The large body of assessment and primary literature cited in the draft NCA4 provides a strong foundation of science that, when communicated well, can serve as a valuable resource for a wide range of audiences. The Committee found that the most effectively communicated sections of the draft report had three elements: foundational science about the relevant climate change drivers, understandable examples of climate change impacts, and clear examples of adaptation or mitigation actions. The draft NCA4 is generally written at a technical level appropriate for a wide range of stakeholders, but there are many opportunities for the draft report’s key messages and supporting information to be conveyed more concisely, with greater cross-referencing between relevant draft NCA4 sections and chapters, and with expanded inclusion of examples of adaptation and mitigation response actions. Additionally, the discussion of uncertainty and risk framing associated with some key messages could be expanded and articulated more clearly. The Committee also recommends identifying the advancements in science and response strategies that have occurred since the publication of the NCA3 to improve the impact of this draft report.
The draft NCA4 builds on the foundational science presented in the CSSR (USGCRP, 2017) to bring the impacts of climate change and response actions into focus for societal decision-making. The CSSR documented the observed 1.0oC (1.8°F) global average temperature increase between 1901 and 2016 that is extremely likely to have been caused by human activities, specifically by the introduction of greenhouse gases into the atmosphere. Beyond increased temperature, the CSSR highlights a wide range of observed changes including the global average rise in sea level of 16-21 cm (7-8 inches) since 1900; the increase in heavy rainfall events, heat waves, and forest fires; the earlier spring snow melt; the decrease in snowpack; the increase in the heat content of the ocean; and associated changes in terrestrial and marine ecosystems, among many other topics. The impacts of these changes in the United States are diverse, complex, and interconnected and are presented in the draft NCA4 around three interrelated themes: national (or sectoral) topics, regional impacts, and responses taken through adaptation and mitigation. Using this structure, the draft NCA4 brings the global and national climate science presented in the CSSR to a scale that is relevant for regional audiences and for groups with a topical focus.
The Committee was impressed by the strength, breadth, and quality of the science presented in the draft NCA4. The draft report provides a thorough and accurate discussion of the predominant aspects of climate change and its impacts, with reasonable reference to the peer-reviewed literature. Specific comments on particular topics and recommended additional citations are provided in the chapter-level comments provided in Chapter 3 and in the line comments included in Appendix B of this review report. A few larger gaps in topical coverage were identified, such as in the draft NCA4 Chapter 7, “Ecosystems, Ecosystem Services, and Biodiversity,” and Chapter 29, “Mitigation: Avoiding and Reducing Long-Term Risks,” where expanded treatment of some subjects is needed. Minor gaps for draft NCA4 chapters are noted in Chapter 3.
While numerous suggestions on ways to improve the assessment are offered in this review report, these should be viewed as constructive criticism on a generally strong draft report. Furthermore, the Committee believes the draft NCA4 report will be a powerful tool for a variety of stakeholders—including federal agencies, policymakers at all levels (local, state, and national), decision makers, the private sector, community members, interested individuals, educators, and students—to learn about the scope of climate change impacts in the United States as well as possible adaptation and mitigation strategies.
The Committee determined that the draft NCA4 meets the intent of Section 106 of the Global Change Research Act (see Box 2.1). The draft report effectively addresses climate change, a critical component of global change, which was found to be an appropriate scope for a national climate assessment. Other aspects of global change, such as land-use change, are introduced with an appropriate level of detail and provide necessary context. Other non-climate stressors that interact with climate (e.g., pollution, aging infrastructure, population growth and associated resource demands) are effectively discussed in some chapters of the draft NCA4 and provide useful context for understanding climate change impacts and societal risks. The Committee encourages greater attention to interacting non-climate stressors when discussing climate change impacts and adaptation strategies where relevant across the draft report. Greater acknowledgement of the limited ability to project future change for some non-climate stressors would also be useful.
In general, the draft NCA4 effectively addresses part 1 of Section 106 in that it integrates, evaluates, and interprets the findings of USGCRP-supported research, including the information synthesized in previously published reports and other data products. It also accurately conveys the scientific understanding of climate change and its impacts to the United States. Evaluation of scientific uncertainties is included in the “Traceable Accounts” section of each draft NCA4 chapter. Suggested improvements related to the uncertainties and traceable accounts content are provided later in this chapter and in Chapter 3 of this review report.
The draft NCA4 also analyzes the effects of global change on the sectors listed in part 2 of Section 106. However, expanded treatment of some of these topics would strengthen the draft NCA4. For instance, the influence of climate change on human social systems should be expanded. Climate change impacts all aspects of society—from food, water, and energy security to transportation and human health—directly and indirectly by amplifying other uncertainties and risks. More explicit discussion of these linkages would help readers to better understand how climate change affects their everyday lives, their region, and the nation.
The treatment of how climate-driven ecosystem change affects ecosystem services and the flow of benefits (or negative impacts) to people needs more discussion. The beneficial role of the natural environment in lessening the impacts of some aspects of climate change should be discussed, such as the value of barrier islands and dunes in reducing effects of sea level rise and storm surges; the role of forests, wetlands, and soils in storing carbon; and the importance of snowpack and natural headwater reservoirs that support downstream water needs. The draft NCA4 could also be improved by highlighting ways in which the natural environment can help to mitigate climate change, such as the replacement of fossil fuel use with renewable energy
sources (e.g., solar and wind, hydroelectric, geothermal energy, etc.) and restoration of forests that sequester and store carbon. A comparison of the positive and negative impacts of shifting energy sources could also be discussed (Hill, 2016).
Topics included in the draft NCA4 that are not specifically indicated in Section 106 of the Global Change Research Act provide a valuable addition to the draft report. These include the regional chapters; national topic chapters on air quality, international interests, and interdependences and compounding stressors; and response chapters on adaptation and mitigation. The rich discussions in the regional chapters could strengthen the cohesiveness of the report with increased cross-referencing with the national topic chapters, as appropriate. In the national topic chapters, the Regional Roll-Ups could be better utilized to enhance this added value. The new national-scale chapter on interdependencies could also be used to better connect impacts and responses among sectors and topic areas.
Overall, analysis of current and projected climate trends for the next 25 to 100 years is well integrated throughout the draft NCA4 (Section 106, part 3). Natural climate variability is discussed in the draft NCA4 Chapters 1 and 2, and elsewhere in the report in relation to specific topics, such as El Niño-Southern Oscillation. More discussion of natural climate variability could be added in the regional chapters, particularly for regions where natural climate variability complicates the detection of climate change impacts and adaptation efforts.
The Committee supports the inclusion of response actions and found the examples of these actions in the draft NCA4 to be very impactful. Their inclusion is consistent with literature on science communication, which emphasizes that effective science messaging includes making information relevant to decisions (Moser and Dilling, 2011; Benz et al., 2014; NASEM, 2017a). Discussing climate change impacts alongside examples of current and planned steps to address those impacts, therefore, can leave readers with a sense of how to respond beyond concern or fear of observed and projected impacts.
Figure 1.5 (also included as Figure 29.1) was identified as a particularly effective graphic because it shows actions at both local and state levels in a straightforward manner and represents most of the regions included in the draft NCA4.
Because of the effectiveness of response examples, greater reference to these activities is recommended where such examples exist. Adaptation is discussed in five of the ten regional chapters and mitigation in only three. Since the NCA3 was published, many new response actions have been implemented and planned. Highlighting these actions provides an opportunity to demonstrate advancement in recent years. In particular, recognition of actions that have occurred in the private sector or through public-private partnerships should be acknowledged in addition to government-led efforts.
Finally, given the importance of adaptation and mitigation and the recommendation that these topics be better emphasized throughout the draft NCA4, the Committee suggests making the adaptation (Chapters 28 in the draft NCA4) and mitigation (Chapter 29) chapters more prominent in the report by placing them earlier in the chapter list. In such a long draft document, having the response chapters at the very end may give the misperception that they are less important or not discussed across the draft report.
Recommendation: Incorporate more examples in the draft NCA4 that highlight new and ongoing adaptation and mitigation activities. These should include actions in the private sector, public-private partnerships, and government at multiple scales.
The Overview Chapter (Chapter 1) of the draft NCA4 was reviewed with particular attention to whether the chapter provides an appropriate and balanced overview of the draft report content and is written at a technical level appropriate for the intended audience. As an overview chapter, Chapter 1 should serve as a go-to for readers interested in gaining a quick understanding of the NCA4 and complement the “Report in Brief” that will be developed by the NCA4 authors. While the chapter is well written and scientifically accurate, the strong emphasis on climate science is out of balance with the impacts focus of the draft NCA4 as a whole. Revising this chapter to focus around the twelve report findings and national topic and regional impacts would strengthen the chapter.
Recommendation: Reframe the Overview Chapter of the draft NCA4 to center around the twelve report findings that reflect the impacts and responses that are discussed throughout the draft report.
Key messages in the draft NCA4 convey relevant information and are well supported by the peer-reviewed literature, but most are long and contain multiple unique points. The Committee recognizes that the NCA4 authors were given instructions to limit the number of key messages and this objective may have led to the complexity of some individual key messages. However, using more concise language and shortening the key messages would make them more impactful. Messages could be prioritized by focusing more directly on the take-home point and listing this first. Key messages could also be shortened to provide only the portions of the
messages where confidence and likelihood statements are provided in the traceable accounts. Making the length of key messages more consistent across the report is also recommended and where appropriate, key messages in national topic chapters should parallel related key messages in regional chapters in order to improve consistency and linkages across the draft NCA4.
Many of the key messages place stronger focus on climate change itself rather than the subsequent impacts of that change or associated response actions. A more balanced treatment of these key messages is suggested in order to place greater emphasis on impacts and responses that are the primary focus of the draft NCA4. The NCA4 authors should also consider strengthening the emphasis on adaptation and mitigation activities associated with key messages as appropriate. This modification will facilitate more solution-oriented messaging, which can be lost when key messages are framed largely around the climate drivers and impacts.
Key messages and supporting text are most compelling when concrete examples are given, including stories where response actions have been shown to reduce climate change impacts. The Committee found the draft NCA4 Chapter 18, “Northeast,” to be a good example of effective key message use and thinks it could serve as a model for other chapters. In Chapter 24, “Northwest,” the framing of vulnerable communities as being on the front lines of climate change was viewed as excellent. It is a way to draw in diverse communities that will be impacted by climate change, with consideration for socioeconomic factors as well as location, livelihoods, cultural practices, distance to services, etc. The Committee recommends that this language be used throughout the report, where appropriate, as it greatly increases the relevance of key messages for diverse audiences.
Recommendation: Key messages should be presented using more explicit and concise language. Examples that align with key messages should be included wherever possible in the supporting text and figures. More of the key messages should be supported by examples of response actions to facilitate solution-oriented communication and information sharing.
Use of Graphics to Support Key Messages
Graphics in the draft NCA4 were found to be a mix of highly informative material that is well aligned with the key messages and figures that are difficult to understand and not well tied to chapter content. It is recommended that the NCA4 authors include figures that align closely with chapter key messages. The Committee encourages identifying opportunities to use similar graphics in multiple chapters (i.e., linking concepts in topic chapters to examples in regional chapters) in order to improve message consistency. In general, graphics that convey enough information on the figure and require only limited caption text are the most effective and allow readers to understand take-away messages easily.
Traceable Accounts to Support Key Messages
The Traceable Accounts section included in each chapter is a critical component of the draft NCA4 and a challenging section to review. This section is where the NCA4 authors use their expert judgement to assign confidence and likelihood based on the available evidence contained in the literature. To rigorously review the confidence and likelihood determinations made by the NCA4 authors would require an expansive panel of experts with expertise in all of the detailed topics included in the draft NCA4, as well as a thorough understanding of how
author judgements were developed. The Committee’s review of the traceable accounts focused largely on whether the NCA4 authors presented an adequate level of detail about the information they used in identifying the content of key messages and determining the appropriate confidence and likelihood designations that they assigned.
Most key messages contain an assessment of confidence and not likelihood (see definitions on page 8 of the draft NCA4), which the Committee deemed appropriate given the generally qualitative nature of the key message content. Minor chapter-specific concerns about confidence and likelihood are provided in Chapter 3 of this review report. Many chapters effectively utilize traceable accounts and provide robust support for key messages presented in a transparent manner, but some inconsistency in the utilization of this section was identified. For instance, some chapters use the traceable accounts largely to detail how the evidence was obtained while the traceable accounts in other chapters focus on an explanation of what evidence supports the key messages. According to the Front Matter “Guide to the Report” section of the draft NCA4, both of these types of information should be provided. In other cases, the traceable accounts introduce new information not previously discussed in the chapter. One of the chapters identified as being particularly effective in developing traceable accounts was the draft NCA4 Chapters 21, “Midwest.”
The traceable accounts section is also an appropriate place to provide sufficient evaluation of topics that are introduced in the main text, but where the scientific evidence is contrary or mixed and where different conclusions could be reached (e.g., changes in likelihood or confidence since the NCA3 or key messages with low or medium confidence). This section could also be used to expand on topics that NCA4 authors deem necessary to include in the report but where confidence is currently low based on available evidence, such as impacts of high consequence where there is currently low confidence.
Other Comments About Key Messages
Broader discussion of equity and environmental justice is needed in the key messages and supporting text in the draft NCA4. Currently, this topic receives very little treatment except in the draft NCA4 Chapter 8, “Coastal Effects,” (most notably Figure 8.4). There is literature that could be drawn on to support wider inclusion of this topic in the draft report (e.g., Brulle and Pellow, 2006; Balazs et al., 2012; Bautista et al., 2015; Bravo et al., 2016).
The Statement of Task charges the Committee with evaluating statistical methods applied to the key messages and supporting text in the draft NCA4. The majority of the information in the draft report is an evaluation of previously published literature and therefore does not contain new analyses or statistical tests. Comments related to analyses for a few specific draft NCA4 chapters are provided in Chapter 3 of this review report.
Note that any revision of key messages would require revision of the main text, traceable accounts, and report findings in order to assure consistency with the revised messages across the draft NCA4 report.
The draft NCA4 report deals with a broad range of climate change impacts that can influence societal risks. These impacts carry uncertainties that arise from predictive capabilities, environmental stochasticity, feedbacks, interactions among climate change impacts and non-climate stressors, and many other factors. By evaluating both the magnitude of potential impacts and the probability of occurrence, while considering known characterized uncertainties, the draft NCA4 can support risk-informed decision-making.
The NCA4 demonstrates an improvement compared to the NCA3 in its treatment and communication of uncertainty and the use of a “risk framing” structure. Steps were made early in the NCA4 development process to inform this advancement, including the convening of a National Academies workshop, “Characterizing Risk in Climate Change Assessments” (NASEM, 2016). This workshop was intended to advance the conversation on “characterizing the risks and clearly framing them in terms of their implications for people and systems,” “conveying clear and accurate information about those risks in ways that are useful and accessible,” and “identifying the connections across sectors and regions that are critical for understanding risks.”
The Committee found the Call Out Box “Why is Risk Framing a Useful Tool for Decision-Makers” in Chapter 1 (pages 48-49 of the draft NCA4) to be useful in explaining the complexity of evaluating uncertainties and risks associated with interconnected systems. Importantly, it also notes that decisions can be (and are) made in the presence of uncertainty. However, an explicit distinction between “risk” and “risk framing” is needed. The term “risk” is not defined in the draft NCA4, such as risk due to potential loss from adverse events as used in engineering or the impacts of uncertainties typically used in other sectors including financial markets. Risk seems to be used more colloquially rather than to provide a formal quantification of risk (i.e., the cross product of the magnitude of an impact/consequence and the probability of occurrence), yet the quantification of risk is listed as an element of risk framing (see “Treatment of Uncertainties: Risk Framing, Confidence, and Likelihood” on page 7 of the draft NCA4). Perhaps as a result of this ambiguity, the individual chapters in the report discuss risk in a somewhat inconsistent fashion. The many climate change impacts, sources of uncertainty, and risks discussed in the draft NCA4 need to be differentiated more clearly. Different types of risk call for different types of actions and risk management solutions. Greater standardization in the treatment of this issue across the draft report is recommended. Similar types of uncertainties and risks should be discussed across chapters and each type should be clearly articulated. Uncertainties and risks that may be unique to an individual sector(s) could also be defined in clear terms and discussed.
Although uncertainty and risk concerns are noted by the Committee, some chapters and topics were handled well in the draft NCA4. Chapter 28, “Near-Term Adaptation Needs and Increased Resiliency,” provides an appropriate template for risk framing through robust reporting on major uncertainties, confidence, and likelihood and could provide an example for other chapters. Additionally, a well-articulated acknowledgement of sectoral interdependencies and their effects on uncertainty and risk, multiple stressors, and complex systems is a primary component of the draft report—although this topic is largely relegated to Chapter 17 rather than embedded more broadly throughout the draft report. Interconnectedness and interdependencies spatially and across sectors increase uncertainty that could lead to negative, cascading impacts. In contrast, spatial interactions among sector activities can also reduce impacts in cases where
synergies are positive or countervailing. The NCA4 authors should consider acknowledging these nuances more extensively in the report.
Additional comments related to the treatment of uncertainty and risk in the draft NCA4 are given in the review of the draft NCA4 Chapter 1 (see Chapter 3 of this review report).
Recommendation: The various types of risk included in the draft NCA4 should be defined more explicitly. Improved consistency in the types of risk discussed and inclusion across chapters is also recommended.
The Committee found the linkages and cross-referencing to relevant information among chapters to be insufficient. In such a large report, it is essential to guide readers through the information in a manner that will allow them to explore their topics of interest, making the report more broadly useful. Many of the national topic chapters are siloed such that it is challenging to find the relevant connections to related issues in other chapters. As understanding of the interconnected nature of climate change impacts and responses improves, the need to make robust connections among topics becomes more important. The traditional structure of and process for developing the NCA reports––particularly the topic/sectoral chapters––may hamper the ability to explore interconnections and interdependencies in a way that captures the complexity of the issues. The national topic chapters have a relatively narrow scope and are authored by relevant experts on the subject matter. Thus, finding intersections is not their primary focus, but rather a secondary consideration. The inclusion of the new chapter on “Interdependencies, Multiple Stressors and Complex Systems” (Chapter 17 in the draft NCA4) begins to address this shift; however, as a stand-alone chapter, the concepts are not well integrated across the draft report.
The “Regional Roll-Up” sections in the national topic chapters demonstrate an effort by the NCA4 authors to bridge chapters. However, missing linkages and inconsistencies in the treatment of topics are evident, which could send mixed messages to readers. For example, the topic of ecosystems shows up in the key messages for each region with a wide range of highlighted impacts. The focus of Chapter 7, “Ecosystems, Ecosystem Services, and Biodiversity,” is narrow, with strong emphasis on invasive species, and does not capture the range of impacts on ecosystems or ecosystem services spanning from the U.S. Caribbean to Alaska. Similarly, the issue of water (Chapter 3 in the draft NCA4) shows up in the key messages in many regions, but the water chapter focuses largely on the impact of changing temperatures on snowpack and runoff while the regional chapters emphasize the timing of precipitation and increasing extreme events as important impacts.
Generally, the topics are connected well within the individual regional chapters, in large part because they provide a relatively holistic treatment of relevant climate change impacts and they discuss interrelated issues together. This approach logically conveys the complexity of impacts that extend across sectors/topic areas.
As a whole, the draft NCA4 lacks a unifying figure that ties together the regional information and highlights the largest impacts, risks, and responses. While it is impractical to think that the entire contents of the draft NCA4 can be condensed into a single figure, it is suggested that the NCA4 authors consider dominant impacts and response activities unique to
different regions and explore how these could be illustrated in a single figure or set of figures. A graphic of this nature presented in the Overview Chapter could be highly effective in communicating the high-level findings of the NCA4. The Committee strongly recommends working with a graphic artist to develop such a product.
Recommendation: Linkages to interrelated topics among chapters should be increased throughout the draft NCA4 to ensure consistent treatment of similar topics and to provide readers with a clearer understanding of how impacts and responses at national to regional scales are connected.
The Committee recommends more clearly distinguishing between what information is new since the publication of the NCA3. Currently in the draft NCA4, citation of new material is often embedded with older references in the text. In situations where new literature provides further support for a finding discussed in the NCA3, this may be appropriate. However, in instances where significant advancements in the science have been made or where new actions or activities have been recently implemented or planned, new information should be identified more explicitly. An example of this is for the Northeast region, where the NCA3 identified adaptation plans but little action could be reported. Now, there is action in most states that is discussed in the draft report. Providing greater emphasis on new knowledge and actions may also facilitate more solution-oriented messaging across the report, as recommended in the “Communicating Key Messages” section found earlier in this chapter of the review report.
A high-level overview of new developments could be included in Chapter 1, “Overview,” of the NCA4. This approach was effective in the CSSR (see Box 1.2, “A Summary of Advances Since NCA3,” USGCRP, 2017). For individual chapters, some possible options for distinguishing this new material would be to color code the key messages or provide notation or language that explains whether a key message has changed since the NCA3 based on new scientific understanding or may be the same or similar to the NCA3 but has new evidence that provides continued support. New key messages could begin with language such as “new evidence [either] suggests a new conclusion be discussed or confirms prior understanding.” If the confidence, likelihood, or conclusions about any key messages have changed since the NCA3, that too should be explained. Alternatively, each chapter could select a few areas where significant progress has been made or where a change in confidence or likelihood has occurred between the third and fourth assessments and focus a portion of the chapter discussion more prominently on those areas, with appropriate context.
A brief description of the relationship between materials included in the NCA3 and the NCA4 would be a beneficial addition to the Front Matter of the draft NCA4. In the spirit of the iterative and sustained assessment process, it is assumed that the NCA4 builds on the content in NCA3, but how this transfer and updating of information was done could be made more transparent. This would provide clarification about the selection of materials that are included in the draft NCA4 that may (or may not) have been in the NCA3. For instance, are there key messages in the draft NCA4 that did not appear in the NCA3 due to a change in the science or a shift in the prioritization of topics?
The draft NCA4 would also benefit by more thoroughly identifying recently-developed state and local climate assessments and adaptation planning efforts, as well as other available climate tools. If appropriate, relevant results from these sources could also be highlighted. An indication of which efforts provide actionable information could also be stated. In general, it seems that the number, depth, and breadth of such assessments has increased substantially since the publication of the NCA3; if so, that aspect should be clearly stated. Additionally, it could be noted that more climate adaptation planning has begun to occur at the community level—in some cases, it was ordained by state legislation, and in other cases, it was by local policy or interest. If possible, linking these resources and planning examples to the draft NCA4 would allow the report to serve as a broader resource for the NCA4 audience. This connection would be particularly effective in the regional chapters. Reference to an expanded list, where possible, of available regional-scale climate projections and assessments would improve comprehensiveness. The Committee also suggests drawing more on relevant gray literature (when it is found to meet the NCA4 information quality guidelines) to provide a more comprehensive evaluation of the state of knowledge and to draw on examples of local and regional adaptation and mitigation strategies. Citations taken from media sources should be avoided; instead, reference should be made to the primary sources.
Recommendation: Authors of the draft NCA4 should explicitly identify significant advancements made since the Third National Climate Assessment, with emphasis on emerging science, impacts, and examples of new response actions.