Attention to environmental issues in the 110 th Congress focused early and heavily on climate change. The shift of control from Republicans to Democrats in the new Congress altered the political dynamic concerning this issue, although it did not result in enacted legislation. Hearings were held by at least 10 committees, and 17 bills to cap emissions of greenhouse gases (GHGs) were introduced. One of the bills, S. 2191 , was reported, May 20, 2008, by the Environment and Public Works Committee, and Senate debate on a modified version ( S. 3036 ) began June 2, 2008. A motion to invoke cloture failed June 6, on a vote of 48-36. On the House side, the Speaker urged quick action on legislation, and established a Select Committee on Energy Independence and Global Warming to highlight the issue, but no bills proceeded to markup. Ten of the 17 GHG cap-and-trade bills would have amended the Clean Air Act, generally establishing a new Title VII to address the issue. (For additional information on climate change legislation, see CRS Report RL33846, Greenhouse Gas Reduction: Cap-and-Trade Bills in the 110 th Congress , by [author name scrubbed], [author name scrubbed], and [author name scrubbed].) Whether or not climate change legislation would amend the Clean Air Act, climate change hearings and markup were among the highest priorities for the committees that have jurisdiction over air issues (principally the Senate Environment and Public Works and House Energy and Commerce Committees). Other clean air issues were not the main focus of attention, but they were addressed, primarily through oversight of Administration actions. This report provides a brief overview of the climate change issue as well as other Clean Air Act issues that were of interest in the 110 th Congress. Climate change (often referred to as global warming) has been of interest to the Congress on some level for more than 30 years. Hearings on the topic occurred as early as 1975, with as many as 250 additional hearings since that time. In 1992, the United States ratified the U.N. Framework Convention on Climate Change (UNFCCC), which established a goal of reducing developed countries' greenhouse gas emissions to 1990 levels by the year 2000. In 1997, the parties to the UNFCCC, as a first step to advance stronger measures, negotiated binding emission reductions for developed countries in the Kyoto Protocol. The United States subsequently rejected the Protocol, focusing instead on research and on voluntary emission reduction programs. Despite these programs, U.S. emissions of greenhouse gases have continued to climb: in 2005, U.S. emissions were 16% higher than in 1990. In recent years, Congress has expressed renewed interest in climate issues for several reasons. Perhaps the most important factor has been the continued strengthening of the science supporting the connection between emissions of greenhouse gases and climate changes, including mounting evidence that glaciers and polar ice caps are shrinking, global average temperatures are rising, and other climate-related phenomena are occurring. (For a summary of the science, see CRS Report RL33849, Climate Change: Science and Policy Implications , by [author name scrubbed].) In response, about two dozen states have entered into regional agreements to address the issue. (For a summary of state actions, see CRS Report RL33812, Climate Change: Action by States to Address Greenhouse Gas Emissions , by [author name scrubbed].) There has also been a shift in attitude on the part of some in industry, prompted in part by the growing patchwork of state-level and foreign requirements. New business coalitions have formed to urge Congress to address the problem, or to influence any legislation that Congress might consider. Congress was already beginning to respond to these changes before the 2006 elections. In the 109 th Congress, the Senate passed a Sense of the Senate resolution that acknowledged a "growing scientific consensus" that human activity is a substantial cause of greenhouse gas accumulation in the atmosphere, causing average temperatures to rise, and called for a mandatory, market-based program to limit greenhouse gas emissions. On a complicated issue such as greenhouse gas limits, the devil is in the details: agreement on general principles does not necessarily presage agreement on detailed legislative proposals. One detailed proposal reached the Senate floor before the 110 th Congress: the McCain-Lieberman bill ( S. 1151 in the 109 th Congress, S. 139 in the 108 th ) would have established a mandatory, market-based greenhouse gas reduction program. It was debated in the 109 th Congress as an amendment to the Energy Policy Act of 2005 ( S.Amdt. 826 ) and defeated by a 38-60 vote; as stand-alone legislation, it was defeated 43-55 in the 108 th Congress. In the 110 th Congress, there was new impetus. In the Senate, the Chairs of both the Environment and Public Works Committee and the Energy and Natural Resources Committee announced their intentions to move legislation; the Environment and Public Works Committee approved S. 2191 , amended, December 5, 2007, by a vote of 11-8. The bill was reported ( S.Rept. 110-337 ) May 20, 2008, and Senate debate on a modified version of the bill ( S. 3036 ) began June 2. A motion to invoke cloture failed, however, June 6, on a vote of 48-36. In the House, the Speaker urged quick action, but markup of legislation did not occur. The Energy and Commerce Committee, which has jurisdiction, held a number of hearings and posted four white papers describing a possible cap-and-trade program on its website, before producing a discussion draft of climate change legislation, October 7, 2008. The same committee has jurisdiction over the related issue of energy policy and focused its efforts in the first session on the passage of landmark energy legislation ( P.L. 110-140 ), which was signed by the President December 19, 2007. There was further attention to climate change bills in the second session of the Congress, but a significant number of questions, both procedural and substantive, remain. The relationship of climate change legislation to the more traditional air pollution programs of the Environmental Protection Agency (EPA) is one such question. In brief, should greenhouse gases (particularly carbon dioxide) be considered air pollutants subject to regulation under the Clean Air Act, or are they more properly considered a side-effect of the use of fossil fuels to produce energy? The answer to this question affects jurisdiction over a climate change program (particularly in the Senate, where both the Energy and Natural Resources Committee and the Environment and Public Works Committee have considered greenhouse gas legislation). It could determine whether EPA, the Department of Energy, or some other agency would administer an enacted climate change program. And it might affect whether states have authority independent of the federal government to control certain greenhouse gas emissions. Over the years, EPA has taken both sides of this issue. Under the Clinton Administration, the agency's General Counsel argued that CO 2 is an air pollutant, and thus could be regulated under the existing authority of the Clean Air Act. The agency did not actually propose such regulation; it simply maintained that it would have the authority to do so if it chose. Under the Bush Administration, a new General Counsel argued the opposite, maintaining that Congress had clearly distinguished CO 2 from other air pollutants and, while authorizing research and data collection under the existing Clean Air Act, had expressly decided not to regulate the pollutant. (For a further discussion of these issues, see CRS Report RL32764, Climate Change Litigation: A Growing Phenomenon , by [author name scrubbed].) The Bush Administration has also intervened in court to argue that controlling CO 2 and other greenhouse gas emissions from automobiles is equivalent to setting fuel economy standards (a regulatory authority that Congress reserved for the federal government), not controlling air pollution (where states have a regulatory role). In its April 2, 2007 decision in Massachusetts v. EPA , the Supreme Court resolved the issue of the Clean Air Act's authority, finding: The Clean Air Act's sweeping definition of "air pollutant" includes " any air pollution agent or combination of such agents, including any physical, chemical ... substance or matter which is emitted into or otherwise enters the ambient air...." ... Carbon dioxide, methane, nitrous oxide, and hydrofluorocarbons are without a doubt "physical [and] chemical ... substances[s] which [are] emitted into ... the ambient air." The statute is unambiguous. Thus, the Court found no doubt that the Clean Air Act gives EPA the authority to regulate greenhouse gases (in this case, from new motor vehicles), although the specifics of such regulation might be subject to agency discretion. (For further discussion of the Court's decision, see CRS Report RS22665, The Supreme Court ' s Climate Change Decision: Massachusetts v. EPA , by [author name scrubbed].) As noted, 10 of the 17 bills introduced in the 110 th Congress to cap greenhouse gas emissions would have amended the Clean Air Act, as would the Energy and Commerce Committee's discussion draft. In order to sidestep the complexities of treating GHGs as traditional pollutants, they generally would have created a new Title VII to establish a separate program for greenhouse gas emissions. In this respect, the bills emulated the 1990 Clean Air Act Amendments, which created separate titles to deal with acid precipitation (Title IV) and stratospheric ozone depletion (Title VI). Most of the bills dubbed "climate change" bills would establish economy-wide programs to reduce greenhouse gas emissions. But recent Congresses, including the current one, have also seen dozens of bills aimed at the emissions of individual sectors, notably electric utilities, cars and trucks, electrical appliances, and commercial or government buildings. Together, these sectors account for the lion's share of energy use and GHG emissions. Electric utilities account for about 40% of U.S. emissions of CO 2 . Transportation (of which the dominant portion is cars and trucks) accounts for about one-third.