The Earth's surface has warmed by 1.1 o to 1.5 o Fahrenheit since the Industrial Revolution and precipitation has increased over the past century, although some regions have become wetter while some have become drier. Increases in ocean temperatures, altered wind patterns, extreme weather events, melting glaciers and sea ice, and timing of seasons have also been observed. The Intergovernmental Panel on Climate Change (IPCC) in 2007 declared that "[w]arming of the climate system is unequivocal...." and that most of the observed change since the 1970s is likely due to greenhouse gases emitted as a result of human activities. Experts project that if greenhouse gas (GHG) emissions are not abated well below current levels, the Earth's climate will warm further—to levels never experienced by human civilizations. If, and as, the climate moves further from its present state, it will reconfigure the patterns to which current human and ecological systems are adapted, and the risk of abrupt changes will increase. Understanding of the magnitude, causes, and implications of climate change continues to grow. But alongside efforts to further that understanding, a sense of urgency is spurring many international, national, regional, and local policymakers, industry leaders, non-governmental organizations (NGOs), and citizens to mobilize toward more concrete actions. Concern about poorly understood but potentially catastrophic impacts of human-induced climate change drives the impetus to identify, evaluate, and initiate concrete policy actions to address human activities—such as the emissions of GHGs, land use changes, and forestry practices—believed to contribute to climate change. In parallel, growing attention is being given to characterizing and supporting adaptations to changes already observed or expected future changes. Domestic actions to address climate change are moving independently across many fronts. In the 110 th Congress, numerous bills have been proposed to address climate change research and policy; one bill ( S. 2191 ) that would cap and reduce greenhouse gas emissions was reported by the Senate Committee on Environment and Public Works and the Senate agreed on June 2, 2008 to consider the bill (now S. 3036 ). The Supreme Court in 2007 ruled that the Administration must consider regulating greenhouse gases from motor vehicles as air pollutants, and President George W. Bush in 2008 proposed a qualified national goal for U.S. greenhouse gas emissions to peak by 2025 and then decline. States and localities have moved forward with their own plans and regulations in lieu of a strong national framework. Primary concerns are the costs, which could reach trillions of dollars over coming decades, depending on policy choices; the distribution of those costs; and the effectiveness of policies, with the knowledge that U.S. greenhouse gas reductions would achieve success only if sufficient international cooperation can be achieved as well. Debate internationally has revived over how nations may commit to mitigation, adaptation, and technology actions beyond 2012, the end of the current commitment period of the Kyoto Protocol to the United Nations Framework Convention on Climate Change. Negotiations are aiming to produce a post-2012 decision by the end of 2009. U.S. domestic policy development could influence and support international cooperation or introduce impediments, depending on how the interplay is managed. The stakes are potentially high: the effectiveness of mitigating human-induced climate change depends on action by all major emitters, while the costs of delay, deferral or ineffectiveness have been projected by some analysts to reach many trillions of dollars over coming decades. This report does not discuss or analyze current legislative proposals. Rather, it introduces the reader to fundamentals of the climate change issue. Part One summarizes current understandings and controversies concerning the science, economics, international cooperation, and other aspects of the climate change policy problem. Part Two is a brief update on the status of domestic and international policies. Part Three outlines the policy toolbox seen as being available to policymakers to address the challenge as they define the emerging legislative agenda. The Earth's surface has warmed by 1.1 o to 1.5 o Fahrenheit since the Industrial Revolution (measured since 1880), with most warming occurring since the 1970s. Precipitation has increased over the past century, although some regions have become wetter while some have become drier. These results are consistent with scientists' understanding of how heightened greenhouse gas concentrations affect climate regionally. Increases in ocean temperatures, altered wind patterns, extreme weather events, melting glaciers and sea ice, and timing of seasons have also been observed. The Intergovernmental Panel on Climate Change (IPCC) in 2007 declared that "[w]arming of the climate system is unequivocal.... Observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes." Although there is significant natural variability in the Earth's climate, scientists recognized more than a century ago that pollution from human activities could theoretically warm the Earth. GHGs in the Earth's atmosphere allow the Sun's short wave-length radiation to pass through to the Earth's surface. Once the radiation is absorbed by the Earth and re-emitted as longer wave-length radiation, GHGs trap the heat in the atmosphere. This is often called the "greenhouse effect." The natural presence of GHGs (especially water vapor and carbon dioxide) in the atmosphere warms the Earth to habitable temperatures. Studies show that solar variability has contributed some of the observed changes in global temperature, especially early in the 1900s. A few studies conclude that, at most, solar variability has contributed 10% to 40% of the observed change of the 20 th century. Further research would be required to quantify how some suggested influences, for example, galactic cosmic rays, contribute to observed climate changes. Most scientists conclude that a majority of the Earth's warming since the 1970s is due to GHG emissions from human activities, especially use of fossil fuels, clearing of land, and some industrial processes. While scientists agree that GHGs in the atmosphere are responsible for trapping the Sun's radiation and raising the Earth's temperature to current levels, some scientists disagree that projected increases in GHG concentrations would raise temperatures significantly. Beyond water vapor (which is thought not to be directly influenced by humans), the best-understood greenhouse gases include carbon dioxide (CO 2 ); methane (CH 4 ); nitrous oxide (N 2 O); and certain fluorinated compounds, including chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC), hydrofluorocarbons (HFC), perchlorofluorocarbons (PFC), sulfur hexaflouride (SF 6 ), and nitrous trifluoride (NF 3 ). These GHGs remain in the atmosphere for decades to thousands of years and are generally well-mixed around the globe; hence, their warming effects are largely global and persist for decades to millenia. The long atmospheric residence also means a long lag between policies to abate GHG emissions and their full effects on the climate system. When emissions of the long-lived GHGs are greater than their removals by, for example, photosynthesis, the GHGs accumulate in the atmosphere: the GHG concentrations increase. The increases of concentrations of specific GHGs since the Industrial Revolution (measured since about 1850) include CO 2 by more than 33%, from about 280 parts CO 2 per million (ppm) to current levels of over 380 ppm; Methane (CH 4 ) by about 150%, although the rate of increase has declined over the past decades to essentially no growth (but variable year-to-year); Nitrous oxide (N 2 O) by 16%; and essentially all concentrations of CFC, HCFC, HFC, PFC, and SF 6 . Human-related GHG emissions are partly offset by human-related carbon removals and sequestration in growing forests, some agricultural soils, and other reservoirs. Such "sinks" offset about 11% of U.S. GHG emissions in 2005. For the year 2005, CO 2 constituted approximately 74% of the global, human contribution to long-lived GHG emissions; CH 4 was about 16%, and N 2 O was about 8% ( Figure 1 ). Globally in 2005, the top 10 emitting countries contributed about 60% of global GHG emissions, and the top 20 emitting countries contributed about 72% of global GHG emissions. , China probably is now the leading emitter of human-related GHGs, likely having recently surpassed the United States ( Figure 2 ). Most experts expect that Chinese and other developing countries' GHG emissions will continue to grow more rapidly than those of the United States and other already industrialized countries. The climate-related impacts borne by human and ecological systems will depend on the combination of natural climate variability plus human-induced climate changes. Scientists project that, during the 21 st Century, it is very likely that rising GHG emissions, as expected with current trends and policies, and the resulting higher concentrations in the atmosphere, will raise the global average temperature above natural variability by at least 1.5 o Celsius (2.7 o Fahrenheit) above 1990 levels. The estimates considered most likely by many scientists are for GHG-induced temperature increases around 2.5 to 3.2 o C (4.5 to 5.8 o F) by 2100. There is a small but not trivial likelihood that the GHG-induced temperature rise may exceed 5 o C (9 o F) above natural variability by 2100. In context, the global average temperature is estimated currently to be approaching or exceeding the highest level experienced since the emergence of human civilizations. Future climate change may advance smoothly or sporadically, and some regions are likely to experience more fluctuations in temperature, precipitation, and frequency or intensity of extreme events than others. Almost all regions are expected to experience warming; some are projected to become warmer and wetter, while others would become warmer and drier. Sea levels could rise between 7 and 23 inches by 2100, not including the effects of possible accelerated melting of the Greenland or Antarctic ice sheets.