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Introduction

The U.S. Climate Change Science Program (CCSP) was established in 2002 to coordinate climate and global change research conducted in the United States. Building upon and incorporating the U.S. Global Change Research Program of the previous decade, the program integrates federal research on climate and global change, as sponsored by 13 federal agencies and overseen by the Office of Science and Technology Policy, the Council on Environmental Quality, the National Economic Council, and the Office of Management and Budget. A primary objective of the CCSP is to provide the best possible scientific information to support public discussion and government and private sector decision making on key climate-related issues.


To help meet this objective, the CCSP is producing a series of synthesis and assessment products that address its highest priority research, observation, and decision-support needs. The CCSP is conducting 21 such activities, covering topics such as the North American carbon budget and implications for the global carbon cycle, coastal elevation and sensitivity to sea-level rise, trends in emissions of ozone-depleting substances and ozone recovery and implications for ultraviolet radiation exposure, and use of observational and model data in decision support and decision making. Each of these documents has been / will be written by a team of authors selected on the basis of their past record of interest and accomplishment in the given topic. A list of the CCSP SAPs is provided in Appendix A.


The National Oceanic and Atmospheric Administration (NOAA) is the lead agency for CCSP Synthesis and Assessment Product (SAP) 1.3 “Reanalyses of Historical Climate Data for Key Atmospheric Features: Implications for Attribution of Causes of Observed Change”. NOAA’s stated purpose for SAP 1.3 is to provide an expert assessment of the capability and limitations of state-of-the-art climate reanalyses to describe past and current climate conditions, and the consequent implications for scientifically interpreting the causes of climate variations and change.



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1 Introduction The U.S. Climate Change Science Program (CCSP) was established in 2002 to coordinate climate and global change research conducted in the United States. Building upon and incorporating the U.S. Global Change Research Program of the previous decade, the program integrates federal research on climate and global change, as sponsored by 13 federal agencies and overseen by the Office of Science and Technology Policy, the Council on Environmental Quality, the National Economic Council, and the Office of Management and Budget. A primary objective of the CCSP is to provide the best possible scientific information to support public discussion and government and private sector decision making on key climate-related issues. To help meet this objective, the CCSP is producing a series of synthesis and assessment products that address its highest priority research, observation, and decision- support needs. The CCSP is conducting 21 such activities, covering topics such as the North American carbon budget and implications for the global carbon cycle, coastal elevation and sensitivity to sea-level rise, trends in emissions of ozone-depleting substances and ozone recovery and implications for ultraviolet radiation exposure, and use of observational and model data in decision support and decision making. Each of these documents has been / will be written by a team of authors selected on the basis of their past record of interest and accomplishment in the given topic. A list of the CCSP SAPs is provided in Appendix A. The National Oceanic and Atmospheric Administration (NOAA) is the lead agency for CCSP Synthesis and Assessment Product (SAP) 1.3 “Reanalyses of Historical Climate Data for Key Atmospheric Features: Implications for Attribution of Causes of Observed Change”. NOAA’s stated purpose for SAP 1.3 is to provide an expert assessment of the capability and limitations of state-of-the-art climate reanalyses to describe past and current climate conditions, and the consequent implications for scientifically interpreting the causes of climate variations and change. 3

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4 Review of CCSP SAP 1.3 The Prospectus (Appendix B) describes the topic, audience, intended use, and questions to be addressed by SAP 1.3, as summarized here: This proposed CCSP report will be in the form of a synthesis and assessment product that (a) summarizes the present status of national and international climate reanalysis efforts, and (b) discusses key research findings on the strengths and limitations of the current reanalysis products for describing and analyzing the causes of climate variations and trends that have occurred during the time period of the reanalysis records (roughly the past half-century). The proposed report will describe how reanalysis products have been used in documenting, integrating, and advancing our knowledge of climate system behavior, as well as in ascertaining significant remaining uncertainties in descriptions and physical understanding of the climate system. By identifying key limitations of the current generation of reanalyses, the report will be useful to policymakers in identifying and understanding the causes for remaining uncertainties, and for climate program managers in developing priorities for future observing, modeling, and analysis systems required to advance national and international efforts to describe and attribute causes of observed climate variations and change. The assessment of the capabilities and limitations of current reanalysis products for different applications will also be of value to users of reanalysis products. The assessment of present uses and limitation of reanalysis products for attribution of causes of observed climate variations and trends will provide a basis for decision makers and policymakers to understand the present level of confidence and uncertainties in describing how the climate system has varied in the recent historical past, and how this has enabled, and in some cases limited, our ability to identify the causes of such variations. The report will also provide useful information to help the scientific community and public to understand the causes of past climate variations, especially for those events that have high societal, economic, or environmental impacts, such as large and prolonged droughts. According to the guidance provided in the prospectus, SAP 1.3 is to be written in a style consistent with major international scientific assessments. To address these purposes and audiences, SAP 1.3 was given 10 key questions to address (see Box 1). In a review of the U.S. CCSP Strategic Plan, the National Research Council (NRC) recommended that synthesis and assessment products should be produced with independent oversight and review from the wider scientific and stakeholder communities (NRC, 2004). To meet this goal, NOAA has requested an independent review of SAP 1.3 by the NRC. The NRC appointed an ad hoc committee composed of 7 members (Appendix C). The committee’s Statement of Task is included in Appendix D. The committee conducted its work by first carefully reading the draft SAP 1.3 report “Re-analyses of Historical Climate Data for Key Atmospheric Features: Implications for Attribution of Causes of Observed Change” (draft dated August 20, 2007). The committee then met with the lead authors to ask questions about the authoring team’s research and formulation of the draft document. During this meeting, the committee also interacted with NOAA personnel, who outlined for the committee

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Introduction 5 their expectations for SAP 1.3. This present document constitutes the committee’s review report, resulting from its careful study of the draft SAP 1.3 document and its interactions with those present at the aforementioned meeting. Herein the committee provides its review findings, with recommendations, suggestions, and options for the authors to consider in revising the draft SAP 1.3. In its review, the committee focused on substantive matters of content and did not exhaustively proofread the document for grammatical or typographical errors. BOX 1-1 Questions to be addressed by CCSP Synthesis and Assessment Product 1.3 According to guidance in the CCSP prospectus outlining the purpose of SAP 1.3, the report will consist of two components. 1. Descriptions of Past Climate Variations and Trends. This section will focus on the strengths and limitations of current reanalysis systems for identifying and describing past climate variations. 2. Attribution of the Causes of Climate Variations and Trends. This section will assess present uses and limitations of reanalysis products for attributing the causes of observed climate variations and trends over North America during the time period (1948 to present) included in present-generation reanalyses. Emphasis will be placed on advances in our understanding of the causes of major climate variations over this region and period subsequent to work included in the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report published in 2001. SAP 1.3 is intended to provide a summary of the present level of scientific confidence and remaining uncertainties in identifying and describing how the climate system has varied over approximately the last half-century. The discussion of limitations of current reanalyses will provide valuable information for developing priorities for data recovery and quality control efforts and future requirements for improving models, data assimilation methods, and observing systems to reduce uncertainties and improve our ability to describe past and ongoing climate variability and change. SAP 1.3 will also provide recommended steps to improve future analyses and reanalyses of the climate system, and discuss how this information can be developed and applied more effectively to increase confidence and reduce uncertainties in interpreting the causes for past and ongoing climate variations and change.

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6 Review of CCSP SAP 1.3 The key questions to be addressed by SAP 1.3 are: 1. What is a climate reanalysis, and what role does reanalysis play within a comprehensive climate observing system? 2. What can reanalysis tell us about climate forcing and the veracity of climate models? 3. What is the capacity of current reanalyses to help us identify and understand major seasonal-to-decadal climate variations, including changes in the frequency and intensity of climate extremes such as droughts? 4. To what extent is there agreement or disagreement between climate trends in surface temperature and precipitation derived from reanalyses and those derived from independent data? 5. What steps would be most useful in reducing spurious trends and other major uncertainties in describing the past behavior of the climate system through reanalysis methods? Specifically, what contributions could be made through improvements in data recovery or quality control, modeling, or data assimilation techniques? 6. What is climate attribution, and what are the scientific methods used for establishing attribution? 7. What is the present understanding of the causes for North American climate trends in annual temperature and precipitation during the reanalysis record? 8. What is the present understanding of causes for seasonal and regional differences in U.S. temperature and precipitation trends over the reanalysis record? 9. What is the nature and cause of apparent rapid climate shifts, having material relevance to North America, over the reanalysis record? 10. What is our present understanding of the causes for high-impact drought events over North America over the reanalysis record?