Conclusion 1 Climate change is occurring is caused largely by human activities and poses significant risks forand in many cases is already affectinga broad range of human and natural systems

This conclusion is based on a substantial array of scientific evidence, including recent work, and is consistent with the conclusions of recent assessments by the U.S. Global Change Research Program (e.g., USGCRP, 2009a), the Intergovernmental Panel on Climate Change's Fourth Assessment Report (IPCC, 2007a-d), and other assessments of the state of scientific knowledge on climate change. Both our assessment—the details of which can be found in Chapter 2 and Part II (Chapters 6-17) of this report—and these previous assessments place high or very high confidence1 in the following findings:

• Earth is warming. Detailed observations of surface temperature assembled and analyzed by several different research groups show that the planet's average surface temperature was 1.4°F (0.8°C) warmer during the first decade of the 21st century than during the first decade of the 20th century, with the most pronounced warming over the past three decades. These data are corroborated by a variety of independent observations that indicate warming in other parts of the Earth system, including the cryosphere (snow- and ice-covered regions), the lower atmosphere, and the oceans.

• Most of the warming over the last several decades can be attributed to human activities that release carbon dioxide (CO2) and other heat-trapping greenhouse gases (GHGs) into the atmosphere. The burning of fossil fuels—coal, oil, and natural gas—for energy is the single largest human driver of climate change, but agriculture, forest clearing, and certain industrial activities also make significant contributions.

• Natural climate variability leads to year-to-year and decade-to-decade fluctuations in temperature and other climate variables, as well as substantial regional differences, but cannot explain or offset the long-term warming trend.

• Global warming is closely associated with a broad spectrum of other changes, such as increases in the frequency of intense rainfall, decreases in Northern Hemisphere snow cover and Arctice sea ice, warmer and more frequent hot days and nights, rising sea levels, and widespread ocean acidification.

As discussed in Appendix D, high confidence indicates an estimated 8 out of 10 or better chance of a statement being correct, while very high confidence (or a statement than an ourcome is "very likely") indicates a 9 out of 10 or better chance.

• Human-induced climate change and its impacts will continue for many decades, and in some cases for many centuries. Individually and collectively, these changes pose risks for a wide range of human and environmental systems, including freshwater resources, the coastal environment, ecosystems, agriculture, fisheries, human health, and national security, among others.

• The ultimate magnitude of climate change and the severity of its impacts depend strongly on the actions that human societies take to respond to these risks.

Despite an international agreement to stabilize GHG concentrations "at levels that would avoid dangerous anthropogenic interference with the climate system" (UN-FCCC, 1992), global emissions of CO2 and several other GHGs continue to increase. Projections of future climate change, which are based on computer models of how the climate system would respond to different scenarios of future human activities, anticipate an additional warming of 2.0°F to 11.5°F (1.1°C to 6.4°C) over the 21st century. A separate National Research Council (NRC) report, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia (NRC, 2010i), provides an analysis of expected impacts at different magnitudes of future warming.

In general, it is reasonable to expect that the magnitude of future climate change and the severity of its impacts will be larger if actions are not taken to reduce GHG emissions and adapt to its impacts. However, as with all projections of the future, there will always be some uncertainty regarding the details of future climate change. Several factors contribute to this uncertainty:

• Projections of future climate change depend strongly on how human societies decide to produce and use energy and other resources in the decades ahead.

• Human-caused changes in climate overlap with natural climate variability, especially at regional scales.

• Certain Earth system processes—including the carbon cycle, ice sheet dynamics, and cloud and aerosol processes—are not yet completely understood or fully represented in climate models but could potentially have a strong influence on future climate changes.

• Climate change impacts typically play out at local to regional scales, but processes at these scales are not as well represented by models as continental- to global-scale changes.

• The impacts of climate change depend on how climate change interacts with other global and regional environmental changes, including changes in land use, management of natural resources, and emissions of other pollutants.

• The impacts of climate change also depend critically on the vulnerability and adaptive capacity of human and natural systems, which can vary widely in space and time and generally are not as well understood as changes in the physical climate system.

Climate change also poses challenges that set it apart from other risks with which people normally deal. For example, many climate change processes have considerable inertia and long time lags, so it is mainly future generations that will have to deal with the consequences (both positive and negative) of decisions made today. Also, rather than smooth and gradual climate shifts, there is the potential that the Earth system could cross tipping points or thresholds that result in abrupt changes. Some of the greatest risks posed by climate change are associated with these abrupt changes and other climate "surprises" (unexpected changes or impacts), yet the likelihood of such events is not well known. Moreover, there has been comparatively little research on the impacts that might be associated with "extreme" climate change—for example, the impacts that could be expected if global temperatures rise by 10°F (6°C) or more over the next century. Thus, while it seems clear that the Earth's future climate will be unlike the climate that ecosystems and human societies have become accustomed to during the last 10,000 years, the exact magnitude of future climate change and the nature of its impacts will always remain somewhat uncertain.

Decision makers of all types, including businesses, governments, and individual citizens, are beginning to take actions to reduce the risks posed by climate change—including actions to limit its magnitude and actions to adapt to its impacts. Effective management of climate risks will require decision makers to take actions that are flexible and robust, to learn from new knowledge and experience, and to adjust future actions accordingly. The long time lags associated with climate change and the presence of differential vulnerabilities and capacities to respond to climate change likewise represent formidable management challenges. These challenges also have significant implications for the nation's climate science enterprise.

A NEW ERA OF CLIMATE CHANGE RESEARCH

Conclusion 2: The nation needs a comprehensive and integrative climate change science enterprise, one that not only contributes to our fundamental understanding of climate change but also informs and expands America's climate choices.

Research efforts over the past several decades have provided a wealth of information to decision makers about the known and potential risks posed by climate change.

Experts from a diverse range of disciplines have also identified and developed a variety of actions that could be taken to limit the magnitude of future climate change or adapt to its impacts. However, much remains to be learned. Continued investments in scientific research can be expected to improve our understanding of the causes and consequences of climate change. In addition, the nation's research enterprise could potentially play a much larger role in addressing questions of interest to decision makers as they develop, evaluate, and execute plans to respond to climate change. Because decisions always involve value judgments, science cannot prescribe the decisions that should be made. However, scientific research can play a key role by informing decisions and by expanding and improving the portfolio of available options.

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