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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 (SAPs) that address its highest priority research, observation, and decision-support issues. 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 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 focus of SAP 3.3 (see Box 1-1 for document outline), the subject of this document, is the identification of key variables or indices that can provide important information on weather and climate extremes and their socio-economic and environmental impacts. The Product seeks to identify recent changes and trends in these variables, and outline potential future changes. Key variables and parameters discussed include droughts, heavy precipitation events, heat waves, damaging freezes, tropical and extra-tropical cyclone frequency and intensity, ice storms, snow cover, snow depth, hail, and severe thunderstorms. This Product focuses on extreme events across Canada, Mexico, and the United States, including its territories and does not address extreme weather and climate events on a global scale.


In a review of the U.S. CCSP Strategic Plan, the National Research Council (NRC) recommended that SAPs should be produced with independent oversight and review from the wider scientific and stakeholder communities (NRC 2004). As part of its



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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” 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 (SAPs) that address its highest priority research, observation, and decision-support issues. 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 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 focus of SAP 3.3 (see Box 1-1 for document outline), the subject of this document, is the identification of key variables or indices that can provide important information on weather and climate extremes and their socio-economic and environmental impacts. The Product seeks to identify recent changes and trends in these variables, and outline potential future changes. Key variables and parameters discussed include droughts, heavy precipitation events, heat waves, damaging freezes, tropical and extra-tropical cyclone frequency and intensity, ice storms, snow cover, snow depth, hail, and severe thunderstorms. This Product focuses on extreme events across Canada, Mexico, and the United States, including its territories and does not address extreme weather and climate events on a global scale. In a review of the U.S. CCSP Strategic Plan, the National Research Council (NRC) recommended that SAPs should be produced with independent oversight and review from the wider scientific and stakeholder communities (NRC 2004). As part of its

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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” efforts to meet this goal, National Oceanic and Atmospheric Administration (NOAA) requested that the NRC provide an independent review of SAP 3.3. The NRC appointed an ad hoc committee composed of 10 members (Appendix C) to provide this review. The committee’s Statement of Task is included in Appendix D. The committee conducted its work by first carefully reading the draft SAP 3.3 document Weather and Climate Extremes in a Changing Climate (draft dated February 27, 2007). The committee then met with the authors, who provided in-depth presentations of their research and material used to formulate the draft document. During this meeting, the co-chairs of the authoring committee also outlined for the NRC review committee NOAA and CCSP requirements and expectations for SAP 3.3. This present document constitutes the committee’s peer review of SAP 3.3, resulting from its careful study of the draft document and its interactions with those present at the meeting. This review includes the committee’s findings, recommendations, suggestions, and options for the authors to consider in finalizing SAP 3.3. In conducting its review, the committee focused on substantive matters of content and did not proofread the document for grammatical or typographical errors. BOX 1-1 Outline of CCSP Synthesis and Assessment Product 3.3 The main body of the assessment product is presented in four chapters: Chapter 1: Why Weather and Climate Extremes Matter 1.1. Why are extremes important? 1.2. Defining extremes in relation to social, economic, and environmental impacts. 1.3. Measures of weather and climate extremes and their data limitations Chapter 2: Observed Changes of Weather and Climate Extremes 2.1. Observed changes and variations in weather and climate extremes 2.2. Key uncertainties related to measuring specific variations and changes Chapter 3: Do We Understand the Causes of Observed Changes in Extremes and What are the Projected Future Changes? 3.1. What are the physical mechanisms of observed changes in extremes? 3.2. Attributing observed changes to external forcing 3.3. Projected future changes in extremes, their causes, mechanisms, and uncertainties Chapter 4: Recommendations for Improving our Understanding