Foreword

• What is the range of natural variability in climate?
• Are concentrations of greenhouse gases and other emissions that contribute to climate change increasing at an accelerating rate, and are different greenhouse gases and other emissions increasing at different rates?
• How long does it take to reduce the buildup of greenhouse gases and other emissions that contribute to climate change?
• What other emissions are contributing factors to climate change (e.g., aerosols, CO, black carbon soot), and what is their relative contribution to climate change?
• Do different greenhouse gases and other emissions have different draw down periods?
• Are greenhouse gases causing climate change?
• Is climate change occurring? If so, how?
• Is human activity the cause of increased concentrations of greenhouse gases and other emissions that contribute to climate change?
• How much of the expected climate change is the consequence of climate feedback processes (e.g., water vapor, clouds, snow packs)?
• By how much will temperatures change over the next 100 years and where?
• What will be the consequences (e.g., extreme weather, health effects) of increases of various magnitudes?
• Has science determined whether there is a "safe" level of concentration of greenhouse gases?
• What are the substantive differences between the IPCC Reports and the Summaries?
• What are the specific areas of science that need to be studied further, in order of priority, to advance our understanding of climate change?

The White House asked for a response "as soon as possible" but no later than early June--less than one month after submitting its formal request.

     The National Academies has a mandate arising from its 1863 charter to respond to government requests when asked. In view of the critical nature of this issue, we agreed to undertake this study and to use our own funds to support it.

     A distinguished committee with broad expertise and diverse perspectives on the scientific issues of climate change was therefore appointed through the National Academies' National Research Council (see Appendix B for biographical information on committee members). In early May, the committee held a conference call to discuss the specific questions and to prepare for its 2-day meeting (May 21-22, 2001) in Irvine, California. The committee reviewed the 14 questions and determined that they represent important issues in climate change science and could serve as a useful framework for addressing the two general questions from the White House.

     For the task of comparing IPCC Reports and Summaries, the committee focused its review on the work of IPCC Working Group I, which dealt with many of the same detailed questions being asked above. The committee decided to address the questions in the context of a brief document that also could serve as a primer for policy makers on climate change science. To aid in the presentation, the questions have been organized into seven sections, with the questions addressed in each section listed in italics at the beginning of that section.

     While traditional procedures for an independent NRC study, including review of the report by independent experts, were followed, it is important to note that tradeoffs were made in order to accommodate the rapid schedule. For example, the report does not provide extensive references to the scientific literature or marshal detailed evidence to support its "answers" to the questions. Rather, the report largely presents the consensus scientific views and judgments of committee members, based on the accumulated knowledge that these individuals have gained--both through their own scholarly efforts and through formal and informal interactions with the world's climate change science community.

     The result is a report that, in my view, provides policy makers with a succinct and balanced overview of what science can currently say about the potential for future climate change, while outlining the uncertainties that remain in our scientific knowledge.

     The report does not make policy recommendations regarding what to do about the potential of global warming. Thus, it does not estimate the potential economic and environmental costs, benefits, and uncertainties regarding various policy responses and future human behaviors. While beyond the charge presented to this committee, scientists and social scientists have the ability to provide assessments of this type as well. Both types of assessments can be helpful to policy makers, who frequently have to weigh tradeoffs and make decisions on important issues, despite the inevitable uncertainties in our scientific understanding concerning particular aspects. Science never has all the answers. But science does provide us with the best available guide to the future, and it is critical that our nation and the world base important policies on the best judgments that science can provide concerning the future consequences of present actions.

     I would especially like to thank the members of this committee and its staff for an incredible effort in producing this important report in such a short period of time. They have sacrificed many personal commitments and worked long weekends to provide the nation with their considered judgments on this critical issue.

Bruce Alberts

President

National Academy of Sciences