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Major Comments

In this chapter, the committee provides its major comments on the draft Synthesis and Assessment Product (SAP) document. In some cases, the specific comments the committee provides on the separate sections of the draft SAP (see Chapter 3 of this report) offer detailed suggestions on how to address the major concerns outlined here.


1. The authors have provided a broad and useful assessment of this important issue. The review committee commends the U.S. Climate Change Science Program (CCSP) and the authors of the draft SAP 3.3 for producing a broadly formulated and generally solid assessment of the scientific underpinnings of this most important topic. Indeed, the committee agrees that potential changes in extreme weather and climate events resulting from global climate change have serious socio-economic and environmental implications. In seeking to address this important issue, the authors have provided a document that addresses the goals, objectives, and intended audiences, including scientists, policymakers, resource managers, stakeholders of climate change science, the media, and the general public, as set forth in the document prospectus; all should benefit from the information provided in this Product.


2. The Abstract and Executive Summary are inconsistent with the document content. The Abstract and the Executive Summary are disconnected from the material presented in the four chapters of the document in tone and scope. The Abstract in particular takes an overly alarmist tone. The authors should ensure that the language is tempered to reflect the implications of changes in extreme events only as supported by the scientific material presented elsewhere in the document. The committee understands that the front material should be understandable and readable for a lay audience with a high school education; however the scope of this material falls short of that target. The committee suggests that the technical level could be increased and the readability would remain appropriate.


3. The content is weighted excessively toward tropical cyclones. The committee understands that trends in tropical cyclones (intensity, number of storms, and other characteristics) are an important topic of considerable interest to many audiences; however, in the context of this SAP, they are but one of several types of extreme events with significant socio-economic consequences. Please see specific comments on the relevant sections of the draft document for the committee’s suggestions on how to reduce and consolidate the discussion of tropical cyclones.



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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” 2 Major Comments In this chapter, the committee provides its major comments on the draft Synthesis and Assessment Product (SAP) document. In some cases, the specific comments the committee provides on the separate sections of the draft SAP (see Chapter 3 of this report) offer detailed suggestions on how to address the major concerns outlined here. 1. The authors have provided a broad and useful assessment of this important issue. The review committee commends the U.S. Climate Change Science Program (CCSP) and the authors of the draft SAP 3.3 for producing a broadly formulated and generally solid assessment of the scientific underpinnings of this most important topic. Indeed, the committee agrees that potential changes in extreme weather and climate events resulting from global climate change have serious socio-economic and environmental implications. In seeking to address this important issue, the authors have provided a document that addresses the goals, objectives, and intended audiences, including scientists, policymakers, resource managers, stakeholders of climate change science, the media, and the general public, as set forth in the document prospectus; all should benefit from the information provided in this Product. 2. The Abstract and Executive Summary are inconsistent with the document content. The Abstract and the Executive Summary are disconnected from the material presented in the four chapters of the document in tone and scope. The Abstract in particular takes an overly alarmist tone. The authors should ensure that the language is tempered to reflect the implications of changes in extreme events only as supported by the scientific material presented elsewhere in the document. The committee understands that the front material should be understandable and readable for a lay audience with a high school education; however the scope of this material falls short of that target. The committee suggests that the technical level could be increased and the readability would remain appropriate. 3. The content is weighted excessively toward tropical cyclones. The committee understands that trends in tropical cyclones (intensity, number of storms, and other characteristics) are an important topic of considerable interest to many audiences; however, in the context of this SAP, they are but one of several types of extreme events with significant socio-economic consequences. Please see specific comments on the relevant sections of the draft document for the committee’s suggestions on how to reduce and consolidate the discussion of tropical cyclones.

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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” 4. Some claims of trends in extreme events are insufficiently supported. In some cases trends are inferred and trend lines are drawn on figures when the data do not appear to justify it. Key issues are whether a given time series is long enough to infer or deduce a trend, whether the underlying data are of sufficient homogeneity to draw conclusions, and whether the trend is statistically significant. In the case of the latter, there are many instances where small changes in the start time for the time series produce changes in the magnitude of the trend that are probably non-significant. One notable example of this is the apparent difference in the trend for Atlantic tropical cyclones when the start time is 1880 versus 1900. In this case and others, such a difference may reflect a problem with the trend assessment technique rather than an actual signal. In general, the word “trend” is used too loosely and often interchanged with “variation” or “increase.” These words should be associated with precise statistical definitions. Furthermore, when statements are made, the authors should indicate whether the claim is based on rigorous statistical analysis of a particular dataset (or datasets), expert elicitation, or the informed judgments of the authors. 5. The levels of uncertainty inherent in the trends should be discussed in more detail. The levels of uncertainty associated with trends (both observed and projected) in various types of extreme events should be elaborated upon. For an observed trend in a particular type of extreme event or variable, please discuss the underlying scientific and technical reasons for that uncertainty, and discuss its implications for projected trends in the extreme event or variable in question. When feasible, compare and contrast the issue for a particular variable to the underlying issues for other variables. As an example, consider that it is reasonable to assert that more is known about observed trends in temperature than trends in heavy precipitation, and even less is known about trends in the frequency and/or intensity of tropical cyclones. What are the technical reasons for this and what are the implications for projected trends? 6. Some cited material is not yet published. Some key literature cited in the report was not available to the peer review committee. In many cases the literature in question was cited as “to be submitted.” The committee understands that all literature cited and used as scientific evidence in SAP 3.3 should have reached at least “in press” status by August 2007. The authors of the SAP should ensure that all cited works are publicly available before the public release of the SAP. The committee further recommends that the authors use caution in drawing too heavily on papers and information that are not yet scientifically mature. The authors should minimize reliance on “grey literature” and non-refereed works. 7. The discussion of drought should be strengthened. The discussion of drought is not consistent among the chapters and sometimes contradictory (in terms of observed trends). In some cases apparent trends for particular geographic regions are used to make statements on broader geographic trends that are not justified. This discussion could be strengthened by including a figure for precipitation analogous to Figure 2 in Chapter 1. Notwithstanding the discussion of observed trends and projections, the background information on droughts should better define the different

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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” types of drought (e.g., meteorological, hydrological, agricultural, etc…) and the uncertainties associated in quantifying drought severity using the Palmer Drought Severity Index (PDSI) and other indices. We note that PDSI is the only index discussed in the draft document. Furthermore, the document should address the uncertainty associated with climate model design (e.g., the model treatment of land-surface processes and parameters) and its impact on model representation of drought conditions. 8. The discussion of ecological impacts should be expanded. The draft briefly describes ecological impacts. It would be helpful to expand on these and carry them through with brief pointers, elsewhere in the document, particularly in discussions of future impacts in Chapter 3. The committee recognizes CCSP SAP Goal 4 addresses the subject of impacts in detail, but those SAPs address impacts of climate change in the broader sense and do not necessarily address per se the impacts of extreme events. SAP 3.3 should acknowledge the impacts addressed in other SAPs and incorporate some by example. Impacts to consider by example include wildfires and heat stress, which are “compound impacts” of temperature and moisture extremes. 9. The continuity and cohesion among the chapters needs improvement. The individual chapters read as stand-alone documents. They should be connected with some common themes and examples that are carried through (not just in terms of impacts as discussed in 8 above). There is a considerable amount of overlap and repetition, particularly between Chapters 2 and 3. Each chapter authorship team should coordinate with the other three teams to ensure that redundancies are eliminated. 10. The recommendations are not properly organized. Recommendations, and statements that are in effect recommendations but are not labeled as such, are scattered among the chapters. Some of these are repeated in Chapter 4 and some are not. Recommendations should be combined and contained only in Chapter 4 and a highlighted (bold) short sentence corresponding to each recommendation should appear in the Executive Summary. 11. The preface should not contain scientific or technical material. The Preface should contain “big-picture” information on the CCSP and the Synthesis and Assessment Products, and some background on the process and motivation pertaining to SAP 3.3. From a technical writing perspective, a preface is the appropriate location for framing and context; it should not contain technical information that is presented elsewhere in the report. Please see Chapter 3 of this review for more specific suggestions for content. 12. The content is limited in geographical scope. The rationale behind the minimal coverage of regions outside the North American mainland (e.g. “Hawaii, Caribbean, and U.S. Pacific Islands”) should be explained. The SAP focuses heavily on North America, but the prospectus and the title suggest some appreciable coverage of other geographic locations.