2
Key Issues

Reanalysis is an important and relatively new method in climate science that can provide a context in which to put new climate observations. The reanalysis technique integrates a diverse array of observations within a physical model of the climate system (or of one of its components, such as the atmosphere, ocean, or land surface) to describe past conditions over an extended time period, typically several decades. An important goal of reanalysis is to provide comprehensive, consistent long-term climate data sets that are reliable on hourly to decadal and longer time scales. Another important aspect of reanalysis is that it is three dimensional through at least the depth of the atmosphere (Kistler et al., 2001). SAP 1.3 deals primarily with global reanalyses, however, regional reanalyses are becoming ever more common (e.g., the North American Regional Reanalysis http://www.emc.ncep.noaa.gov/mmb/rreanl/).


By comparing recent surface observations with the corresponding atmospheric dynamical fields estimated by reanalysis we can begin to assess whether current conditions are unusual, whether they are part of a long-term trend, or a result of climate variability that may be expected to reverse over months, seasons, or years. In addition, reanalysis can help determine whether similar or related changes are occurring in other parts of the globe and help identify the processes and mechanisms that can explain current conditions, and how are they similar to, or different from, what has occurred in the past.


This report addresses the strengths and limitations of current reanalysis products in documenting, integrating, and advancing our knowledge of the climate system. It also assesses current capabilities and remaining uncertainties in our ability to attribute causes for climate variations and trends over North America during the reanalysis period (1948-present), and discusses the uses, limits and opportunities for improvement of reanalysis data applied for this purpose.



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2 Key Issues Reanalysis is an important and relatively new method in climate science that can provide a context in which to put new climate observations. The reanalysis technique integrates a diverse array of observations within a physical model of the climate system (or of one of its components, such as the atmosphere, ocean, or land surface) to describe past conditions over an extended time period, typically several decades. An important goal of reanalysis is to provide comprehensive, consistent long-term climate data sets that are reliable on hourly to decadal and longer time scales. Another important aspect of reanalysis is that it is three dimensional through at least the depth of the atmosphere (Kistler et al., 2001). SAP 1.3 deals primarily with global reanalyses, however, regional reanalyses are becoming ever more common (e.g., the North American Regional Reanalysis http://www.emc.ncep.noaa.gov/mmb/rreanl/). By comparing recent surface observations with the corresponding atmospheric dynamical fields estimated by reanalysis we can begin to assess whether current conditions are unusual, whether they are part of a long-term trend, or a result of climate variability that may be expected to reverse over months, seasons, or years. In addition, reanalysis can help determine whether similar or related changes are occurring in other parts of the globe and help identify the processes and mechanisms that can explain current conditions, and how are they similar to, or different from, what has occurred in the past. This report addresses the strengths and limitations of current reanalysis products in documenting, integrating, and advancing our knowledge of the climate system. It also assesses current capabilities and remaining uncertainties in our ability to attribute causes for climate variations and trends over North America during the reanalysis period (1948- present), and discusses the uses, limits and opportunities for improvement of reanalysis data applied for this purpose. 7

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8 Review of CCSP SAP 1.3 While current reanalysis products provide a foundation for climate research, reanalysis data are now used in an increasing range of commercial and business applications, such as energy (supply/demand analysis, assessing locations for wind power generation), agriculture, water resource management, insurance and reinsurance. Thus the Climate Change Science Program’s (CCSP) Synthesis and Assessment Product (SAP) 1.3 will potentially be very beneficial to all stakeholders of climate change science. The committee commends CCSP and the National Oceanic and Atmospheric Administration (NOAA) for emphasizing the need to address this important topic. This chapter outlines the major issues that, from the point of view of the review committee, the authors should strongly consider addressing in the revised version of SAP 1.3. In some cases, findings are simply noted without explicit recommendations. In other cases, the committee provides either a direct recommendation or alternatives for the authors to consider as they address the review findings. In subsequent chapters of this report, the committee provides further overarching thoughts on the draft document and findings and recommendations specific to individual chapters of the draft. Comments regarding key issues follow. The committee finds that in general, the authors nicely summarize the capability of current reanalysis for quantifying climate variations and long-term trends. SAP 1.3 appears to be scientifically objective and policy neutral. In cases where the results of SAP 1.3 are compared with existing peer-reviewed literature, the SAP 1.3 results are consistent with existing data. However, a significant fraction of the SAP 1.3 results is not compared with peer-reviewed literature and the authors are encouraged to compare their results with the peer-reviewed literature whenever possible. The authors correctly point out that the strength of current atmospheric reanalysis is its global coverage and complete description of atmospheric states, and that the reanalysis is best used for quantifying atmospheric processes at synoptic to decadal time scales (including describing the El Niño-Southern Oscillation (ENSO) and the spatial patterns of atmospheric modes such as the Pacific-North America Oscillation (PNA), the North Atlantic Oscillation (NAO)) , but not appropriate for longer-term changes (for example trends in precipitation, atmospheric water vapor or even surface air temperature). The authors rightly state that substantial efforts are needed to correct biases and discontinuities in various observational data before they are assimilated into reanalysis. The committee commends the authors for clearly stating their goals and their intended audience and for their fidelity in following the prospectus. The primary issues identified by the committee focus on the effectiveness of the document’s presentation, level of technicality and organization, as well as accessibility of the document to its target audiences. The committee identified the following issues and offers suggestions on how to improve the document. 1. The title and contents of the document are not entirely consistent. The title of the present draft, “Reanalyses of Historical Climate Data for Key Atmospheric Features: Implications for Attribution of Causes of Observed Change,” correctly suggests that reanalysis data is useful for attributing the causes of observed climate change. This link, however, is often missing and attribution is not tied to reanalysis directly. All

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Key Issues 9 elements of the document (title, table of contents, chapters and recommendation) fail to link reanalysis to attribution to various degrees. In particular, there is a mismatch between chapters 2 and 3. The document would greatly benefit from tying these two chapters together more strongly. This could be accomplished by the following. The document should stress that climate science needs a more quantitative way to bring multiple lines of evidence together, and that reanalysis provides an important means to do so. This point should be the theme throughout the report and should be evident in the abstract, preface, summary, introduction and in each chapter and recommendation, as appropriate. Highlighting this point often will help emphasize the message of the report. The document should clearly explain why reanalysis is needed and how reanalysis is connected to attribution. This discussion should emphasize what aspects of this work, such as diagnosing and assessing climate model output, both in terms of model veracity and aspect of simulated climate change and variability, could not have been done without reanalysis. The document should highlight the difficulties in connecting attribution to reanalysis and what data and steps are needed to overcome these challenges. For example, the attempt at attribution with reanalysis surface variables appears to indicate that model based reanalysis does not aid in the attribution problem. This may indicate a real need to better assimilate the available surface data in order to address the surface attribution problem. Other challenges include assimilation of analyzed surface data such as gridded temperature and precipitation data into the reanalysis, minimizing spurious changes in the reanalysis data due to changes in input data, and improved data quality control for raw observations assimilated into the reanalysis. 2. The document is not accessible to all intended audiences. The committee finds that the draft is written largely for a technical audience. The intended audiences as outlined in the prospectus include those people engaged in scientific research, the media, policymakers, and the general public. Policy and decision-makers in the public sector (e.g., congressional staff) need to understand the implications of reanalysis and its role in attribution, in contrast to the research science community, who may be more interested in the actual outcomes. The draft would benefit from including more information for an audience of non-technical readers, particularly information that could be used as guidelines for effective communication techniques. In general, the draft would greatly benefit from revisions to highlight the essential points of the document. Some specific suggestions follow. The committee was informed by the authors that a major goal for this document was to provide education for both a general audience and a scientific audience. The education function of this document could be improved by explicitly stating why reanalysis is needed in plain language. The document should clearly state how climate science is done and how reanalysis fits in a more broad perspective. Risk type language may be a better

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10 Review of CCSP SAP 1.3 way to attribute causes of climate change and the committee suggests using a probabilistic approach to explain uncertainties. This could be illustrated in call out boxes illustrating these scientific concepts, which may improve communication without interrupting the flow of the document. The committee finds that the medical analogy was not helpful and in some ways misleading, and suggests that a concrete example to illustrate how attribution is done in a step by step manner should be substituted. This approach would be beneficial to policy makers and to scientific program managers. Although this is primarily an agency-written government report, a key mission of many federal agencies is to provide education and outreach for their programs. At present, the document does not clearly mention the academic community, which is a key audience and principal contributor to reanalysis work. An example of explaining the benefits of using reanalysis and attribution in conjunction is highlighted in Recommendation A2 (An important focus for future attribution research should be on developing capabilities for better explaining climate conditions at regional to local scales, including the roles of changes in land cover/use and aerosols, as well as changes in greenhouse gases, sea surface temperatures, and other forcing factors.) This recommendation illustrates the benefits of combining reanalysis and attribution for societal benefit and should be mentioned in the introduction. The organization of the report could be improved by a stronger tie between chapters 2 and 3. It is important to explain how these chapters fit together and why the reanalysis period was chosen. The key findings are summarized at the beginning of Chapters 2 and 3, and also in the Executive Summary, which helps the flow of information, but the summaries are not consistent. The consistency needs to be improved, bearing in mind that many readers will consult the individual chapter summaries for additional detail after the executive summary. The main points of the chapters should be highlighted in these summaries. The committee finds that the lack of a non-technical executive summary hinders the document’s accessibility to the audiences named in the prospectus. A concise and readable summary of the document, including key findings and recommendations, would enable all audiences -- producers of synthesis and assessment products, scientific researchers, decision-makers, media, and the public -- to glean the main points and to locate further information that may be of interest to them. The document should include a short executive summary for non-technical readers, such as congressional staff, local and regional governmental decision makers. The summary should be clearly labeled as such (non-technical or other indication) and not be merely descriptive, but informative on the main points of the document. The summary should use

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Key Issues 11 plain language to describe the goals of the report, the principal findings and why reanalysis is important to attribution, as well as to highlight the strengths and limitations of reanalysis. It may also be beneficial to explain to the lay reader that it is the limitations of reanalysis that drive future research directions. An alternate approach could be to add a box explaining the differences between these topics and explaining how climate science is done within the current executive summary. A technical summary written for an informed general scientific audience should be included. This should be written using clearly defined technical language (without acronyms) so that the general scientific community, not just atmospheric scientists, can understand the goals, findings and relevance of the study. If some chapters are to use technical language, the introduction chapter should contain a section with advice on “How to read this document” – a paragraph that describes the intent of each chapter and its target audience. For instance, the paragraph may state: Chapter 1 provides an introduction to the study and relevant findings from previous studies and is intended to provide all audiences with a general overview. Chapters 2 and 3 provide detailed technical information about specific models, model runs and trends and are intended primarily for the scientific community. Chapter 4, which is intended for all audiences, provides a summary of the major findings and identifies new opportunities for future research. 3. Introductory material is lacking. The draft would be improved if the introduction section (either the preface or Chapter 1) provided a clear framework and context for the rest of the document. At present, the scope of and motivation for the study are not well explained. The authors should stress that the topic is directly related to some of the most basic and frequently asked questions by the public and decision-makers. For example: What do we know about past climate? What are our uncertainties? What do we know about the causes of climate variations and change? What are our uncertainties on causes? Reanalysis addresses science challenges at the heart of CCSP Goal 1: “Improve knowledge of the Earth’s past and present climate environment, including its natural variability, and improve understanding of the causes of observed variability and change.” This topic is directly relevant to core questions on our current scientific abilities to detect and attribute causes of climate variability and change. Some specific suggestions follow. At present the public perception of reanalysis is woefully inadequate. The introduction would benefit from a discussion of the usefulness of reanalysis in understanding climate variation, which is a compelling national need that is of vital interest to the nation and society. The introduction should educate the reader about how reanalysis fits into climate science and include a general description of how climate science is done and how the models, observations, and theories are related to the ultimate goal of reanalysis. The introduction should ultimately highlight the benefits and limitations of reanalysis and

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12 Review of CCSP SAP 1.3 should provide information to all readers, including the public, congressional staff, science program managers and, hopefully, a new generation of students. In the current draft of the document, the transition to technical material is far too abrupt. Specific ways to improve the introduction follow. The introduction should explain what analysis is, distinguish reanalysis from climate data analysis, and explain how reanalysis fills in a data and knowledge gap. The strengths and limitations of the reanalysis approach should be stated and the unique contributions that reanalysis makes over existing climate analyses should be highlighted. It should stress that reanalysis may in some cases be the best tool for obtaining information about critical aspects of the climate system. In addition, the document should clearly state what is needed to be done to make reanalysis more powerful. The introduction needs to clearly explain the motivation for ad hoc choices such as why the German rainfall data was used in Chapter 2. The introduction should also clearly convey the limitations of current reanalysis and that reanalysis could be used for future studies provided specific improvements were made. It is also important to mention that reanalysis has the potential to be an effective way of assessing long-term climate change. The introduction should also highlight that because society needs to have the clearest picture of climate, reanalysis activities should continue in order to provide a way to evaluate the majority of climate information. In addition, there is a need to explain why there are no more effective options for such evaluation. The introduction (either the preface or Chapter 1) should outline the charge to the authors as they perceived it, and clearly define the goals and objectives of the document. The foreword or introduction should also state explicitly what the document does not address. 4. Details about the methods, data sources and assumptions used are lacking. Specific details about the methods, data and assumptions used in this assessment need to be provided within the document to enable a meaningful interpretation of the data, especially those that are not compared to the peer-reviewed literature. The committee suggests that the report be revised to rely more on the published literature as opposed to the authors’ original research. At present there is no discussion about how statistical significance was determined. The statistical significance of certain trends is discussed and judgments are made about the relative significance, yet there is no description of how this was calculated. This is particularly important for the unpublished results calculated by the authors. This information could be provided in an appendix and should clearly describe the statistical approaches used to determine the relative significance of trends and explain the rationale behind why and how judgments were made. The committee suggests that technical details regarding the previously-unpublished calculations and syntheses of climate model output in Chapter 3 either be included in the text of Chapter 3 or in a separate appendix to

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Key Issues 13 enable an interested and motivated reader to draw his/her own conclusions. More general information about the methods, data sources and assumptions used could be included in the introduction (see reviews of specific chapters for suggestions). 5. The document heavily relies on original research and does not include sufficient peer-reviewed literature. Although the committee finds that the results presented in the document are consistent with scientific literature and that the document appears to be objective, the document relies too heavily on original, non-peer-reviewed work. The authors should include more discussion of findings in the scientific literature and how the unpublished findings compare with previously published findings (especially for sections 3.1 through 3.3). While the authors provide compelling evidence supporting their recommendations, greater lengths are needed to distinguish their work from peer-reviewed literature. At present, it is difficult to determine how much of the document is collective opinioned. The authors should explicitly distinguish the findings from the peer-reviewed literature from those derived from original work. The report should give precedence to peer-review literature whenever possible. 6. The document and its language should be clarified. The committee notes that the document lacks a suitable table of contents and that section and subsection headings are generally too wordy. At the level of language, the phrasing regarding attribution is awkward. It is more correct to say that one attributes climate variations to particular causes, not that one attributes causes to climate variations. Also, one should speak of causes of variations, not causes for variations.