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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product on Temperature Trends in the Lower Atmosphere Major Review Comments Understanding temperature trends in the lower atmosphere over recent decades is important to our overall understanding of the Earth’s climate and its evolution. Such understanding requires both analysis and comparison of changes derived from different observing platforms and comparison of observed changes to those simulated by models. The vertical profile of temperature changes is particularly important because it provides a fingerprint of the mechanisms responsible for past changes. This topic is of substantial interest to the climate research community, especially the key issues of data quality, data-model agreement, and monitoring of future climate change. Thus the Climate Change Science Program (CCSP) synthesis and assessment product on this topic stands to be a timely and useful document, and the committee commends the CCSP for initiative and leadership in this area. Temperature Trends in the Lower Atmosphere: Understanding and Recognizing Differences is a good first draft that covers an appropriate range of issues. It reflects extensive effort and coordination by a team of experienced, knowledgeable authors. By nature, this review of the report is critical, but our comments are intended as constructive input to strengthen the document. This review begins with a summary of the most important, major comments for the document as a whole. Subsequently comments and specific suggestions for each of the chapters of the CCSP synthesis and assessment product, called the Temperature Trends report for brevity, are presented. 1. The Temperature Trends report should include an improved discussion of the motivation for this report, which will increase the report’s effectiveness for a variety of audiences. The committee also suggests more explicit clarification of the context and intended audience for this report. This background should occur in the preface or in an introduction and address the specific scientific issues that motivated the work (which surprisingly go unmentioned in the draft), what has been done previously, and what are the key outstanding issues. This section should be accessible to general educated readers and also scientifically sound. 2. To help the report communicate effectively to an array of audiences, the committee suggests changes to the presentation style within each chapter. The key findings for each chapter, should be brought to the front of that chapter possibly in the form of bulleted highlights, with a one-sentence summary and brief discussion for each key point (similar to the format within Intergovernmental Panel on Climate Change and
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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product on Temperature Trends in the Lower Atmosphere World Meteorological Orginization Assessments). The key points should be based on the detailed discussions within each chapter. The chapters should in some cases include more scientific rigor (as detailed in the chapter reviews that follow) and be aimed at the broad climate science community. One possible strategy for including scientific rigor in the chapters is to use footnotes to describe technical details, as was done in Chapter 5. 3. More explicit discussion of the statistical characterization of uncertainty in trends is needed, with emphasis on several specific topics. a. The conclusions reached in the report are often based on estimates of trends, neglecting uncertainty levels, and many statements on comparisons are inaccurate because of this. Uncertainties are relatively large because of small trend signals, large natural variability, and data records that are relatively short. Conclusions within the report should more accurately reflect the uncertainties inherent in the statistical trend calculations. b. A more thorough discussion of the detailed statistical trend calculations for the various data sets is needed. This discussion might be appropriately placed within an appendix. c. When comparing trend differences between two estimates of the same quantity (e.g., tropospheric temperatures from radiosondes and satellites), it is more appropriate to examine the trend of the difference time series, rather than trends for each time series individually (because the data contain similar overall variability). Specific instances where this is relevant are identified in the chapter reviews that follow. 4. The report would benefit from a more critical evaluation of the trend differences between the University of Alabama, Huntsville (UAH) and Remote Sensing Systems (RSS) satellite microwave data sets. The report presents results from UAH, RSS, and University of Maryland (UMD) satellite data and effectively treats the UAH and RSS results as equal independent estimates with a lesser discussion of UMD results because the latter do not include a diurnal correction and because their newest analysis appeared only recently. An expanded discussion and critical assessment of the UAH and RSS differences is needed in particular, as well as a discussion of the implication of the differences. The committee would like to see a resolution of this difference or, if that is not possible within the current report, some discussion of what sorts of analyses would be required to resolve the differences between the two groups’ trend estimates. Similar scrutiny of the UMD product and its associated uncertainties would be valuable. 5. Satellite trends derived from combining separate microwave channels (the so-called Fu et al. technique, as described in Fu and Johanson [2004 and 2005], and Fu et al. [2004a and b]) should be discussed in more detail. The Fu et al. results are potentially of key importance to the issue of tropospheric temperature trends and should be discussed more thoroughly. Currently, there is minimal discussion, and the data are labeled “controversial”. A more thoughtful and balanced appraisal is necessary, and more recent references should be included. 6. A more thorough discussion of biases in trends derived from historical radiosonde data is needed. The report concludes that stratospheric trends derived from radiosondes are biased so badly as to be unbelievable (a result for which there appears to be a
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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product on Temperature Trends in the Lower Atmosphere community consensus). However, details of the causes of these biases should be clearly described and a stronger explicit case made for discounting the stratospheric results. More critically, it is important to assess if the stratospheric bias problem extends into the middle and upper troposphere, especially in the tropics. The vertical range for which the radiosonde data have least the uncertainties for trend analyses should be more carefully discussed. Finally, the prospects for removing the biases or more effectively utilizing these data should be discussed. 7. The recommendations in Chapter 6 are too non-specific, unprioritized, and largely disconnected from the findings in Chapters 1-5. We suggest that Chapter 6 be reorganized into two parts: a. The first part should take findings from Chapters 1-5 to recommend specific opportunities to improve understanding of vertical temperature trends. These recommendations should focus on understanding remaining uncertainties in existing satellite and radiosonde data sets. b. The second part should focus on future measurement opportunities in the context of the specific goals of the report for reconciling observation and understanding of temperature trends. 8. Changes in the presentation and content of the Executive Summary are needed to make key results more accessible to a wide audience and ensure traceability to the results in Chapters 1-6. A possible strategy is to bring forward key bullet points from each chapter as answers to the six main chapter questions, followed by brief explanatory text, key figures, and implications for understanding within each chapter. The Executive Summary should reflect an appropriate balance of new results and outstanding uncertainties. The first page of the Executive Summary should concisely summarize the key results of the report in a short abstract. One further summary comment is relevant for the CCSP synthesis and assessment reports in general. The committee feels that the current report suffered to some degree from the author group assessing their own work and excluding other independent work. This is evidenced by a lack of critical evaluations on some key data issues and numerous citations to their own work. To the extent possible, the authors should not be put in a position of assessing work where they have a vested interest in the outcome. While it is not reasonable to revise author teams for this report, those preparing future CCSP reports should carefully consider this issue.
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