Introduction

This chapter provides an overview of the potential impacts of climate change affecting the United States. The chapter also summarizes current measures and future adaptation and response options that are designed to increase resilience to climate variations and reduce vulnerability to climate change. The chapter is not intended to serve as a separate assessment in and of itself, but rather is drawn largely from analyses prepared for the U.S. National and IPCC Assessments, where more detailed consideration and specific references to the literature can be found (see NAST 2000, 2001 and IPCC 2001d, including the review of these results presented in NRC 2001a and IPCC 2001a).

As indicated by the findings presented here, considerable scientific progress has been made in gaining an understanding of the potential consequences of climate change. At the same time, considerable uncertainties remain because the actual impacts will depend on how emissions change, how the climate responds at global to regional scales, how societies and supporting technologies evolve, how the environment and society are affected, and how much ingenuity and commitment societies show in responding to the potential impacts. While the range of possible outcomes is very broad, all projections prepared by the IPCC (IPCC 2001d) indicate that the anthropogenic contribution to global climate change will be greater during the 21st century than during the 20th century. Although the extents of climate change and its impacts nationally and regionally remain uncertain, it is generally possible to undertake "if this, then that" types of analyses. Such analyses can then be used to identify plausible impacts resulting from projected changes in climate and, in some cases, to evaluate the relative plausibility of various outcomes.

Clear and careful presentation of uncertainties is also important. Because the information is being provided to policymakers and because the limited scientific understanding of the processes involved generally precludes a fully quantitative analysis, extensive consideration led both the IPCC and the National Assessment experts to express their findings in terms of the relative likelihood of an outcome's occurring. To integrate the wide variety of information and to differentiate more likely from less likely outcomes, a common lexicon was developed to express the considered judgment of the National Assessment experts about the relative likelihood of the results. An advantage of this approach is that it moves beyond the vagueness of ill-defined terms, such as may or might, which allow an interpretation of the likelihood of an outcome's occurring to range from, for example, 1 to 99 percent, and so provide little basis for differentiating the most plausible from the least plausible outcomes.

In this chapter, which uses a lexicon similar to that developed for the National Assessment, the term possible is intended to indicate there is a finite likelihood of occurrence of a potential consequence, the term likely is used to indicate that the suggested impact is more plausible than other outcomes, and the term very likely is used to indicate that an outcome is much more plausible than other outcomes. Although the degree of scientific understanding regarding most types of outcomes is not complete, the judgments included here have been based on an evaluation of the consistency and extent of available scientific studies (e.g., field experiments, model simulations), historical trends, physical and biological relationships, and the expert judgment of highly qualified scientists actively engaged in relevant research (see NAST 2000, 2001). Because such judgments necessarily have a subjective component, the indications of relative likelihood may change as additional information is developed or as new approaches to adaptation are recognized.

Because this chapter is an overview, it generally focuses on types of outcomes that are at least considered likely, leaving discussion of the consequences of potential outcomes with lower likelihood to the more extensive scientific and assessment literature. However, it is important to recognize that there are likely to be unanticipated impacts of climate change that occur. Such "surprises," positive or negative, may stem from either

1. unforeseen changes in the physical climate system, such as major alterations in ocean circulation, cloud distribution, or storms; or
2. unpredicted biological consequences of these physical climate changes, such as pest outbreaks.

 For this reason, the set of suggested consequences presented here should not be considered comprehensive. In addition, unexpected social or economic changes, including major changes in wealth, technology, or political priorities, could affect society's ability to respond to climate change.

This chapter first describes the weather and climate context for the analysis of impacts, and then provides a summary of the types of consequences that are considered plausible across a range of sectors and regions. The chapter then concludes with a brief summary of actions being taken at the national level to learn more about the potential consequences of climate change and to encourage steps to reduce vulnerability and increase resilience to its impacts. Although the federal government can support research that expands understanding and the available set of options and that provides information about the potential consequences of climate change and viable response strategies, many of the adaptation measures are likely to be implemented at state and local levels and by the private sector. For these reasons and because of identified uncertainties, the results presented should not be viewed as definitive. Nonetheless, the more plausible types of consequences and impacts resulting from climate change and the types of steps that might be taken to reduce vulnerability and increase adaptation to climate variations and change are identified.