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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 Review of the Executive Summary The Executive Summary is intended to present the key findings from the main body of the report, written in a style intelligible to a technically literate lay audience. The most obvious take-home messages and quotable text should be presented in a clear and concise manner. An effective summary is especially important because many readers of the report will carefully read only the Executive Summary, merely browsing the individual chapters. In the committee’s opinion, the draft Executive Summary does not communicate the most important points in an effective manner. It is quite long and inefficient in conveying information, with many restatements of various methodological issues that result in no conclusions. In fact, it tends to read as if it were a chapter itself, not a crisp summary of the key major conclusions, accomplishments, and future challenges in this important topic. Changes in the length, presentation, and content of the Executive Summary are needed to make key results more accessible to a wide audience and to ensure traceability to the results developed in Chapters 1-6. MAJOR COMMENTS 1. A possible strategy to structure the Executive Summary 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 conclusions and outstanding uncertainties. The first page of the Executive Summary should concisely summarize the key results of the report in a short abstract. 2. The first two paragraphs of the Executive Summary say essentially the same thing—that models and observations are now more “consistent”. It is not clear what these statements really mean. A clear statement about what the observations show, independent of model projections, including specifically what is new in this report that has not been presented in previous reports cited would provide clarification. A second conclusion might address how new observations and new model runs affect our perception of whether the models are consistent with observed trends or basic theory. As currently written, the observational work and the modeling work seem muddled together.
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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 3. It appears that some subjectivity is necessary in making the optimistic-sounding statements in the first two paragraphs. Given the very small absolute changes that have been observed over the 20 years of the focus period of 1979-1999 (~ 0.2K) and the larger natural variability that has occurred over that time (~ 0.5K), it seems that the data can neither reject nor confirm the hypothesis that the models are in some sense reliable. The Executive Summary focuses almost exclusively on the 1979-1999 period, a rather short period for which the trends discussed would not be significant at the 5% level, if the significance testing were done in the usual way. The choice to present the results without any statistical significance testing or confidence intervals is highly questionable and ordinarily not allowed in the scientific literature. 4. Some additional support for the conclusions in the first two paragraphs might be gained if the issue of separating the stratospheric and tropospheric signals in the Microwave Sounding Unit (MSU) channels were addressed more directly in the body of the report and carried forward to the Executive Summary. In this regard, the Fu et al. results appear important but are basically ignored in the explicit arguments used to formulate the conclusions. This should be remedied. The Fu et al. work may be central to the issue of measuring and interpreting the vertical profile of temperature change and is no more difficult or controversial than addressing the spurious radiosonde trends or the differences between the Remote Sensing Systems (RSS), University of Alabama, Huntsville (UAH), and University of Alabama (UMD) interpretations of the MSU data. 5. The traceability of the conclusions in the Executive Summary to the detailed arguments presented in the chapters is not entirely clear. In some cases the Executive Summary does not appear consistent with the corresponding chapters where the conclusions should have been developed. A way to clarify the source of the conclusions might be to develop major conclusions in the chapters and then move these statements forward to the Executive Summary. SPECIFIC COMMENTS 1. The Executive Summary should start with a statement about why the reader should care about the subject of the report. 2. The first two paragraphs appear to be the abstract of the document and contain the main takeaway messages. Both paragraphs conclude with statements about the comparison of models with observations. In the second paragraph, it is not clear what consistent means. The observations seem to give qualitatively different conclusions on several key points. Does consistency merely mean that the observations and the model results have overlapping uncertainties? 3. In lines 71-73, the conclusion that the report increases confidence in our understanding of recent climate change seems optimistic and inconsistent with the supporting evidence. The evidence concerning this conclusion needs to be spelled out more clearly in the document and succinctly summarized in the Executive Summary. It must be possible and convenient to trace these conclusions back to arguments that can be evaluated scientifically. 4. Figure 1 is not effective. For example, the color scheme should be changed because the “multi-colored line” is mostly just one shade of blue.
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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 5. Figure 2 is the basis for the major new conclusions summarized in the first two paragraphs of the report; however, it is not developed in detail and appears only in the Executive Summary and not in the supporting chapters. It seems that additional analysis or logic is being applied in the Executive Summary that is not present in the supporting chapters, suggesting a lack of traceability between the major conclusions of the Executive Summary and the supporting chapters. A number of critical decisions were made about which data sets to include and exclude and whether or not to show sampling uncertainty. These decisions are not adequately discussed in the report. Figure 2 deals only with the 20-year period from 1979-1999. This is a rather short period so the sampling errors are large relative to the absolute changes. However, Figure 2 has merit in that it does include error bars that are generally missing from the report’s figures. 6. Figures 2-5, and 7: All the arguments are formulated in terms of the satellite weighting functions. A rather strange nomenclature is developed based on the MSU weighting functions shown in Figure 3, Low-Trop, Trop-UW, Mid-Trop and Low-Strat. The labeling is misleading because Mid-Trop has a significant contribution from the stratosphere, and Low-Strat has a considerable contribution from the troposphere. It is potentially misleading to call these estimates tropospheric and stratospheric as they are a blend of tropospheric and stratospheric trends. It might be more accurate to use the satellite nomenclature T2LT, TFU, T2 and T4. Projecting the radiosonde data onto these weighting functions forces one to combine potentially good lower tropospheric radiosonde data with potentially bad radiosonde data above 200 mb. 7. An important statement in the report is “the climate models simulate greater warming in the troposphere than at the surface which is not apparent in the observations” (lines 67-68). Whether this statement is true depends on whether the results by Fu et al. are correct. As discussed elsewhere, the Fu et al. results should be discussed in the report chapters and then distilled in the Executive Summary. a. The Fu et al. retrieval is mentioned briefly in Chapters 3, 4, and 5, but it is not included in the broader discussion. Given that the Fu et al. results appear in the peer reviewed literature, it is inadequate to dismiss them as “controversial”. b. It is only fair to include the published Fu et al. 850-300 mb temperature trend estimate in the figures presented in Chapter 5 and in the Executive Summary (and to modify the text accordingly). The authors may state their reservations about the Fu et al. method, provided that they distinguish clearly between what is published and what is (as yet) unpublished, and that they incorporate Fu et al.’s published reply to the one published paper (thus far) that is critical of their method (Tett and Thorne, 2004). c. Failure to account for the stratospheric contribution in the comparisons between data and models may compromise the report’s conclusions. This should be addressed in Chapter 5 and also summarized in the Executive Summary. 8. In Figure 4, why were the negative weights for TLow-Trop truncated off the plot at the left edge? 9. Figure 5 should include error bars. 10. lines 123-153 The section on suggests “Motivation for this Report” that the main purpose of the report is to address the single issue of surface versus tropospheric temperature trends over the past 20 years, yet the six questions that were to be addressed
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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 and the main body of the report seem to be a somewhat more general approach to the question of temperature trends. Is the Temperature trends report intended to be a summary and extension of the 2000 NRC report or a more general statement of knowledge about temperature trends? 11. In lines 163-169, it would be helpful to specify exactly which are the new data sets that have lead to new interpretations. This information is not clearly stated anywhere in the Executive Summary. What specifically happened since NRC (2000) and IPCC (2001) and how has this changed perceptions? The two paragraphs on this page should be replaced with simple, direct statements of fact, if possible. 12. In lines 353-359, it seems misleading to use the phrase: “at any one level”. The comparison has not been made for levels, but for very deep layers as specified by the MSU channels. 13. In Section 3.2, “Radiosonde data” no specific information is given about uncertainties in the two radiosonde datasets. 14. Does the statement on lines 426-427 include an assessment of the sampling uncertainty and a statistical confidence level? 15. In lines 557-579, rather, discuss succinctly and in common language the meaning of Figures 8 and 9. The discussion of fingerprinting is not really necessary. Technique description, jargonizing, and philosophizing should be eliminated in favor of straightforward, accurate, descriptions of the significant results or conclusions. 16. In line 586 the statement that not including the indirect effect of aerosols is the most important deficiency of the global model simulations should be better justified. In particular, other uncertainties, such as those associated with cloud or water feedbacks, could be of similar magnitude. 17. Because Figure 6 is repeated in the top half of Figure 8, perhaps Figure 6 can be removed. 18. In lines 620-626 the statistical significance is very important, so it would be helpful to highlight the statistical significant areas in Figures 8 and 9, if they are known. If they are not known, then that is important too. Throughout the Executive Summary, the reader is invited to take the values presented literally, even though they may have very large statistical uncertainty in addition to residual structural uncertainty. 19. In Figure 9, it is somewhat of a misnomer to call this “Mid-Tropospheric Temperature”. It contains a significant contribution from stratospheric trends that increases in magnitude with latitude. Much of the apparent agreement in this figure is simply the result of negative trends in the stratosphere and the increasing fraction of the stratosphere that the weighting function samples as one moves toward the poles. 20. In lines 620-626, are the only trends that are statistically significant the ones above 100 mb, and are they believed to be spurious trends associated with the radiosonde instrument? If so, how are the conclusions about different trends in the surface and troposphere supported? It seems reasonable to suppose that the spurious negative trend in the radiosonde data extends below 100 mb, albeit with decreased magnitude. 21. In lines 639-640, given the uncertainties in both the direct aerosol forcing and the indirect aerosol effects, the report should provide better justification for the conclusion that the aerosol effects have almost certainly been underestimated. 22. Are the statements in lines 646-652 true for the radiosonde era, or just for the satellite era?
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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 23. Section 6 “Improving our understanding” starts with rather general philosophical statements, whose connection to the recommendations that follow is unclear. The recommendations are broad and unspecific, despite the fact that the report raises some very specific problems. Some of the recommendations are not argued clearly elsewhere in the report, in particular Chapter 6. For example, it is not clear that Chapter 6 calls for “efforts to better understand and reconcile differences between climate data records that purport to measure the same variable,” the second of the specific recommendations listed on page 32 of the Temperature Trends report. More specific recommendations that could immediately be acted upon could be to initiate an action to determine whether the RSS-UAH MSU discrepancy can be resolved or to better characterize the instrumental contribution to the radiosonde trends. In addition, more recognition that planning activities have gone on internationally to design an effective global climate monitoring system should be given. 24. It would be useful to have a summary table showing all datasets and models used in this report to avoid long figure captions.
Representative terms from entire chapter: