3
Review of Individual Sections

This chapter provides detailed comments on the front matter (Abstract, Preface, and Executive Summary) and the four chapters of the draft Synthesis and Assessment Product (SAP). The review of each section begins with the committee’s overarching thoughts, followed by a list of specific suggestions. The overarching thoughts at the beginning of the review of each section/chapter generally relate the review to the issues raised in the Major Comments (Chapter 2 of this peer review report). In some cases, the specific suggestions that follow the overarching thoughts further relate to issues raised in the Major Comments; in other cases, these specific suggestions are targeted and the committee considers the issue a relatively minor one.

ABSTRACT, PREFACE, AND EXECUTIVE SUMMARY

Abstract

The Abstract should read as a summary of the Executive Summary or a “one-pager” for policymakers. In its current form, the Abstract provides a sufficient summary of the key issues; however, the tone is inconsistent with the balanced and objective tone projected elsewhere in the document. The authors of the document’s four chapters should ensure that the language of the revised abstract is not alarmist and that statements are supported by the scientific content provided.


Specific Suggestions:

  • Line 10: Make this sentence consistent with the Intergovernmental Panel on Climate Change (IPCC) language it alludes to and refer to the source IPCC material. For example, begin the sentence with “The IPCC 4th assessment concluded…” In addition, replace “planet’s” with “Earth’s atmosphere and oceans,” or a similar phrase.

  • Line 11: The second or third sentence should state that, unlike the IPCC assessment, the SAP focuses on North America (this information is currently withheld until Line 17).

  • Line 16: The committee feels that it is unjustified to refer to trends in drought since 1950 over North America. If the authors of the SAP disagree, they should



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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” 3 Review of Individual Sections This chapter provides detailed comments on the front matter (Abstract, Preface, and Executive Summary) and the four chapters of the draft Synthesis and Assessment Product (SAP). The review of each section begins with the committee’s overarching thoughts, followed by a list of specific suggestions. The overarching thoughts at the beginning of the review of each section/chapter generally relate the review to the issues raised in the Major Comments (Chapter 2 of this peer review report). In some cases, the specific suggestions that follow the overarching thoughts further relate to issues raised in the Major Comments; in other cases, these specific suggestions are targeted and the committee considers the issue a relatively minor one. ABSTRACT, PREFACE, AND EXECUTIVE SUMMARY Abstract The Abstract should read as a summary of the Executive Summary or a “one-pager” for policymakers. In its current form, the Abstract provides a sufficient summary of the key issues; however, the tone is inconsistent with the balanced and objective tone projected elsewhere in the document. The authors of the document’s four chapters should ensure that the language of the revised abstract is not alarmist and that statements are supported by the scientific content provided. Specific Suggestions: Line 10: Make this sentence consistent with the Intergovernmental Panel on Climate Change (IPCC) language it alludes to and refer to the source IPCC material. For example, begin the sentence with “The IPCC 4th assessment concluded…” In addition, replace “planet’s” with “Earth’s atmosphere and oceans,” or a similar phrase. Line 11: The second or third sentence should state that, unlike the IPCC assessment, the SAP focuses on North America (this information is currently withheld until Line 17). Line 16: The committee feels that it is unjustified to refer to trends in drought since 1950 over North America. If the authors of the SAP disagree, they should

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” justify disregarding data prior to 1950. The observational record over North America is sufficient to infer trends beginning prior to the 1930s, when the most significant drought in the record occurred. Line 18: Please see comment on Line 220 regarding the use of the phrase “hurricane activity.” Line 21: Longer duration aggravates the impacts of heat waves, droughts, downpours, and to some extent tropical cyclones. This paragraph should be modified to reflect this consideration, consistent with statements in the body of the SAP. Lines 29-30: Erosion and inundation of coastal lands are arbitrary selections of impacts. The committee suggests either removing these two impacts or expanding the list. The Abstract should mention conclusions made within the body of the report on mid-latitude cyclones (blizzards or “nor’easters”), severe thunderstorms, and tornadoes. All other phenomena discussed in the report are mentioned in the abstract. Preface The committee suggests that a preface should not contain scientific or technical material; rather, it should provide background information and outline the process that led to the formulation of the document. In this case, the background information should outline purpose and goals of the U.S. Climate Change Science Program (CCSP) and the SAPs, and provide a brief summary of the purpose of SAP 3.3. The Preface in its current form does address the purpose of SAP 3.3; however background on the CCSP is absent. Remaining material of a technical nature should be moved to other sections of the document. Specific Suggestions: Display and discuss Figure 1 in Chapter 1 and in the Executive Summary, but not in the Preface. Place Figure 2 in a “Box” and compare it to the analogous table in the IPCC assessments, by displaying the IPCC figure or via a description of that figure. Acknowledge the similar Figure included in SAP 5.2, which was originally conceived to provide guidance for communicating uncertainty in the formulation of other SAPs. Note that SAP 5.2 is currently under revision but should be released publicly before SAP 3.3 is finalized.

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” Executive Summary The Executive Summary is more balanced in tone than the Abstract; however, the committee notes several instances where statements in the Executive Summary are not well-supported in the four chapters. The committee does not necessarily disagree with these statements, but only recommends that the authors ensure they are supported if they are included in the Executive Summary. In particular, and in keeping with Major Comments 4 and 5, claims of trends (increases or decreases), which are listed at length, should be rooted consistently in statistical significance and the underlying uncertainties summarized. The Executive Summary contains several sections with seemingly random, single-sentence paragraphs that should be consolidated, when possible, into coherent themes. Section 6 should be revised to concisely and adequately reflect the recommendations provided in Chapter 4 (see Major Comment 10). Specific Suggestions: Lines 26-29: This paragraph is vague and may be interpreted as alarmist; it should focus on some specific extremes that definitely appear to be changing (e.g., maximum temperatures and precipitation intensity). At a minimum, the committee suggests inserting “some” before “extremes” on line 26. Line 66: This sentence on problems of climate models simulating extremes is correct, but it is not obviously related to the previous two sentences, which describe that changes in climate averages imply changes in the tails of the distribution and hence in climate extremes. Perhaps a more direct connection could be made or the sentence put in a separate paragraph where the difficulties for models in simulating extremes are elaborated a little, including both resolution and process limitations. Line 87: What is the conclusion of this paragraph? Line 113: What does “are likely to be attributable to” mean? Does it mean that they have been attributed to, or does it mean they would be attributed to, but the relevant studies haven’t been done yet? There are no formal attribution studies that attribute the global changes in these phenomena to anthropogenic forcing. There is a single attribution study on these by Christidis et al. (2005) but it only considers limited global coverage, as there are no data in many regions. Line 118: In this case there are a number of studies that have detected and attributed observed changes in Sea Surface Temperature (SST) to increasing greenhouse gases. Does this statement refer to global average SST or regional SST in the tropics?

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” Line 120: There is a new study by Christdis et al. (2007) that attributes observed increases in growing season length (GSL) (based on a simple temperature threshold to define GSL) globally and in North America to increasing greenhouse gases. The main changes are associated with earlier spring onset and are consistent with observed changes in the frost-free period. This should be mentioned as it is just as robust as the other attribution studies on changes in temperature extremes. Line 121: Sections 2.3 and 3.1 list societal and ecological impacts for some forms of severe weather but not others. All paragraphs could benefit from some examples of impacts. Line 164: There appears to be no basis for this statement in Chapter 2 on observed changes. Line 170: If the authors agree that the material presented elsewhere in the document illustrates increases in extreme precipitation, substitute “increases” for “changes.” Lines 192-197: These statements concerning drought are acceptable, but they are inconsistent with the Lines 202-203, “… it is likely that the increasing temperatures are already contributing to droughts that are longer and more intense.” The committee could find no evidence in the material presented (the observed record) of longer, more intense droughts relative to the 1930s and 1950s droughts over North America. These statements also do not comport with the perception conveyed in the Abstract that drought is increasing over North America since 1950. Line 205: Section 4.3 boldly asserts increased evapotranspiration and decreased spring runoff in the mountains without providing any compelling evidence. First, it is likely that potential evapotranspiration will increase because of warmer air temperatures but actual evapotranspiration will decrease because of decreased soil moisture. Second, the authors state elsewhere (Lines 182-183) that increased spring snowmelt (in higher latitudes) may contribute to extremes in river flooding. Does the first assertion apply only to mountains at low latitudes? Does the second apply only to lowlands at high latitudes? Please clarify this. The peak in spring snow melt run-off is likely to occur earlier, exacerbating the problems of summer water availability, but it unlikely to decrease in magnitude. Line 220: This section concludes that it is “very likely” that hurricane activity is increasing. The case seems weaker in light of the general uncertainty in the database and Figure 2.30. Moreover, hurricane activity is defined here specifically in terms of frequency and destructive power yet the phrase is used throughout the document in seemingly less definitive ways. The authors should

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” ensure that when the phrase “hurricane activity” is used anywhere in the document, it refers to a precise and consistent concept. Line 227: If the significance of the trends in hurricane frequency is sensitive to a small change in the start date of the period, it may mean that the statistical estimation of the trend significance has not been performed correctly or that assumptions in the noise model are inappropriate. The data do not appear to support a real significant trend in hurricane frequency. This sentence is not appropriate for the Executive Summary. Line 245: Over what time period? Line 248: The document provides a case for increasing severity of hurricanes but the case for increasing frequency is much less compelling. Thus the changes in activity (see comment above) may reflect changes in severity. Line 302-303: This trend is not supported by an analysis for statistical significance. Lines 310-315: The authors state that data are not adequate to make definitive statements on observed changes in tornadoes and severe thunderstorm (Lines 310-312). Then, in an apparent contradiction, they state (lines 312-313) that data related to severe thunderstorms ‘are reliable’ (suggesting that the presence or absence of trends in conditions can be determined). The trends in conditions for severe thunderstorms (lines 313-315) have not been shown in the text, and in fact are inconsistent with material presented in Chapter 2 (Lines 144-145; 154-157). Unless the trends in conditions are shown, this statement should be restricted to simply state that the data are not adequate to make definitive statements about trends in severe thunderstorms and tornadoes. Lines 327-329: This paragraph does not specify a particular region within the Pacific basin. It seems unlikely that the statement applies to the entire basin. Furthermore, this paragraph relies on Figure 5 to justify its conclusions. If Figure 5 applies globally, the authors should briefly state why this figure supports their conclusion regarding increased extra-tropical cyclone severity. Chapter 2 suggests extra-tropical cyclone severity off the U.S. east coast has decreased because the tracks have shifted northward. The authors should comment on whether this trend is expected to continue. Line 379: The inflation-adjusted curve in Figure 1 does not compellingly support the notion extreme events are changing because of a changing climate. If 2005 (Katrina) is removed, there is no significant upward trend. Moreover, any increase in damage (in terms of dollars) could be attributed to increased vulnerability (e.g., expanding infrastructure and population near the coastlines) as much as changes in extreme events. The report needs something more compelling. One possible alternative is to disaggregate the data by phenomenon

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” (e.g., provide inflation-adjusted trends corresponding to the parameters presented in Figures 2-5). CHAPTER 1 Why Weather and Climate Extremes Matter Weather and climate extremes matter primarily because of their socio-economic and environmental impacts, yet the Chapter does not discuss socio-economic impacts in any significant detail. A good starting place for such a discussion would be to include an improved version of Figure 1 from the Executive Summary in this chapter (see above for specific suggestions on how to improve this figure). The authors should also consider providing supporting material for an improved Figure 1. Supporting material could include facts and figures illustrating the migration of the U.S. population to vulnerable coastal areas and time series of deaths due to extreme weather events. Chapter 1 provides material on many topics but the topics are not well connected. The lack of cohesion is apparent in the architecture of the sub-sections, wherein there are multiple one-sentence paragraphs that do not always support a more general theme. One remedy would be to combine these short paragraphs; another is to enhance each of them by developing a stronger lead sentence and providing supporting material. In addition, the boxes at the end should be integrated into the chapter rather than grouped together. In some cases, these boxes could be moved into other chapters (e.g., Box F could be moved to Chapter 2). Specific Suggestions: Line 31: This is an excellent recommendation but it should be augmented to explain why it is important. For example, Chapter 4, Figure 1, which states “Mitigation of adverse impacts through better planning and decision making.” The recommendation on Line 31 should include language to this effect. Line 66: This sentence is unclear and Figure 1 is not very effective. Figure 1 should be removed or revised to differentiate between natural and human systems. Line 68: This chapter should emphasize a key point: Changes in extremes are key mechanisms by which climate change affects society and the environment. IF these changes can be forecast reliably society can adapt to minimize their impacts. This point could be the lead-in sentence to the paragraph starting on line 68. The authors should consider moving the resulting paragraph to the beginning of the chapter. Line 83: Section 1.3 is a summary but not a catalogue of extreme event indicators. Moreover, there probably should be a catalogue to help explain what extremes were considered and why. This report should provide a first pass at

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” such a catalogue, as the authors imply that future work should refine a catalogue based on dialogue with the stakeholder community (lines 34-37 and Chapter 4, lines 255-258). Line 103: The authors should replace “determine the probability” with “estimate.” Line 109: Insert “some” before “extreme events”. Line 110: Figure 2 is a valuable figure but its value is diminished because of the lack of discussion. The authors should walk the reader through the figure more carefully, perhaps looking at some specific numbers and examples (e.g. in the previous climate the probability of seeing a 10­° temperature was 10%. When the mean shifts, the probability becomes 20%.) Line 127: The title of Section 1.1.2 is ambiguous. One possibility might be to rename it, “Important Characteristics of Extremes.” Line 133: The authors raise important points but then illustrate them with examples that are relatively unimportant. For example, it may not be important that the tornado season shifts if season duration is unchanged. More important examples might include: (a) time of the first snow melt (earlier in the Sierras means longer dry season with far reaching impacts on the ecology); (b) time of the wild-fire peak (earlier might pose threats to certain species). Line 144: The authors introduce the term “morphology” and use it very ambiguously throughout the report. The authors seem to define morphology to mean the detailed characteristics and properties of an extreme. The authors could in some cases employ more direct terminology; perhaps “characteristics” or “properties” would suffice. Line 151: The details of this discussion are quite good but the organization needs improvement. One possibility is to move Sections 1.1.3-4 into 1.2 since these sections focus largely on vulnerabilities. Line 167: “Enormous” may be an overstatement if one considers the numbers of deaths resulting from other natural hazards and disasters (e.g., the Indian Ocean Tsunami). Line 170: Consider combining sections 1.1.3 and 1.1.4. Line 208: This sentence fails to summarize the important point. Consider replacing “the relationship between climate and society” with a phrase similar to “calculating losses due to extremes”. In addition, “statistics” can be deleted.

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” Line 212: This is a powerful statement but the authors offer no justification or evidence to support it. Line 222: Consider substituting “probability of extremes” for “magnitude of the exposure to which the system is subjected” if this statement refers to extremes but not vulnerability. Line 243: The authors should point out that there is a lack of quantitative proof that adaptation and mitigation saves money. Many decision makers will not support the cost of mitigation unless the cost-benefit relationship is known. Line 272: The concept of a binary classification is introduced yet it is not employed elsewhere in the report (e.g., in the boxes at the end of this chapter other chapters). Consider employing the concept elsewhere in the report. Please clarify the concept when it is first introduced and explain how a binary tree would work. Line 325: The last six words are emphatically policy prescriptive and should be removed altogether. The authors should not recommend courses of action for policymakers. Line 355: This paragraph should be re-written using simpler terminology. At a minimum, “evolutionary” in first sentence should be replaced with “behavioral.” The present paragraph implies that while some species have shown behavioral adaptation to on-going climate change, there are no species that have shown genetic adaptations (presumably because they have not had time to evolve). If that is true then the paragraph deserves its own section and would not belong in Section 1.2.3 on Thresholds. Line 500: A recent study in press in the Proceedings of the National Academy of Sciences (PNAS) with Tom Knutson as a co-author has attributed the observed increase in coral bleaching in the Caribbean to anthropogenic forcing (Donner et al. 2007). This could be mentioned in this box. Line 562: The sentence beginning on this line could be interpreted as a value judgment that compares human worth and suffering to animal or plant worth and should be deleted or phrased more precisely. CHAPTER 2 Observed Changes of Weather and Climate Extremes In this chapter the authors address the key issues set forth in the document’s prospectus regarding what is known about observed changes in extreme events. In several cases, however, the authors overstate the case for an observed trend. Such claims

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” should be supported by the data, provided here or elsewhere in the peer-reviewed literature, with uncertainties quantified and the analyses of datasets subjected to tests for statistical significance. If the claims are based on expert elicitation and not data per se, this should be noted explicitly. The material on tropical cyclones is too lengthy. The committee provides several specific comments to indicate what material should be retained and what should be omitted in the revised SAP. In considering these comments, the authors should strive to capture the dimensions of the ongoing scientific debate vis-à-vis trends in tropical cyclones and climate change, while considering the need to limit the length of the discussion. Specific Comments and Suggestions: Line 69: This finding is inconsistent with earlier statements about drought (Line 17 of the Abstract and Line 193 of the Executive Summary). Line 89: The change in start time from 1880 to 1900 is a small percentage of the overall length of the time series. Such a relatively small change should not impact the trend in a statistically significant manner. This difference likely reflects a problem with the trend assessment technique more than a difference in any actual trend in nature. Line 103: There is no quantitative discussion accompanying the phrase “very unlikely” of the key uncertainties related to measuring the decadal variability. Line 121: This does not rise to the level of a “key” finding. Line 141: This key finding is repeated on line 1551 without any evidence. Please provide citations (if they exist) to support this. Line 164: Are these regime changes associated with climate change issues? Is there evidence for a naturally varying climate system? Please provide citations (suggestion: Bell and Halpert 1995)? (Note: this would be a good location for Box F from Chapter 1). Line 182: Delete “exactly.” As is, the sentence implies that exactness is the norm or at least is not rare. Line 184: What is meant quantitatively by “above to much above?” The National Oceanic and Atmospheric Administration (NOAA) defines these terms quantitatively. What are these definitions? Line 187: Insert anomalies after temperature[s].

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” Line 224: This statement may be inconsistent with lines 242-243. How do “very extreme” heat episodes differ quantitatively from “warm spells?” (Note: Peterson et al. 2007 is not yet published). These data do not support the tone of certainty projected by sentence that begins on line 12 of the Abstract. Line 261: To minimize confusion, please characterize all changes in terms of either frost day occurrence or length of the frost season, but not both. Line 338: Define Palmer Drought Severity Index (PDSI). Provide a reference. Discuss its strengths and weaknesses. Line 424: Please quantify what is meant by “disproportionately” or strike it. Line 426: It would be useful to note parenthetically that 101.6 millimeters is equivalent to 4 inches. If there are other instances in the document where seemingly random metric quantities relate to “round” English system numbers, please provide the equivalents. Line 491: Cavazos 2007 is not yet published and may not yet have even been submitted. Line 539: Please better define “90-days duration precipitation episode.” This may be a misstatement (e.g., 90-day accumulation may be the intended phrase). Line 544: How is the temporal behavior similar? Does the comment refer to multi-decadal variability? Line 643: The annual-average global total of 90 with a variance of approximately 10 is consistent with Poisson statistics, and indeed a range of studies provide no compelling evidence to reject a Poisson model (Gray 1968; Katz 2002; Frank and Young 2007). Since the process of tropical cyclogenesis involves many disturbances each with low probability of becoming a storm one would expect this sort of model to work reasonably well. This should be acknowledged. Line 656: The more commonly quoted damage total for Katrina is $80B; $110-120B is the generally quoted cost for the entire 2005 season (NHC 2006). Line 676: The discussion of data limitations on pp. 27-30 is generally balanced and accurate. Note that routine reconnaissance began in 1944 (not in 1945 as stated on line 715), and the National Weather Service (NWS) attributed the limited loss of life ashore in the “Great” Hurricane of that year to this reconnaissance. Lines 744: Although cyclone-size data are invaluable for many aspects of impacts modeling, this discussion is not essential here and could be removed. At

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” a minimum, the concluding statement of this paragraph is inflammatory (“failure of governments to take seriously…”) and should be removed. Lines 754: The discussion of spectra of hurricane occurrence is not essential to the thrust of this chapter. It should be reduced to mention El Niño/Southern Oscillation’s (ENSO) modulation of Atlantic activity and the existence of substantial multidecadal component. Lines 773: Please define the Power Dissipation Index (PDI) and comment that statistical significance of PDI trends is dependant upon corrections applied to the records, listing all references. The detailed accounts of dueling corrections will make a fascinating review paper at some point, but they do not contribute to the message here. The assessment of the science is that eminently qualified investigators cannot yet forge a solid consensus. Nevertheless, the paragraphs that begin on lines 798 and 804 should be retained verbatim. Line 812: The material beginning here and ending on line 891 reflects a level of detail entirely appropriate for a review article, but not for the document at hand. The readers (at the level of a “Scientific American” readership) need not know the details of the argument, but need to know that different, but reasonable detrending strategies can yield either large trends or trends at the margins of detectability. Line 906: This paragraph provides too much detail for the proposed audiences. Line 924: This section should be condensed into one page or less, incorporating key references that span the range of evidence and informed opinion. The committee does not necessarily disagree with the statements in this section but that the level of detail is not appropriate for a SAP. These are issues that should mature in the peer-reviewed, technical literature before they are presented in an assessment document. Line 930: Figure 2.27 would be improved if it showed tropical cyclone numbers subject to different adjustment strategies and resulting uncertainties in the trends, such as is shown in 2.30 (note: Figure 2:30 is “in preparation”). Line 965: The literature documents that historical SST data for these time periods constitute a well maintained database with clearly defined, acceptably small errors. Again, this is a point that would be essential in a review targeted at a professional audience, but it does not serve the purpose here. Line 998: The lack of an increasing trend in landfalling Atlantic tropical cyclones is important. The committee recommends that the material on observed trends in Chapter 3 (e.g., Pielke) be combined with this material and that it all be placed here in Chapter 2. The lack of trend due to signal-to-noise problems has implications for projection into the future. The message is that landfalls may be described by a Poisson process, thus a low mean implies a larger standard

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” deviation relative to the mean and hence a large stochastic component, regardless of any trends with basin-wide numbers and intensities. Line 1069: Fig 2.30 contributes significantly. A paragraph based upon lines 1069-1086, but without the details of the significance testing should be retained. Line 1087: The material presented from here through line 1165 should be reduced because it is largely redundant with the discussion that begins on line 976. Line 1166: The review panel endorses the recommendation to pursue paleotempestology; it is a promising method to extend the short, heterogeneous instrumental record. Historical investigations of existing, but obscure, written archives also show promise. However, this section could probably be reduced to a paragraph or two. (Note: please clarify here or elsewhere that as used in the SAP, “paleo” refers primarily to the Holocene). Line 1227: The discussion of the climatic role is interesting, but it may not belong in this document. This is about extreme events, not about the maintenance of the general circulation by hurricanes. Line 1247: Is Hart et al. 2006 yet published? Line 1296: The committee believes that in fact there are data over the central North Pacific to analyze extratropical cyclones; it may be limited, but it exists. Line 1352: This statement is not supported by Figure 2.36, which indicates no trends whatsoever. Line 1355: It would help to insert a sentence or two on the importance of increasing sea level as it relates to the ‘perfect storm’ nor’easter. Line 1475: Table 2 should indicate whether there are significant trends, despite the textual reference to the five largest wave occurrences each year. Line 1573: Please clarify what is meant by W-shaped. Does this refer to a figure in the Changnon and Karl reference? Line 1612: If there is not a trend, please reconcile this with the statement that severe weather environments have increased and then decreased (Lines 314-315 of Executive Summary and Figure 2.42). Line 1630: These changes do not appear to be significant and may just be natural variations. The results here may be the justification for the statement in Exec Summary lines 313-315 but since the trends are likely not significant, they do not justify being brought forward to the Executive Summary.

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” Line 1648: Please clarify what is meant by the climate shift in 1976-77. If this shift is important, why is it not discussed elsewhere? Line 1639: Enhance this section with follow-up discussion from lines 662-752 of Chapter 1. The discussion should relate suppression of hurricane activity in the Atlantic to the enhanced ENSO index. The observed changes that have occurred in Pacific-North America pattern (PNA) and North Atlantic Oscillation (NAO) are related to the surface storm track that you have already discussed earlier (lines 1249-1354). Line 1768: This section could be moved to Chapter 3. It is about projections (not the observed record) and combining models to predict future extremes. CHAPTER 3 How Well Do We Understand the Observed Changes in Extremes, and What Are the Projected Future Changes? The authors provide a good assessment of the scientific understanding of extremes and projected future changes; however, the chapter is too long. Moreover, the discussion on tropical cyclones is excessively lengthy while drought receives less attention than it should. The committee understands that although hurricanes are of considerable interest to a wide variety of audiences, the socio-economic implications of increases in droughts and heat waves are also very serious. Consider, for example, tens-of-thousands who died in the European heat wave of summer 2003, or the 739 excess deaths in Chicago during the 1995 heat wave. Given these implications, the committee believes that the SAP 3.3 should expand the discussion of drought, particularly in regards to projections and uncertainties in those projections. The authors should coordinate with the authors of Chapter 2 to eliminate the many redundancies with Chapter 3. Much of the detection section (3.2) should be moved to Chapter 2 to reduce redundancies. The authors should discuss the differences among detection in observed changes and their implications for projections. If these differences and the causes for them are presented adequately in Chapter 2, the discussion of attribution in Chapter 3 would flow more logically than in the present structure of the document. To reduce the material on tropical cyclones, the authors should focus on significantly reducing Subsections 3.2.4 and 3.3.3. Subsection 3.2.4 is twice as long as all other parts of Section 3.2 and reads as a text on mechanisms. Much of this material is not appropriate for an assessment. The revised versions of these subsections should outline the basic arguments and summarize the range of informed opinion without providing the fine details of every researcher’s arguments, in keeping with a style and

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” level of detail appropriate for a Scientific American readership. The committee provides some specific suggestions in the comments below. Specific Comments and Suggestions: Line 36: The number of key findings (34) dilutes the effectiveness of each. The authors should reduce the number. Line 43: What period of time is being referenced in the first two bullets? Line 48: The paper by Christidis et al. (2007) supports a stronger conclusion for the attribution of GSL reductions in North America and globally over the last 50 years. Line 73: What is meant by “tropical cyclone activity”? Does activity refer to a combination of intensity, number of storms, and other characteristics, or something more specific? Line 108: Increased frequency of droughts is not supported by the evidence provided in this document. Insert “potential” before evapotranspiration. Line 128: Is this statement supported by published literature or is it based on expert elicitation? The conventional wisdom in the hurricane community is that shear is a bigger factor in the Atlantic and that as a result, Atlantic hurricanes often form under marginally favorable conditions. Thus, hurricane activity in the Atlantic is sensitive to small changes in environment near the threshold of formation. Other basins may exhibit similar sensitivity, but this sensitivity is geographically larger and less influenced by mid-latitude windshear (apart from the northern Indian Ocean). Line 135: If this is a key finding based on one study that provides an ensemble mean of climate model output, the uncertainty (spread of the ensemble) should be discussed. This statement on the changes in vertical wind shear appears to be based on the ensemble mean model result from one study. Given the likely poleward shift in the mid-latitude jets and the large variability between different models, there may be good reasons for being less confident about this statement. Line 198: Mesoscale models (used in a regional climate modeling mode) can be used to help address these issues. The committee suggests explicitly mentioning this in this recommendation. Line 261: In Table 1, it may be incorrect to assert better than even odds that global drought is attributable to anthropogenic forcing considering that this information is (1) based only on one study; (2) based only on one model, (3) the

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” reference isn’t available in the published literature; and (4) is based only on the PDSI, which has its limitations. Line 301: This reference is not listed. The U.S. Climate Extremes Index (CEI) uses annual mean maximum and minimum temperatures, not daily temperatures, so the observed increase in the U.S. CEI is largely due to an increase in the area of the U.S. with much above normal mean maximum and minimum temperatures and an increase in the area of the U.S. experiencing a much greater than normal proportion of precipitation from heavy one day events. Line 324: This paragraph is a little misleading, as the main external forcing of the observed global precipitation changes is the volcanic forcing. This should be stated explicitly, as elsewhere in this chapter, the main forcing is the increase in anthropogenic greenhouse gas and aerosol concentrations. Line 427: In addition to comparing observations to the ensemble mean of climate model output, these types of comparisons should address ensemble spread and differences in the spatial patterns among ensemble members. All that is required for the observations to be consistent with the model ensemble is that the observed trend and spatial pattern of change lies within the full range of the model ensemble members, but not necessarily close to the ensemble mean. It appears that the authors may be neglecting to take account of the contribution of internal climate variability to the spatial pattern of observed changes, which could be very large regionally. Line 482: This result only applies globally, but there can be large spatial variations in the decrease in duration or frequency of precipitation events, with some locations having increases. Line 766: Insert “observational” before “studies.” Line 1024: This section needs more subheadings. Some suggestions include frost, snow, drought, and lake effect snow. In the current format, all of these events are lumped under “precipitation” and “temperature.” Line 1041: The uncertainty ranges (shaded sigma bounds) are plotted incorrectly in Figure 1. It may be that the authors did not assess the trends in the control runs and eliminate models with significant control run drift before assessing uncertainty ranges in the projections. Line 1043: The statements on effects of soil moisture change are difficult to follow. Consider simplifying them to state that changes in soil moisture and land-surface parameters may differently affect changes in extremes for maximum and minimum temperature.

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” Again, the comparison of the observed spatial pattern of warming and decrease in frost days in Figure 2 with the ensemble mean pattern from models seems to expect the observed pattern of change to be close to the ensemble mean. However, consistency with the model forced changes plus natural internal variability just requires that the observed pattern of change be well correlated with at least one member of the model ensemble, not necessarily with the ensemble mean. Differences in the spatial patterns are likely due to natural climate variations at regional scales. Line 1262: The introductory material is excellent, although here and elsewhere, the authors should reconcile the disconnect between model predictions of reduced tropical cyclone numbers due to increased shear with the subset of the observations that support increased numbers. Line 1271: At some point, either here or (preferably) in the discussion of middle latitude cyclones and storm surge (Chapter 2), the authors should quote the amount of historical sea-level rise as measured by instruments like tide gauges. Note the apparent acceleration detected with satellite altimetry, and mention scenarios (e.g., West Antarctic Ice Sheet, Greenland Ice Sheet, reduction in formation of Atlantic Intermediate Water) that could lead to acceleration (Alley et al. 2005; Shepherd and Wingham 2007). Line 1318: This citation is not in the references section. Is it yet published? Lines 1362: It would be useful to quote the intensity increases in terms of wind speed in addition to (or instead of) pressure. Wind speed is more meaningful to a broader readership and also avoids the issues of ambient environmental pressure and pressure-wind relationships. Line 1425: The discussion of modeling lapses into far too much detail near this location. From here through line 1466, it needs to be condensed and simplified. Line 1467: The material from here through line 1506 could be reduced to one or two paragraphs. The intensity changes per degree Celsius should be quoted in terms of velocity and pressure fall, or at least consistently in terms of one or the other. Line 1542: Beginning here, the material presented in Subsection 3.3.3 could be condensed. Line 1600: The caption for Table 3 is incorrect. The table provides percentages of 20th Century occurrence, not percentage changes. Line 1636: The summary that extends through should be reduced to a few lines and appended to the paragraph beginning on line 1675, which should be retained,

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” because these paragraphs are to a large extent redundant with the discussion that begins on line 1599. Line 1642: The material beginning here and through line 1655 should be removed because it addresses non-tropical cyclone convection over the oceans. While this connection may be real, it is too speculative for inclusion here. Line 1694: The authors probably mean accumulated rainfall at a locality in the storm’s path. Radar meteorologists use the term “total storm-lifetime precipitation,” but for broad readership it implies a Lagrangian (not Eulerian) concept. Line 1702: A citation to personal communication may be problematic, given the requirements of the prospectus. Line 1725: Subsection 3.3.3.5 is well-written but the scientific support for it is perhaps the weakest subsections of 3.3.3. This committee suggests that this subsection in particular should be greatly condensed. CHAPTER 4 Recommendations for Improving Our Understanding The committee generally concurs with the recommendations of the authors but offers several specific comments to sharpen their impact. From a formatting perspective, the committee recommends that the authors consolidate all recommendations scattered among the chapters into Chapter 4. Stylistically, using both bold face and italics for the entire text of a recommendation statement dilutes its impact. Each numbered recommendation statement (e.g., line 115) should begin with a concise, high-impact sentence (in bold) followed by supporting (plain) text. Each of these first sentences should appear verbatim in the Executive Summary. The committee noted that although the draft document devotes a considerable amount of space to tropical cyclone issues, there are no recommendations regarding this topic. Notwithstanding the recommendation to reduce the amount of discussion within the chapters, the committee suggests that the authors add a recommendation to support research that seeks to improve our understanding of what governs hurricane intensity. Current theory (e.g., the Maximum Potential Intensity) does not adequately explain the correlation between higher sea-surface temperature and hurricane intensity. The mechanisms that govern intensity must be understood better in order to understand better the potential impacts of a warming world on hurricane intensity.

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” Specific Comments and Suggestions: Line 11: The sentence beginning on this line should be deleted. It’s a parenthetical remark without much meaning that detracts from the point of the paragraph. Line 30: Substitute “approximate” for “produce” (also on line 59). Producing a completely homogeneous time series is impossible. Line 57: This is a particularly balanced statement on the ongoing scientific debate and could be included in the Executive Summary. Lines 74: Is there a reference that supports this conclusion (in Chapter 2 or 3 of the draft SAP)? Line 77: The authors may mean 1905 instead of 2005. Line 96: Please see comment on line 74. Line 109: Please see comment on line 74. Line 119: Why is this recommendation applied only to severe thunderstorms and tornadoes? Do observations of other types of events not suffer from inconsistent standards of data collection? Line 123: The authors could add to line 123 “Recover, digitize” before “homogenize.” In the four recommendations surrounding this line, there is nothing about digitization or recovery of data Line 129: Recommendation 3 does not rise to the level of generality as the other recommendations and could be incorporated into recommendation 2, which should emphasize long term analysis of observational data. This combined recommendation should be sufficiently overarching and include a statement to the effect that many improvements are needed in many types of observing systems in order to address the issues set forth in the SAP. In a combined and broader recommendation, extra-tropical Cyclones (ETCs) and extreme wave heights could be mentioned as two types of extremes for which better analysis of long-term data is required. If the authors are raising ETCs and extreme wave heights for a particular reason and they wish for Recommendation 3 to remain separate, then those particular reasons should be further elucidated. Line 133: This recommendation should be broadened to included paleo datsets that can be used to infer time series of extreme hydrologic flows (paleogeological datasets) and droughts (tree rings and other paleohydrologic datasets). These datasets could provide information in addition to time series of temperature and precipitation.

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a Changing Climate” Line 140: The sentence beginning on this line should be a dependent clause of the one that immediately follows. Please provide a reference. Line 144: The acronym is WGCM, not WGNE. Line 167: What is “this” resolution, in kilometers? Line 177: The Hurricane Rainband and Intensity Change Experiment (RAINEX) results show that approximately 2km may be adequate (Houze et al. 2007). Line 184: In this recommendation, replace “with only minor modifications” with “within the same conceptual framework.” Insert “of weather and climate extremes” after predictions. Insert “to enhance spatial and time resolution” before “to recreate.” In addition, this recommendation should be consistent with what appears in the document (see text beginning on line 175 of the Executive Summary) and should further recommend use of, or consideration of, regional climate models. Line 192: A better first sentence could be “We recommend that modeling groups make available data at the highest spatial and temporal resolution from existing simulations of the climate of the 20th and 21st century.” Line 240: This example has been explored adequately in previous chapters. The committee suggests deleting the sentence on lines 240-241, and everything from “threshold” in line 244 through the end of the paragraph on line 247. Line 242: What changes are “these” changes? Line 250: Remove “Considering the rapid pace of climate change,” because this sentence is a statement of fact regardless of the current or future pace of climate change. Line 259: The summary, while it is a straightforward statement of what needs to be done, needs editing to make it more vigorous and emphatic. Also, need to emphasize the need not only for scientists and users to communicate, but in this case, the weather and climate communities need to learn to talk to one another. Line 329: Figure 1 would be improved if the text in the boxes is replaced with single words, such as “Observations”, “Models”, “Understanding”, “Impacts”, “Adaptation and Mitigation”.