasive species, wildlife transmitted diseases), as well as the socioeconomic challenges facing gateway communities, should be discussed. Headwaters Economics offers several important studies that could be considered by the chapter authors.⁹

Key Message 4 is fairly speculative and weakly supported by the provided references. A large percentage of the statements derive from U.S. Department of Energy reports, which are not very detailed or referenced on the points about infrastructure and transport that are emphasized in the key message. Whereas some of the impact pathways listed in this section are fairly logical and plausible, it is not easy to discern them as measurable impacts on energy supply or prices relative to the huge extraneous forcings on the energy sector from global supply, demand, and geopolitics. Economic modelling studies that could be cited to boost the case made here would strengthen the statement.

In terms of greenhouse gas mitigation, the chapter might mention the relative importance of coal-fired power plants in this region. The U.S. Environmental Protection Agency data suggest that Wyoming and North Dakota have the highest per capita emissions in the country. The authors should refer to the draft NCA4 mitigation (Chapter 29) and energy sector (Chapter 4) chapters, including Department of Energy initiatives related to carbon capture and sequestration.

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⁹ See https://headwaterseconomics.org/economic-development/trends-performance/montanas-economy-and-protected-lands and https://headwaterseconomics.org/dataviz/national-park-service-units.

Key Message 5 includes good supporting evidence for anticipated and current impacts to culturally-important resources and the livelihoods of Indigenous Peoples. However, it is important to clarify which examples and citations come from the Northern Great Plains region. The invasive species case study and paragraph about proactive adaption add useful context.

Inclusion of Chapter 22 Materials in Chapter 1, “Overview”

The chapter’s key messages are broadly covered in the draft NCA4 report findings and Chapter 1, “Overview.” Inclusion of the Northern Great Plains in Chapter 1 could be expanded to better reflect issues of concern in the region. For example, on page 43, lines 32-34, declines in snowpack and shifts to more precipitation falling as rain is a clear component of climate projections for the Northern Great Plains and mountains of Montana and Wyoming, but this region is not identified in the discussion of this topic.

Comments on Graphics

Figure 22.1 should show the regional patterns and changes in annual and seasonal temperature and precipitation so that the reader has easy reference to this information at the regional scale. Referring back to draft NCA4 Chapter 2 national-scale Figure 2.6 is not convenient for this regional information. See “Other Recommended Changes” section for expanded discussion related to this figure recommendation.

In Figure 22.6, it is unclear what the map is supposed to be showing. More explanation is needed so that it can be understood as a stand-alone image.

Comments on Literature Cited

The chapter provides many recent and helpful citations from the peer-reviewed scientific literature. The Committee suggests that the authors consider other relevant literature (see Appendix B in this review report).

Comments on Traceable Accounts

The traceable accounts section is quite short—but provides some evidence that the findings are documented in a transparent and credible way. It is noted that each chapter in the draft NCA4 handles traceable accounts differently, with some, like this chapter, providing new information not previously discussed in the chapter and being relatively short. The Committee suggests better alignment of key message text and the traceable accounts to avoid seeming like afterthoughts, and where possible, more inclusion of specific examples of adaptation with the key messages.

Key Message 1 is assigned “very high confidence,” but this does not align well with the text that follows (page 946), which notes uncertainties in changes in precipitation and run-off and precipitation variability. Moreover, the key message as written focuses on water management, whereas the analysis of uncertainty focuses on climate projections. The “Description of confidence and likelihood” states high confidence for warming temperatures, which is not specifically part of Key Message 1.

For Key Message 2, the points discussed in “Major uncertainties” and “Description of confidence” refer to climate change metrics but not the key message, which addresses producer responses. To call out more specifically the current adaptation measures that are underway by a “subset of producers” to support Key Message 2 would greatly strengthen the message and evidence base.

Key Message 3 text does not discuss “future government policies that could exacerbate or mitigate climate-induced losses,” so this point seems poorly supported. Similarly, Key Message 4 does not discuss renewable energy, biofuel production, or low-level ozone production in any substantial way, although they are emphasized in the “Major uncertainties” section.

Other Recommended Changes

The opening “Background” section of Chapter 22 would benefit from a more detailed geographic description. For example, reference is made to three distinct geographic features—Red River Valley, Upper Missouri River Basin, mountains of Montana and Wyoming—but other subregions are not identified (e.g., Sand Hills, High Plains, North Platte River basin, etc.) and some are cited in the text but not shown on a map (e.g., the Snake River drainage, the Prairie Potholes region, and the Columbia River drainage). It would be helpful to have a map that shows the three identified geographic features, as well as a description of the other parts of the Northern Great Plains not covered by these three features. Figure 22.1 should show the regional patterns and changes in annual and seasonal temperature and precipitation so that the reader has easy reference to this information at the regional scale.

Table 22.3 does not include Montana prairie pothole data and it is unclear why this is the case.

More specific line comments for this chapter are provided in Appendix B.

CHAPTER 23: SOUTHERN GREAT PLAINS

Summary

Chapter 23 surveys the diverse set of anticipated climate change impacts on the industries, communities and ecosystems of the Southern Great Plains region. The chapter maintains an appropriate balance of attention across these issues by emphasizing what are likely to be the most sensitive and costly impacts, including those related to Gulf Coast infrastructure and industry, regional agricultural systems, and human health. The chapter is very readable for a general audience, making good use of discussions that telescope from big picture context down to engaging local case studies, including several from high-profile extreme events experienced in

the region in recent years. The chapter does a good job of reflecting current scientific understanding via references to relevant research, including peer-reviewed literature and gray literature reports. It also links directly and clearly to findings of the CSSR and helpfully incorporates estimations of the economic costs of regional climate change impacts.

Suggested improvements to Chapter 23 are mostly around clarification and contextualization. For example, better descriptions are needed of baselines and how climate changes are expected to affect deviations from those baselines. Tables, figures and case study boxes should be better used to reinforce key messages. The Chapter 23 authors are also encouraged to ensure that there is appropriately balanced representation of locations across the region.

Comments Related to the Statement of Task

Comments on Key Messages

Key messages are not comprehensive, but generally provide well-chosen key takeaways for the chapter.

In Key Message 1, the authors should reconsider whether the “nexus” language is necessary. It will come across as jargon to most general-audience readers. The main point is that disparate water users from different sectors, including food and energy production sectors, experience increased tension and tradeoffs during periods of water scarcity. Some further interaction effects are referenced but are not well-developed (e.g., water needed for electricity generation, which is needed for irrigation, which is needed for food production). Unless the Chapter 23 authors think that readers need to view these potentially interacting effects in a sophisticated way, the “nexus” term does not seem appropriate and it would suffice to point out that these industries are interdependent on each other and potential competitors for scarce water resources. Also, the Committee suggests considering adding language to Key Message 1 that clearly distinguishes it as a paragraph that is specific to the Southern Great Plains region. As currently worded, it could potentially be applicable to several U.S. regions. Adding a few words of geographic specificity would make it more engaging.

In Key Message 2, authors should consider adding a timeline for the statement on “up to four feet” sea level change. It would also be useful to make the sea level rise impacts in this key

message specific to the Southern Great Plains region, since sea level rise impacts may be broadly applicable to U.S. coastlines.

Comments on Graphics

Tables, figures, and case study boxes should be better aligned with key messages. The case study boxes for Key Messages 1 and 2 and the specific examples for Key Message 3 and 4 are helpful. Key Message 5 should include a case study.

Figures 23.1 and 23.2 come across as overly-specific and somewhat arbitrary.

A map or infographic that show geographically the patterns and features of the region described in the “Background” section of the chapter would also be useful.

Comments on Literature Cited

Generally, the chapter reflects the peer-reviewed literature on the covered topics. It would be beneficial to provide a discussion of how the state of knowledge on regional climate change impacts has advanced since the NCA3, to provide better continuity.

Comments on Traceable Accounts

In contrast to some other chapters, the traceable accounts section is not used to provide an adequate set of references. The “Description of evidence base” explains only the general source of information for a particular key message, whereas most chapters provide specific evidence. The Chapter 23 authors should make better use of the traceable accounts section for strengthening supporting evidence for key messages, increasing transparency, and providing resources for readers wishing to explore any particular topic in the chapter in more detail.

The justifications for assignments of likelihood and confidence levels are not always clear and could be strengthened with additional references and comments on how the literature supports the claim. Descriptions of confidence and likelihood for Key Message 1 (page 992) and Key Message 2 (page 993) are the main examples where increased treatment is needed, but all justifications could be strengthened.

Other Recommended Changes

For readers not already intimately familiar with the geography of the Southern Great Plains, the “Background” section (first five paragraphs) of the chapter helps set baselines and provide basic context. The Chapter 23 authors should consider expanding this part of the chapter to include background descriptions of some of the other attributes of the region that are referred to in the key messages, such as tribal communities, natural habitats, agriculture, and population centers. In other words, make sure that background paragraphs are well-aligned with key messages.

The Committee suggests showing region-specific climate projections or anomalies (temperature, precipitation, days over 90^oF, drought) where possible. The draft NCA4 Figure 2.6 shows projections for the entire United States, but it would be good to have a closer view of this

region for easier reading of the linkages between climate projections and the impacts discussed in the chapter.

Local details provided in the chapter are heavily geared toward Texas. It is recommended that the material either be rebalanced somewhat, or explicitly state the reasoning for this approach. Is the emphasis on Texas a reflection of the relative states-of-knowledge across the region, or distributions of populations or economy size? The main relevant adaptation efforts of each state should be mentioned. For example, the Texas State Water Plan is noted, but efforts in Oklahoma or Kansas are not. Similarly, the discussion about managing risk with respect to ecosystems discusses programs for Texas only.

Message consistency with the draft NCA4 Chapter 10, “Agriculture and Rural Communities,” should be reviewed and edits made accordingly. Chapter 23 currently includes at least one impact pathway that is not addressed in Chapter 10—grain quality—and conversely Chapter 10 includes some impact pathways that are relevant to the Southern Great Plains region that are not touched on here.

Also see line-by-line comments in Appendix B.

CHAPTER 24: NORTHWEST

Summary

Overall, the Committee found Chapter 24 to be strong and effective in communicating climate impacts, risks, and response actions in the Northwest. The framing of vulnerable communities as being on the front lines of climate change is excellent and should be considered as a framing approach for other chapters of the draft NCA4. The chapter draws appropriately on recent literature, provides clear and consistent messaging, and highlights examples well in included boxes. Graphics are clear and effective. More balanced discussion of species distribution changes and expanded discussion of demonstrated benefits of climate adaptation and mitigation actions in the region would improve this chapter.

Comments Related to the Statement of Task

Comments on Key Messages

Organization and content of the key messages is clear, consistent, and makes the findings accessible. The information is linked to observed climate and regional risks, future climate relevant to regional risks, the challenges, response actions, and success stories for reducing risk, and emerging issues. The language is accessible and effective at making climate impacts and possible actions to address them tangible and relatable to readers.

The supporting text for Key Message 1 needs to provide evidence for the statement that “new business opportunities…” are provided for parts of the economy. Additionally, the Chapter 24 authors should point out potential effects of management and variability in adaptive capacity (i.e., “what can humans do about this?”). Evaluation of fishery management under climate change is cited in the draft NCA4 Chapter 9, “Oceans and Marine Resources,” and mentioned in this chapter. Proactive management will help in some areas or sectors, but is harder to realize in others. This can be documented, even generally if no regional examples exist. The mention on page 1018 of the draft chapter needs citation.

Key Message 2 should include a tie-in to observed consequences from extreme climate events, and what they indicate for the future. This is done in the text on page 1013.

In Key Messages 3, 4, and 5, the framing of vulnerable communities as being on the front lines of climate change is excellent. It is a way to draw in diverse communities who are most likely to be affected, which is not just based on socioeconomics, but also on livelihood dependency on ecosystems, location, etc. The Committee recommends this language be used elsewhere in the NCA4, as it helps greatly in orientating readers to the direct relevance of the key messages to them, or to communities they know. See also “Other Recommended Changes” section of this chapter review for additional suggestions related to Key Message 3.

Comments on Literature Cited

The chapter does a good job of reflecting current understanding and literature and is systematic in the treatment of climate science issues raised in the draft NCA4 Chapter 2, “Our Changing Planet,” and how these issues affect the Northwest region. Furthermore, the report provides robust documentation of findings since the publication of the NCA3, although explicitly noting what is new could be improved.

Comments on Traceable Accounts

The “Description of evidence base,” “Major uncertainties,” and “Description of confidence and likelihood” sections do a good job for each key message in documenting statements, their uncertainties, and confidence associated with summaries. Highlighting new information provided since the NCA3 would help emphasize how knowledge about and confidence in climate impacts and adaptation and mitigation strategies has evolved in recent years.

Other Recommended Changes

More minor concerns that should be addressed include:

• Examples provided in the chapter imply that shifts in species distributions will all be detrimental to human communities and economies, which is not always the case. For example, in commercial fishery impacts introduced on page 1017, species are entering or will likely enter new regions, becoming available for catch where they were not previously. For tribes and others with accustomed fishing grounds, there will be winners and losers. Cheung et al. (2015) is cited later in the chapter, which is appropriate, but the point should be acknowledged consistently. Citations included in the draft NCA4 Chapter 9 also contain examples (e.g., Ianelli et al., 2001; Seung and Ianelli, 2016).
• The Northwest is one of a few regions in the United States that is strongly influenced by interannual and interdecadal climate variability associated with the Pacific Ocean. It would be useful to refer to Box 2.1 (in the draft NCA4 Chapter 2) on climate variability and discuss specific challenges of detecting climate impacts in the midst of the large climate variability in the Northwest, and what challenges this presents for developing adaptation strategies.

• The chapter should provide expanded discussion of the interactions among processes which are well documented in the region. For example, fire, water quality, and hydropower generation are inter-connected phenomena and sectors. This chapter could also link to more specific treatments of those individual topics in other relevant NCA4 chapters.
• It is suggested that the authors consider more clearly articulating that many of the “success” stories for climate adaptation are technological in nature. This provides an opportunity to also point out use of ecosystems as part of solutions, and many co-benefits that can be provided. For example, in the text describing Key Message 3 (infrastructure), the threat is well discussed, but the authors should also include in the “solutions” discussion the opportunities to use hybrid green and gray infrastructure to reduce flood and erosion risk. See The Nature Conservancy in Washington “Floodplains by Design”¹⁰ work and citations provided in the draft NCA4 Chapters 8, “Coastal Effects,” and 25, “Southwest.”
• Mitigation gets little treatment this chapter, except in Key Message 4. There is available literature to cite that can be found in the draft NCA4 Chapter 8, “Coastal Effects,” Chapter 9, “Ocean and Marine Resources,” and possibly others, on integrative, positive effects of habitat and ecosystem protection and restoration on mitigating carbon emissions, in addition to the other adaptation co-benefits that such habitat-based strategies provide. See page 1026 for an example of where this could be discussed.
• Cross-border issues with Canada could receive more attention in this chapter. A straightforward way to do this could be to cite relevant information in the draft NCA4 Chapter 16, “Climate Effects on U.S. International Interests.”
• The chapter could refer readers to state and local climate assessments and climate plans.
• It would be helpful if urban climate issues were given more emphasis.

CHAPTER 25: SOUTHWEST

Summary

This regional chapter is informative, up to date, and well written. It builds from the Southwest Chapter in the NCA3 and presents essential climate-related issues under seven key messages and selected highlight boxes. Traceable accounts effectively describe the evidence base and uncertainty of the key messages. The chapter gives excellent treatment to a full range of biophysical, socioeconomic and cultural impacts and provides strong examples of adaptation and mitigation strategies that give the reader tangible cases to increase understanding.

The Committee recommends more consistent discussion across the main text and traceable accounts sections of the chapter and increased citation of relevant literature for select topics. Some topics would benefit from increased treatment in the chapter, including coastal habitats and their role in protecting shorelines, urban climate issues, and interactions among processes that are well documented in the region such as fire, flooding, sedimentation, and cost of hydropower generation.

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¹⁰ See http://www.washingtonnature.org/floodplains.

Comments Related to the Statement of Task

Comments on Key Messages

Key Message 1 neglects to mention flooding. Extreme events (droughts and floods) are more likely under future climate. It is fair to highlight drought in this key message, but flooding should not be neglected in the narrative of the main chapter body. Currently, flooding is mentioned in the traceable accounts section, but it is buried in the final “Evidence” section, making the documentation and key message narrative inconsistent. Besides an increase in the number of dry days, occasional wetter very wet days are expected to occur (Das et al., 2013; Polade et al., 2015, 2017). This key message could also draw linkage between shorter- and longer-term dryness—more dry days in projected future climate leads to more dry years (see Berg and Hall, 2015; Polade et al., 2017). Alternatively, Key Message 1 could be framed around extreme events, saying mostly these will be manifest as drought, but as evidence base discussion elaborates, depending on assumptions in models, flooding can sometimes occur, as recent observed events support (Odigie and Warrick, 2017).

Supporting text for Key Message 1 states that, “Models project substantial changes in snowpack, which supplies almost all of the water in the region…” (page 2091, lines 34-35). Stating, “almost all” is an exaggeration. Considerable runoff and stream discharge in California and other Southwest states is from rainfall. The fraction of water supplied varies considerably over the landscape, so it would be more appropriate to say something similar to “snowpack, which supplies a major portion of the water used in the region.”

Supporting text for Key Message 1 would also benefit from discussion of desalinization as an alternative water supply and the high cost and environmental impacts of this process (see Cooley and Phurisamban, 2016).

Key Message 2 could note that the seasonal occurrence and variability of downslope winds in California and elsewhere are usually accompanied by anomalously low humidity (Guzman-Morales et al., 2016).

The supporting text for Key Message 3 should acknowledge the solid work done by California on protecting and restoring coastal habitats, and the resilience benefits they provide for erosion, flood reduction, nursery habitat, etc. The text could also discuss climate change impacts on physical, chemical, and biological components of the coast and estuaries (e.g., multiple facets of the Santa Barbara Coastal Ecosystem Vulnerability Assessment; Myers et al., 2017) and a cascade of possible impacts in the San Francisco Bay, where altered regional patterns of temperature, precipitation and sea level could cascade to provoke local impacts such as modified water supplies, increasing risks of coastal flooding, and growing challenges to sustainability of native species (see Cloern et al., 2011).

Some expanded information on the implications of variability in ocean acidification for ecosystems and people would be useful in the supporting text for this key message. Also, in the traceable accounts “Major uncertainties” it is unclear why there is no mention of variation in ocean acidification levels and vulnerability of species, similar to what is provided for sea level rise, especially since it is mentioned in the main body of the chapter.

Key Message 4 presents little to no documentation for how livelihoods will be affected, despite highlighting it in the message. In the traceable accounts “Description of evidence base” (page 1118), no citations directly estimating such effects are provided. Livelihood diversification is notoriously difficult to estimate, and the chapter authors do not cite literature showing whether vulnerabilities in livelihoods occur, or if substitutes can buffer communities from nature-based economic changes. References should be cited, or acknowledgement provided that the connection is inferential, with discussion qualified as uncertain or a current research gap.

Key Message 5 might benefit from mention of daily to multi-year variation in coastal cloud cover, which would affect solar electricity generation along the California coast. See Appendix B for specific references that could be cited for this topic.

For Key Message 6, uncertainty around the connection between climate impacts and both food security and rural livelihoods should be revised. It would be more accurate to include “and may increase future vulnerabilities of food security and rural livelihoods.”

Similar to the comment on livelihoods made for Key Message 4, discussing “rural livelihoods” in Key Message 6 as increasingly vulnerable is not well supported by documentation in the narrative following it. Are there estimates of number of farmers, laborers, and other input providers, or transport and processing jobs associated with changes in agricultural production? How much of agricultural production in the region is from small holder producers versus larger industrial operations possibly more buffered from climate impacts?

Some additional recent literature that should be cited to support Key Message 7 is listed in Appendix B comments for this chapter.

Comments on Graphics

Overall, Chapter 25 makes good use of simple visualizations and examples in figures and boxes.

For Figure 25.8, it is suggested that the time period of projected changes be inserted into the figure caption, e.g., “Projected increases in extremely hot days, (2036-2065 versus 1976-2005).”

Comments on Literature Cited

The chapter provides systematic and robust treatment of the topics discussed in the draft NCA4 Chapter 2, “Our Changing Climate” and appropriately discusses these topics in the context of the Southwest region. Recent literature produced since publication of the NCA3 is well cited throughout the chapter, with additional references suggested in relevant locations throughout this chapter review.

Comments on Traceable Accounts

Overall, the key messages are documented in a consistent, transparent, and credible way. However, some information is missing or inconsistently treated among main chapter text and traceable accounts sections. Specific comments on this are provided in the “Comments on Key Messages” section for this chapter review.

Other Recommended Changes

The chapter would benefit from expanded discussion of a few additional topics relevant to the region.

• Little treatment is given to coastal habitats (dunes, wetlands, seagrasses, etc.) and their role in protecting shorelines from erosion and flooding. As sea levels rise and king tide

• flood events increase, actions of California Coastal Commission and others in protecting coastal habitats are an important adaptation strategy.

• Increased discussion of interactions among processes in the region such as fire, flooding, sedimentation and cost of hydropower generation would be valuable (see Writer et al., 2014; Sankey et al., 2017 for examples).
• Mention of riverine and coastal flooding interactions is also suggested (Bromirski and Flick, 2008; Cayan et al., 2008), with an example of how longer-period climate changes and short-period weather extremes that arise from both oceanic and atmospheric processes may conspire in creating large impacts (such as in the San Francisco Bay/Delta).
• More attention could be given to United States-Mexico cross-border issues relevant to the region (see GNEB, 2016), such as drought, flood and wildfire occurrence (Westerling et al., 2004).
• The Chapter 25 authors should make it clear that “availability of food” is a general vulnerability due to potential declines in production of specialty crops, as stated on page 1122, and not a vulnerability specific to the Southwest region, which is not dependent on locally grown food for subsistence.
• Page 1097, lines 16-18 in the draft NCA4 state “Under the highest emissions scenario, climate change could raise sea level at San Francisco 30 in (76 cm) ± 11 in (28 cm) by 2100 (Griggs et al. 2017) to a maximum of 6.2 ft (1.9 m) (Jevrejeva et al. 2016).” The maximum stated here is misleading: the highest sea level rise reported in Griggs et al. (2017) approaches 3 meters.
• Page 1089, lines 32-34 in the draft NCA4 state “Hotter temperatures have already contributed to a 20% reduction of snowpack and its water content since 1950 (Pierce et al. 2008; Fyfe et al. 2017), with more than half of this attributed to human-caused climate change (Pierce et al. 2008; Pederson et al. 2011; Fyfe et al. 2017).” The Committee generally agrees with this statement, but 20% is debatable.
• Urban climate issues, including changes and barriers to adaptation, might be given more emphasis in the chapter (see Auffhammer and Aroonruengsawat, 2011; Ekstrom and Moser, 2014; Sun et al., 2015; Vahmani et al., 2016).
• A number of web-accessible items could also be considered. For readers interested in adaptation and planning, Chapter 25 might point to regional and community level efforts and climate data resources that have been conducted or are ongoing. Websites and other examples for these topics are listed in Appendix B.
• Readers could also benefit from reference to available state and local climate assessments and climate plans resources. Reference to other relevant draft NCA4 chapters should also be added, including those focused on water, oceans, and ecosystem services.

CHAPTER 26: ALASKA

Summary

This chapter clearly describes the effects of climate change in Alaska and is accurate, up-to-date, and rigorous. It provides substantial detail on the nature and geographic variation in climate change impacts. It also points to the recent and likely future changes but is cautious not

to claim more than can be said from the existing data in both the main text and traceable accounts. Like other regional chapters, Chapter 26 is relatively detailed and perhaps more technical than some chapters. This makes it particularly helpful to Alaska-focused policymakers, managers, and the public, but perhaps less accessible to some readers from other regions. For the general reader, additional details, such as a definition of permafrost, would be helpful.

One suggestion from the Committee is that the chapter be more explicit about the types of adaptation measures being implemented and the extent to which these currently meet adaptation needs. Much of the adaptation section is about the tools that are available and it provides fewer specifics than other parts of the chapter.

Comments Related to the Statement of Task

Comments on Key Messages

The key messages accurately reflect the understanding described in the text of the chapter with respect to past and projected changes and the associated risks for ecosystems and society. The key messages convey the main topics of the chapter, but are vaguer than the main text and traceable accounts.

The statement in Key Message 1 that retreating sea ice plays an important role in marine habitats seems somewhat vague; what exactly are the impacts? In the second sentence, is the intended message that future carbon dioxide emissions are expected to continue unabated? Or does it mean that if emissions continue unabated, these effects are expected to happen? The use of more concise and explicit language is recommended. Consider also whether the time interval (e.g., next 25 years or next 100 years) is important to specify.

Key Message 2: Local Alaskan residents, communities, and their infrastructure continue to be affected by permafrost thaw, coastal and river erosion, increasing wildfire, and glacier melt. These changes are expected to continue into the future with increasing warming temperatures, which will directly impact how and where many Alaskans will live.

Climate change impacts on infrastructure are described in Key Messages 2-5. Perhaps these could be consolidated in Key Message 5, which is explicitly about infrastructure damage. For example, in Key Message 3, why is damage to infrastructure listed as a human health risk in the same sense as injuries, smoke inhalation, and infectious disease?

In the last sentence in Key Message 3, is it not the implementation of adaptation (rather than the strategies) that would reduce harm? Clarification is needed.

For Key Message 5, “effects are likely to grow with continued increases in emissions.” Is this sentence intending to state that there will be continued increases in emissions? See also comment for Key Message 1.

In Key Message 6, the first sentence states that adaptation can reduce costs and have other positive outcomes. However, it is unclear whether this is a statement of belief about consequences of adaptation in general or is a conclusion based on evaluation of adaptation actions that have been taken in Alaska; the traceable accounts indicate high confidence. The supporting text related to Key Message 6 states that adaptation actions are under way but provides little information on what adaptation actions and consequences have been observed. The supporting text should be expanded to provide concrete examples to support the text and confidence given.

Comments on Graphics

Most figures show changes in the physical environment (climate and erosion), not impacts on ecosystems or people (except for reduced heating costs), whereas the key messages emphasize effects on ecosystems and communities. Graphics should be selected to more clearly illustrate the key messages.

Comments on Literature Cited

The chapter does an excellent job of synthesizing the key literature, especially the literature since the NCA3 was published. The chapter makes one important misinterpretation of the literature though, which should be corrected. It states that climate change is causing conversion of forest to shrubland. The cited study (Mann et al., 2012) describes a change from conifer forests to deciduous vegetation (such as the aspen mixed-wood of Alberta). The statement that forests are changing to shrublands may be true, but the Mann et al. reference does not make this claim.

Comments on Traceable Accounts

The findings are documented in a consistent, transparent, and credible way, with traceable accounts supporting the key messages and likelihood evaluated appropriately, but see comments on key messages.

Other Recommended Changes

The chapter describes how adaptation could be done and what tools are available, but it does not describe explicitly what adaptation implementation is occurring. Some documentation of adaptation actions that have been implemented would be helpful.

See Appendix B for specific line comments for Chapter 26.

Key Message 1 refers to results from different forms of statistical and dynamical downscaling. These methods may not yield consistent outcomes since statistical downscaling assumes stationarity of underlying processes and dynamical downscaling less so. Dynamic downscaling may be better suited to understanding Pacific Island climate changes, especially precipitation changes, because it can potentially better represent fine scale topographic influences. See also the Other Recommended Changes section of this chapter review.

The supporting text for Key Message 4 could be clarified. For instance, the text states that, “If the current emissions trajectory continues, coral reefs will experience annual bleaching beginning in about 2035 in the Marianas Archipelago, in about 2040 in American Sāmoa and the Hawaiian Islands, and in about 2045 at other equatorial reefs” (page 1257, lines 5-7). Many readers may not equate an emissions trajectory to a given amount of warming. Also, the amount of warming projected during the next few decades is roughly the same for all emissions scenarios. This statement could be modified to, “under projected warming of approximately 0.5^oF per decade, coral reefs will experience…” or something similar.

Comments on Graphics and Boxes

For Box 27.1, additional context should be provided for the statement “El Niño-Southern Oscillation (ENSO) is the main source of year-to-year climate variability in Hawai’i and USAPI.” ENSO does have a strong influence on interannual variability, but sea level fluctuations have also been linked quite strongly to multi-year variability, such as the Pacific Decadal Oscillation and decadal trade wind strengthening and weakening (Bromirski et al., 2011; Merrifield, 2011; Hamlington et al., 2016). These lower frequency fluctuations and links to sea level (and probably rain and snowfall) should be discussed. Additionally, rainfall has been decreasing in Hawai’i, presumably because of changes in trade wind regime or North Pacific storm tracks. It would be informative to readers to know what mechanisms are involved and what season(s) have experienced the greatest changes. In general, in addition to reporting projected local changes a description of causal mechanism provides useful insight.

Figure 27.7 may mislead readers because it is presented in rates of sea level change (inches per decade), in comparing a short period (2012-2015) with a longer period (1993-2011). Granted, the rates have changed dramatically, but the overall amount of anomalous sea level rise in 1993-2011 is large and that in 2012-2015 is relatively small. One possible way to address this could be to instead show the time average sea level anomaly for the two periods.

For Figure 27.9, “The Marshallese Traditional Agroforestry Calendar,” the fact that such a calendar exists and may be used is interesting, but the Committee doubts that most readers will be able to decipher this diagram. It would also take a lot of mental effort to distinguish the El Niño version from the traditional version; showing one of these versions is more than enough.

Comments on Literature Cited

The literature cited reflects both literature that demonstrates the advancement of data and information availability in climate science in the region, as well as relatively recent research studies that were published before 2013 addressing topics specific to island issues that do not have recent updates. In general, presentation is linked to findings of the CSSR and its summary in the draft NCA4 Chapter 2.

Comments on Traceable Accounts

Overall, the key messages are documented in a consistent, transparent and credible way. In the “Description of confidence and likelihood” section for Key Message 3, it is stated that: “There is very high confidence that a continued rise in global temperature will lead to increases in the rate of sea level rise.” The Committee agrees with this, but it is suggested that the authors add that while there is high confidence that sea level will rise, there is not much confidence in the actual amount of sea level rise—the increased rate could fall within a wide range of possible outcomes.

The process that was used to prepare this chapter seems to have effectively enlisted scientists (Workshop) and Stakeholders (Town Hall meetings), and the evidence base is well documented by reference to numerous sources.

Comments on Data and Analyses

In general, there is consistency in the way that data and analyses are handled. But for the most part, these are not new analyses. It should be noted that on page 1257, line 25, the work cited from Bell et al. (2013) reports a 20% decline in coral reef fish production under model projections from an SRES A2 emissions scenario (still relevant, but rather dated) in contrast to the RCP approach being used more commonly throughout the draft NCA4 report.

Other Recommended Changes

• As an overview, it would be useful to have U.S. Island systems on both sides of the globe (Hawai’i Pacific Island, this chapter, and the U.S. Caribbean, Chapter 20) affirm the uniqueness and similarities that islands face in addressing climate change issues. This chapter handled the Pacific side, but in a self-contained fashion. It is important that linkages or comparisons to the U.S. Caribbean chapter of the draft NCA4 be made, with cross-referencing between the two included, as appropriate. An issue that both might address is whether the temperature and precipitation changes projected for these island systems is to large extent the same as changes projected over the broader sweep of

• oceanic area within which they are placed, or whether the island landscape and topography creates important differences from those larger-scale open ocean changes.

• The “Summary Overview” section of this chapter addresses the topics and information at a reasonable level; however, the last paragraph that begins on page 1233 (lines 35-38) and ends on page 1234 (lines 1-5) might benefit from additional attention to flow and connectivity of ideas and information. Chapter 27 is longer than some of the other chapters and a case could be made for reducing the number of pages, but the Committee would not recommend it if doing so would mean a loss of the sense of process and an understanding of the complexity of the area and its challenges.
• In the reporting of temperature (e.g., page 1243, line 15 and page 1271, lines 6-7), the authors report an air temperature increased by 0.42°C (0.76°F) in the past 100 years. This is surely an estimate from a limited set of weather stations, which probably have some degree of uncertainty. The way it is stated could be interpreted as being highly confident with a two decimal place accuracy. Care should be taken here and elsewhere to ensure that statements do not falsely exude high confidence.
• Page 1247, lines 3-5 states that, “Throughout the region, the number of climate and water resources monitoring stations has declined (Oki, 2004; Keener et al., 2012; Giambelluca et al., 2013), reducing the ability of researchers to project future changes in climate.” This might also be inserted in the traceable accounts section for Key Message 1, since it introduces uncertainty (or more certainty if observations are shored up).

CHAPTER 28: NEAR-TERM ADAPTATION NEEDS AND INCREASED RESILIENCY

Summary

This chapter on adaptation response is one of the stronger chapters in the draft NCA4 report. It is balanced, easy to read, and frames the discussion well in terms of a challenge that can be tackled by investing and adapting. The tables and figures are effective and relatable for conveying important messages for the intended audience. As noted elsewhere in this review report, expanded and integrated treatment of climate change responses across the draft NCA4 is strongly encouraged, including discussion and examples of adaptation. Chapter 28 provides a robust foundation for that discussion. The risk management conversation is timely and positive. This is an important message for the intended audiences—adaptation is a form of risk management. The key messages are effective and accurate, but could be improved by incorporating stronger language that ties in concrete examples.

Comments Related to the Statement of Task

Comments on Key Messages

The key messages reflect current understanding and current and emerging conversations, especially in terms of risk management and risk reduction. They are clear and consistent, and reflect a balance of challenges and options for responding. However, there are stronger

statements in the body of the chapter that could be woven into the key messages to strengthen their impact.

Page 1309, lines 36-39 states, “Mainstreaming of climate adaptation into existing decision processes has begun in many areas, for instance in financial risk reporting, capital investment planning, engineering standards, military planning and disaster risk management.” This sentence is strong and effective in that is uses concrete examples.

Page 1311, line 20 states, “Adaptation is a form of risk management.” This is a simple, yet strong sentence. It puts adaptation in familiar risk management terms. Intended audiences can relate to this as a manageable challenge when framed this way.

Comments on Graphics and Tables

Figure 28.1 is a simple yet effective graphic and illustrates action and progress well. Any organization could scale and use this information.

Table 28.1 is also a simple, effective, and relatable tool for intended audiences, including decision-makers.

Comments on Literature Cited

Overall, Chapter 28 accurately reflects the peer-reviewed literature and appears to illustrate progress since the NCA3 was published. The chapter should rely on peer-reviewed literature, gray literature, or direct source data from cities, states or agencies that meets the NCA4 quality standards. It is suggested that the authors avoid quoting newspapers (e.g., page 1318 cites the Miami Herald) that may not fully capture a policy, project, or situation.

Comments on Traceable Accounts

Chapter findings are documented in a consistent, transparent and credible manner with high to medium levels of confidence. The findings are communicated effectively and rather easily for intended audiences.

Other Recommended Changes

The Southeast Florida Regional Climate Change Compact is a model of regional climate adaptation planning.¹¹ Recent developments include the creation of the 2017 updated Regional Climate Action Plan.¹² This activity would be a strong example to emphasize in this chapter. The Miami Beach stormwater infrastructure program is also a good example which demonstrates progress from regional planning to local adaptation action.¹³ Miami Beach is often noted as a city investing and adapting and not just planning. It is recommended that the NCA4 authors review the work of the Compact and the City of Miami Beach for possible reference and inclusion.

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¹¹ See http://www.southeastfloridaclimatecompact.org.

¹² See http://www.southeastfloridaclimatecompact.org.

¹³ See http://miamibeachfl.gov/risingabove.

Many other adaptation efforts at local scales have been established across the U.S., which could be drawn on for examples in the draft NCA4, including the Bay Area Regional Collaborative,¹⁴ the King County-Cities Climate Collaboration,¹⁵ the Metropolitan Mayors Coalition,¹⁶ and the Climate Compact of Colorado Communities,¹⁷ among others.

On page 1321, line 16, the statement about fuel treatments reducing risks of wildfire should be qualified by adding “in some forests.”

CHAPTER 29: MITIGATION: AVOIDING AND REDUCING LONG-TERM RISKS

Summary

Mitigation is aimed at reducing the magnitude of human-caused climate change, which complements the use of adaptation to cope with climate change impacts. The draft chapter provides an overview of how mitigation efforts can affect climate change risk and impacts. It makes the appropriate points that (1) adaptation and mitigation are complementary efforts; (2) the scale of risks and impacts can be reduced through mitigation; and (3) the timing and magnitude of emission reductions are important for reducing risk.

In the “State of Mitigation” section of this draft chapter, too much focus is placed on the U.S. government’s withdrawal from the Paris Agreement. A large number of mitigation responses have been undertaken beyond the federal government—by individuals, local and state governments, large corporations, and many other institutions—and these efforts are insufficiently covered in this chapter. An expanded inclusion of important examples of a range of mitigation initiatives would offer a solutions-oriented message, a perspective that would align more clearly with the broader messaging approach recommended in this review report.

The Chapter 29 authors are encouraged to describe the timescales associated with mitigation versus adaptation, namely that mitigation provides benefits on the longer timescales of interest (approximately 100 years) and probably has a limited effect in the near term (approximately 25 years). Placing focus on the timing of mitigation is necessary to highlight the important point that without significant emission reductions, the United States will experience substantial and far-reaching impacts, especially in the latter half of the century. This could be emphasized and discussed more explicitly in the chapter.

Chapter 29 focuses largely on economic values and essentially argues from a cost-benefit framing. The Committee sees the value in including economic risks and impacts in the draft NCA4, which are primarily only included in this chapter. As noted in the comments in the “Front Matter: Report Findings” section earlier in Chapter 3 of this review, the revised NCA4 should address economic impacts more broadly in the national topic and regional chapters to complement the discussion of impacts. That said, the draft Chapter 29 focuses too narrowly on economic impacts, and thus does not reflect the risk-based framing of the rest of the report.

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¹⁴ See http://bayarearegionalcollaborative.org/projects.html.

¹⁵ See https://kingcounty.gov/services/environment/climate/strategies/k4c.aspx.

¹⁶ See https://www.mapc.org/our-work/expertise/climate/mmc.

¹⁷ See https://www.compactofcoloradocommunities.org.

Review Comments Related to the Statement of Task

Comments on Key Messages

For the most part, the chapter’s key messages are clear, but they are incomplete and not anchored in a risk framing around likelihood and consequence. Messages could also be framed in a more solution-oriented manner to make it more accessible to broad audiences. Generally, it is recommended that the messages place greater emphasis on response actions than on potential impacts.

It may also be worthwhile to include a key message that emphasizes that mitigation operates on time scales that span multiple decades that may have mid-course iteration while investments in adaptation can expect more immediate or short-term benefits.

Key Message 1 focuses on the risk of inaction, imprecisely applied. The message would be clearer if it started with “Without significant mitigation efforts, impacts on the United States are expected to be substantial,” rather than starting with impact quantification.

Key Message 2 would be improved by revising to “the magnitude and timing of avoided risks varies by sector, region and population adaptive capacity (or socioeconomics).” The second paragraph in the support text (page 1355, lines 20-29) supports this additional caveat.

The Committee suggests a thorough rethinking the content of Key Message 3. The first two sentences only convey information on the well-known complementarity of adaptation and mitigation. The last sentence about recent studies simply attributes high confidence that progress has been made in recent research.

Comments on Treatment of Risk

The focus on economic valuation in Chapter 29 does not reflect the risk management perspective of the rest of the report. The chapter acknowledges that adaptation efforts will reduce impacts, but it does not directly estimate or account for how these efforts will reduce impacts and net costs. This is understandable though, because data to support calibrating the value of adaptation on an aggregate level is lacking. The selected approach essentially argues from a cost-benefit framing and only calibrates risks and benefits in dollars. Adopting more calibrations of risk, and recognizing the importance of timing of mitigation action, would make this chapter much more powerful, up to date, and broadly consistent with the treatment of risk in other chapters of the draft NCA4. To be more specific, economic damages are only one part of the equation. The chapter reports aggregate economic damages without recognizing that even the most current literature is incomplete in coverage and inadequate in reflecting adaptation. Some mention of this fundamental caveat is needed and would make the discussion more consistent with recent literature on this topic (Hsiang et al., 2017).

Inclusion of additional approaches to quantifying aggregate economic damages would strengthen the chapter. It is recommended that the “Reasons for Concern” framework (Schneider et al., 2007; O’Neill et al., 2017) be used. This framework is used to communicate risks associated with climate change based on scientific evidence and expert judgement and would move the chapter content beyond the economics. Aggregate damages under multiple emissions scenarios can be estimated in decadal increments driven by transient temperature change (see Yohe, Climatic Change, November 2017; Hsiang et al., 2017; O’Neill et al., 2017). This work includes uncertainty in emissions and associated temperature change for each temperature target, and emphasizes dependence of damages and concerns on observed transient temperature change. More broadly, considerable research has been published recently for inclusion in the forthcoming IPCC Special Report on 1.5^oC and should be cited in this chapter, which could support and inform Key Messages 2 and 3.

Comments on Graphics and Tables

Figure 29.1 is effective and a welcome addition to the draft NCA4. See specific comments provided in the review of the draft NCA4 Chapter 1, “Overview,” found earlier in Chapter 3 of this review report, where this figure is included as Figure 1.5.

Figure 29.2 is interesting and effective at communicating the relative ranking of impacts from mitigation, but confidence in the estimates is overstated by not giving ranges and reporting too many significant figures. The figure reports estimates of year 2090 damages along RCP8.5 running from hundreds of billions of dollars per sector down to 1 million. That reflects six significant figures in accuracy from top to bottom, which is not credible. This should be clearly explained.

Figure 29.3 is interesting, but will be challenging for many readers. It should be revised to better highlight the differences across the three RCP scenarios and link them to mitigation, if possible.

Table 29.1 is an annotated bibliography of studies which conveys no content and is not comprehensive. This table could be useful if it provided information on what the noted studies concluded.

Comments on Literature Cited

As noted elsewhere in this chapter review, there are some gaps in recent literature on risk management, mitigation values beyond economic measures, time frame differentiation, and recognition of the iterative nature of responding to climate change.

Section 29.5.2, “Reducing Risk Through Climate Intervention,” is appropriate for this chapter. The conclusion about carbon dioxide removal, however, should be modified. It is incomplete in stating only that carbon dioxide removal is “estimated to have high costs and long implementation times....” It should also mention that several studies of long term climate mitigation strategies suggest that without carbon capture and storage, the long-term costs of addressing climate change are much higher. The IPCC Fifth Assessment Synthesis Report (IPCC, 2014) concluded that carbon capture and storage technology will be essential to meet more near-term climate goals, such as the mid-century climate goal of keeping global temperature rise within 2^oC agreed to in the Paris Agreement. In fact, IPCC suggests that without carbon capture, utilization, and storage, mitigation costs will rise by 138%. Finally, the Chapter 29 authors might want to mention that considerable effort is underway to reduce the cost of carbon capture, utilization, and storage, including finding alternative uses of the captured carbon, (e.g., the Carbon XPrize¹⁸).

The chapter could also better link to topic chapters in some instances, such as the discussion of agricultural carbon sequestration could also note that this would improve soil water retention (page 1358, line 23).

Comments on Traceable Accounts

The findings are consistent and transparent with what has been presented. However, there is a larger body of available literature that could be draw on to support the chapter text, which the Committee recommends the Chapter 29 authors consider including to bolster the traceable accounts.

Comments on Data and Analyses

The data and analyses included in this chapter are consistent, transparent, and credible, to a point. The assessment is limited and narrow because it does not critically evaluate the data presented. Critical evaluation of weaknesses, strengths, omissions, and other caveats reported in the cited literature should be noted.

Other Recommended Changes

In the draft chapter discussion of aggregate economic damages, the text should be clear that reported totals are dominated by health impacts derived from the value of statistical life (not a well-accepted concept, though better within a country than across the globe).

Readers may be confused when the topics switch between impacts and mitigation. For example, Section 29.4, jumps from the previous section on emission reductions to impacts

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¹⁸ See https://carbon.xprize.org/teams.

quantification. To help with this transition, it would be useful for the opening paragraph to mention this connection. Specifically, “to understand how mitigation can help reduce impacts, it is useful to look at how the impacts change under various emission scenarios.” In addition, headers that always mention mitigation or make the connection between mitigation and impacts would reduce confusion. Organizing in terms of likelihood (confidence) and consequence (including adaptation in the reported literature) would also be useful.

While the ancillary benefits of mitigation are mentioned in Section 29.5.1, this section is relatively short and could be expanded to explain that the immediate benefits are also ones that tend to be especially beneficial for vulnerable populations and tend to have large public support.

Given that it is challenging to determine how adaptation will impact the net cost of climate change calibrated in dollars, human lives, likelihoods of extreme weather events, the distribution of impacts, and the potential of crossing irreversible tipping points, the chapter provides only the beginning of an adequate overview of how mitigation can reduce long-term climate risk. While expanded discussion of the synergy between adaptation costs and mitigation is likely infeasible in the NCA4, it would be helpful to provide a brief overview of this topic. As stated in the IPCC Synthesis Report (2007), “Responding to climate change involves an iterative risk management process that includes both adaptation and mitigation and takes into account climate change damages, co-benefits, sustainability, equity, and attitudes to risk.”

This chapter could benefit from framing existing and needed mitigation and adaptation efforts (and their interactions) in an adaptive iterative risk-management framework, with cross-reference to the draft NCA4 Chapter 17, Sectoral Interdependencies, Multiple Stressors, and Complex Systems. Reference to other national topic chapters where mitigation is possible is also suggested (e.g., Chapter 7, “Ecosystems, Ecosystem Services, and Biodiversity,” Chapter 10, “Agriculture and Rural Communities,” etc.).