Climate change is generally viewed as a global issue, but proposed responses generally require action at the national level. In 1992, the United States ratified the United Nations Framework Convention on Climate Change (UNFCCC), which called on industrialized countries to take the lead in reducing the six primary greenhouse gases to 1990 levels by the year 2000. For more than a decade, a variety of voluntary and regulatory actions have been proposed or undertaken in the United States, including monitoring of power plant carbon dioxide emissions, improved appliance efficiency, and incentives for developing renewable energy sources. However, carbon dioxide emissions have continued to increase. In 2001, President George W. Bush rejected the Kyoto Protocol, which called for legally binding commitments by developed countries to reduce their greenhouse gas emissions. He also rejected the concept of mandatory emissions reductions. Since then, the Administration has focused U.S. climate change policy on voluntary initiatives to reduce the growth in greenhouse gas emissions. In contrast, in 2005, the Senate passed a Sense of the Senate resolution on climate change declaring that Congress should enact legislation establishing a mandatory, market-based program to slow, stop, and reverse the growth of greenhouse gases at a rate and in a manner that "will not significantly harm the United States economy" and "will encourage comparable action" by other nations. A number of congressional proposals to advance programs designed to reduce greenhouse gases have been introduced in the 110 th Congress. These have generally followed one of three tracks. The first is to improve the monitoring of greenhouse gas emissions to provide a basis for research and development and for any potential future reduction scheme. The second is to enact a market-oriented greenhouse gas reduction program along the lines of the trading provisions of the current acid rain reduction program established by the 1990 Clean Air Act Amendments. The third is to enact energy and related programs that would have the added effect of reducing greenhouse gases ; an example would be a requirement that electricity producers generate a portion of their electricity from renewable resources (a renewable portfolio standard). This report focuses on the second category of bills. (For a review of additional climate change related bills, see CRS Report RL34067, Climate Change Legislation in the 110 th Congress , by [author name scrubbed] and [author name scrubbed].) In the 110 th Congress, Members have introduced 12 bills that include provisions to impose or permit some form of market-based controls on emissions of greenhouse gases. General descriptions of those bills follow, beginning with S. 2191 , which was reported, with amendments, on May 20, 2008, by the Senate Committee on Environment and Public Works. The major provisions of the seven Senate bills are compared in Appendix A . The major provisions of the five House bills are compared in Appendix B . S. 2191 , as introduced October 18, 2007, by Senators Lieberman and Warner, would cap greenhouse gas emissions from the electric generation, industrial, and transportation sectors (for facilities that emit more than 10,000 metric tons of carbon dioxide equivalent—mtCO2e). As introduced, the cap is estimated by the sponsors to reduce emissions to 15% below 2005 levels in 2020, declining steadily to 63% below 2005 levels in 2050. The program would be implemented through an expansive allowance trading program to maximize opportunities for cost-effective reductions. Credits obtained from increases in carbon sequestration and acquisition of allowances from foreign sources could be used to comply with 30% of allowance requirements. The bill would also establish a Carbon Market Efficiency Board to observe the allowance market and implement cost-relief measures if necessary. (For recent action on S. 2191 and for modifications to the provisions, see the next section.) S. 3036 , introduced by Senator Boxer on May 20, 2008, is identical to the reported version of S. 2191 , except that S. 3036 contains a budget amendment aimed at making the bill revenue-neutral. This would entail devoting a percentage of auction revenues—increasing from 6.1% in 2012 to 15.99% in 2031 and thereafter—to offset budget deficits that are projected to occur due to the cap-and-trade program. This bill was considered by the Senate the week of June 2, 2008. S. 280 , introduced January 12, 2007, by Senator Lieberman, would cap emissions of the six greenhouse gases specified in the United Nations Framework Convention on Climate Change at reduced levels from the electric generation, transportation, industrial, and commercial sectors—sectors that account for about 85% of U.S. greenhouse gas emissions. The reductions would be implemented in four phases, with an emissions cap in 2012 based on the affected facilities' 2004 emissions (for an entity that has a single unit that emits more than 10,000 metric tons of carbon dioxide equivalent); the cap steadily declines until it is equal to one-third of the facilities' 2004 levels. The program would be implemented through an expansive allowance trading program to maximize opportunities for cost-effective reductions, and credits obtained from increases in carbon sequestration, reductions from non-covered sources, and acquisition of allowances from foreign sources could be used to comply with 30% of reduction requirements. The bill also contains an extensive new infrastructure to encourage innovation and new technologies. S. 309 , introduced January 16, 2007, by Senator Sanders, would cap greenhouse gas emissions on an economy-wide basis beginning in 2010. Beginning in 2020, the country's emissions would be capped at their 1990 levels, and then proceed to decline steadily until they were reduced to 20% of their 1990 levels in the year 2050. EPA has the discretion to employ a market-based allowance trading program or any combination of cost-effective emission reduction strategies. The bill also includes new mandatory greenhouse gas emission standards for vehicles and new powerplants, along with a new energy efficiency performance standard. The bill would establish a renewable portfolio standard (RPS) and a new low-carbon generation requirement and trading program. S. 317 , introduced January 17, 2007, by Senator Feinstein, would cap greenhouse gas emissions from electric generators over 25 megawatts. Beginning in 2011, affected generators would be capped at their 2006 levels, declining to 2001 levels by 2015. After that, the emission cap would decline 1% annually until 2020, when the rate of decline would increase to 1.5%. The allowance trading program includes an allocation scheme that provides for an increasing percentage of all allowances to be auctioned, with 100% auctioning in 2036 and thereafter. The cap-and-trade program allows some of an entity's reduction requirement to be meet with credits obtained from foreign sources and a variety of other activities specified in the bill. S. 485 , introduced February 1, 2007, by Senator Kerry, would cap greenhouse gas emissions on an economy-wide basis beginning in 2010. Beginning in 2020, the country's emissions would be capped at their 1990 levels. After 2020, emissions economy-wide would be reduced 2.5% annually from their previous year's level until 2031, when that percentage would increase to 3.5% through 2050. The allowance trading system includes an allocation scheme that requires an unspecified percentage of allowances to be auctioned. The bill also includes new mandatory greenhouse gas emission standards for vehicles, along with a new energy efficiency performance standard. The bill would establish a renewable portfolio standard (RPS), increase biofuel mandates under the Renewable Fuels Standard, and mandate new infrastructure for biofuels. Finally, the bill expands and extends existing tax incentives for alternative fuels and advanced technology vehicles, and establishes a manufacturer tax credit for advanced technology vehicle investment. S. 1766 , introduced July 11, 2007, by Senator Bingaman, would set emissions targets on most of the country's greenhouse gas emissions. Greenhouse gas emitting activities such as methane emissions from landfills, coal mines, animal waste, and municipal wastewater projects, along with nitrous oxide emissions from agricultural soil management, wastewater treatment, and manure management, are not included under the targets, although credits for use by covered entities are available or may be generated by verified GHG reductions in these areas. Beginning in 2012, covered entities would have emissions targets set at their 2006 levels in 2020. The emissions targets would decline steadily until 2030 when the emission target would be set at the entities' 1990 levels. Compliance can be secured either through an allowance trading program or by paying a safety valve price (called a Technology Accelerator Payment or TAP). Under the trading program, allowances are allocated according to various categories, including covered entities; eligible facilities, such as coal mines and carbon-intensive industries; states; and sequestration activities. Initially, 24% of all allowances are auctioned, a percentage that increases over time. The TAP is set at $12 a metric ton of carbon dioxide equivalent; it increases 5% annually above the rate of inflation. The bill also requires countries that do not take comparable action to control emissions to submit special allowances (or their foreign equivalent) to accompany exports to the United States of any covered greenhouse intensive goods and primary products. H.R. 620 , introduced February 7, 2007, by Representative Olver, is a substantially modified version of S. 280 . Using the same basic structure as S. 280 , the emission caps under H.R. 620 are more stringent. Reductions from affected sectors (electric generation, transportation, industrial, and commercial) would be set at 2004 levels in 2012 and then steadily decline until the cap is equal to about one-fourth of facilities' 2004 levels. Although H.R. 620 permits affected entities to comply with the reduction requirements with credits from foreign sources, sequestration, and reductions from non-covered entities, these credits are limited to 15% of the source's reduction requirement. H.R. 1590 , introduced March 20, 2007, by Representative Waxman, is similar to S. 485 . H.R. 1590 would cap greenhouse gas emissions on an economy-wide basis beginning in 2010. Beginning in 2020, the country's emissions would be capped at their 1990 levels.