Review of the U.S. Climate Change Science Program's Draft Synthesis and Assessment Product 2.4: Trends in Emissions of Ozone Depleting Substances, Ozone Layer Recovery, and Implications for Ultraviolet Radiation Exposure (2007)

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Summary The National Research Council (NRC) empanelled an ad hoc committee to review the U.S. Climate Change Science Program’s (CCSP) draft Synthesis and Assessment Product (SAP) 2.4, Trends in Emissions of Ozone Depleting Substances, Ozone Layer Recovery, and Implications for Ultraviolet Radiation Exposure (draft dated August 20, 2007). The National Oceanic and Atmospheric Administration asked the NRC committee to review SAP 2.4 based on criteria from the NRC’s own review process for NRC reports. This committee has prepared the present report to point out the strengths of SAP 2.4 and to provide some suggestions for improving it. SAP 2.4 is noteworthy as the first-ever attempt to assess the specific contribution of the United States to ozone-depleting substances and ozone recovery. Scientifically objective and policy neutral, the analysis is well-grounded in the international consensus of recent ozone assessments. The SAP authoring team has incorporated more information on ozone-climate interactions than has been done in previous assessments, advancing this approach in the right direction for future assessments. An impressive expert team has been assembled to provide comprehensive coverage of the scientific basis, and the committee acknowledges the unprecedented degree of interagency coordination involved with this effort. In particular, the committee appreciates the major effort required to provide the amount of detail in Chapter 2 of the SAP and to bring the information together. Chapter 2 of the SAP represents an excellent synthesis of the available material and adds value to what is already known. The committee presents five overarching comments to address issues that span more than one section of the SAP. Therefore, a number of important major comments that are technical and specific to a single chapter are not mentioned in the following overarching comments. • The authoring team should revise its discussion of the climate effects of ozone. In Chapter 4 of the draft SAP, the authoring team brings up the issue of the effects of ozone on climate without fully pursuing this topic because it is beyond the scope of this SAP. Nonetheless, the authoring team should include a more complete discussion that clearly states that ozone is part of the climate system and that explains the ozone-climate connection at the process level. Ozone is important to the climate system both through its absorption of solar radiation and as a greenhouse gas through its absorption of infrared radiation. Ozone is not only a greenhouse gas, but is the third most important greenhouse gas in the natural climate system after water vapor and carbon dioxide. • The authoring team should revise its approach in estimating U.S. contributions to production, consumption, and emissions of ozone-depleting substances (ODSs). Specifically, for emissions of chlorofluorocarbons (CFCs) 1 

2 Review of CCSP SAP 2.4 prior to 1975, the U.S. contribution should be estimated as a minimum of one- third and a maximum of two-thirds of global emissions for each type of CFC; post-1975 estimates should also be adjusted accordingly. The justification for this approach begins with the fact that the United States was a major consumer of fluorochemicals before EPA estimates became available in 1985. After the publication of the ozone depletion theory in 1974, there was a discontinuity in the use patterns of CFCs (namely, CFC-11 and CFC-12)—of which 70% of global use was as an aerosol propellant. Although U.S. consumption data are not available prior to 1985 there is information on which to base a more realistic estimate. There was almost a complete elimination of the use of CFCs as a propellant in the United States after 1974, while many other countries continued use of the CFCs as propellants. Therefore, the U.S. fraction of global emissions of CFCs for 1985-1990, as represented in the SAP, is not an accurate representation of the U.S. fraction of emissions prior to 1975, contrary to the approach presented in the draft SAP. The committee’s recommended changes will provide a more realistic range of the U.S. contribution and will affect how U.S. contributions are estimated throughout the SAP. • To address the SAP goal to “describe how these findings relate to human activities, with a particular focus on the United States,” the authoring team should include a discussion of scientific issues that have policy implications. The draft SAP contains policy relevant information that should be stated more clearly as such in Chapter 6 and should be brought forward to the Executive Summary in a policy neutral manner. This proposed approach will make the SAP more useful for policymakers. • To address the SAP goal to “identify where research supported by CCSP agencies is critical for future assessments,” the authoring team should highlight research needs in each chapter where the research is discussed. Suggestions for accomplishing these goals appear in the committee’s specific comments for each SAP chapter. • The authoring team should specifically identify and accommodate the intended audiences of the SAP. The committee suggests some ways of reorganizing and editing the sections of the draft SAP in the following chapters of this report, including suggestions about the use of introductory material and the explanation of technical concepts. The committee commends the SAP authoring team on developing a comprehensive document covering the scientific basis of this important climate issue. For the final SAP document, the authoring team would meet its goals and provide rigorous answers to its key questions by incorporating the committee’s suggestions detailed in this report.