Climate change is occurring. It is very likely caused by the emission of greenhouse gases from human activities, and poses significant risks for a range of human and natural systems. And these emissions continue to increase, which will result in further change and greater risks. America's Climate Choices makes the case that the environmental, economic, and humanitarian risks posed by climate change indicate a pressing need for substantial action now to limit the magnitude of climate change and to prepare for adapting to its impacts. Although there is some uncertainty about future risk, acting now will reduce the risks posed by climate change and the pressure to make larger, more rapid, and potentially more expensive reductions later. Most actions taken to reduce vulnerability to climate change impacts are common sense investments that will offer protection against natural climate variations and extreme events. In addition, crucial investment decisions made now about equipment and infrastructure can "lock in" commitments to greenhouse gas emissions for decades to come. Finally, while it may be possible to scale back or reverse many responses to climate change, it is difficult or impossible to "undo" climate change, once manifested. Current efforts of local, state, and private-sector actors are important, but not likely to yield progress comparable to what could be achieved with the addition of strong federal policies that establish coherent national goals and incentives, and that promote strong U.S. engagement in international-level response efforts. The inherent complexities and uncertainties of climate change are best met by applying an iterative risk management framework and making efforts to significantly reduce greenhouse gas emissions; prepare for adapting to impacts; invest in scientific research, technology development, and information systems; and facilitate engagement between scientific and technical experts and the many types of stakeholders making America's climate choices.

In the three decades since the U.S. Environmental Protection Agency (EPA) was created, the agency's scientific and technical practices and credibility have been independently assessed many times in reports from the National Research Council (NRC), EPA Science Advisory Board, General Accounting Office, and many other organizations; in congressional oversight and judicial proceedings; and in countless criticisms and lawsuits from stakeholders with interests in particular EPA regulatory decisions. As a previous independent panel put it in the 1992 report Safeguarding the Future: Credible Science, Credible Decisions, EPA's policy and regulatory work receives a great deal of public attention, but the agency's scientific performance typically receives a similar degree of attention only when the scientific basis for a decision is questioned. Thus, strong scientific performance is important not only to enable EPA to make informed and effective decisions, but also to gain credibility and public support for the environmental protection efforts of EPA and the nation. This report is the fourth and final one in a series prepared by two independent expert committees convened by the NRC in response to a request from Congress and to subsequent, related requests from EPA. The Committee on Research Opportunities and Priorities for EPA-the companion committee in this study-was charged to provide an overview of significant emerging environmental issues, identify and prioritize research themes most relevant to understanding and resolving those issues, and consider the role of EPA's research program in the context of research being conducted or supported by other organizations. That committee published an interim report in 1996 and a final report, Building a Foundation for Sound Environmental Decisions, in 1997. The Committee on Research and Peer Review in EPA was charged to evaluate research management and scientific peer-review practices in the agency. The committee published an interim report in 1995 and this final report.