The debate in Congress over whether and how to address possible future climate change is intensifying. Often, the role of the U.S. agriculture sector is invoked in this debate. Agriculture is a source of greenhouse gas (GHG) emissions, which many scientists agree are contributing to observed climate change. Agriculture is also a "sink" for sequestering carbon, which partly offsets these emissions. Carbon sequestration (the capture and storage of carbon) in agricultural soils can be an important component of a climate change mitigation strategy, limiting the release of carbon from the soil to the atmosphere. Congress is considering a range of climate change policy options, including GHG emission reduction programs that would either mandate or authorize a cap-and-trade program to reduce GHG emissions. In general, the current legislative proposals would not require emission reductions in the agriculture and forestry sectors. However, some of these proposals would allow farmers and landowners to generate offsets in support of a cap-and-trade program. Other proposals that Congress has considered would give farmers and landowners a share of available allowances (or credits) for sequestration and/or emission reduction activities. These offsets and allowances could be sold to facilities (e.g., power plants) covered by a cap-and-trade program. Some bills have also specified that the proceeds from auctioned allowances be used to promote certain activities, including farmland conservation and developing bio-energy technologies. This report is organized in three parts. First, it discusses the extent of GHG emissions associated with the U.S. agriculture sector, and cites current and potential estimates for U.S. agricultural soils to sequester carbon and partly offset national GHG emissions. Second, the report describes the types of land management and farm conservation practices that can reduce GHG emissions and/or sequester carbon in agricultural soils, highlighting those practices that are currently promoted under existing voluntary federal agricultural programs. The Appendix provides a summary primer of the key background information presented in these first two sections. Finally, the report provides a brief overview of legislative action within the ongoing energy and climate debate in Congress to enact changes to existing laws and regulations affecting primarily the energy-producing sectors and other sectors thought to be contributing to GHG concentrations. Many of these energy and climate bills include provisions that could involve farmers and landowners by allowing agriculture and forestry-based carbon offsets and allowances and/or by allowing for farm-based biofuels, biomass residues, and dedicated renewable energy crops. More detail on these bills is available in other CRS reports. This report also describes provisions enacted in the 2008 farm bill (Food, Conservation, and Energy Act of 2008, P.L. 110-246 ) that could expand the scope of existing farm and forestry conservation programs in ways that could more broadly encompass certain aspects of these climate change initiatives. This farm bill provision is also invoked in most energy and climate bills in order to establish an advisory committee to oversee implementation of agricultural and forestry carbon offsets. The report concludes with a discussion of some of the types of questions that may be raised regarding the role of the U.S. agriculture sector in the broader climate change debate. This report does not address the potential effects of global climate change on U.S. agricultural production. Such effects may arise because of increased climate variability and incidence of global environmental hazards, such as drought and/or flooding, pests, weeds, and diseases, or temperature and precipitation changes that might cause locational shifts in where and how agricultural crops are produced. This report also does not address how ongoing or anticipated initiatives to promote U.S. bioenergy production may effect efforts to reduce GHG emissions and/or sequester carbon, such as by promoting more intensive feedstock production and by encouraging fewer crop rotations and planting area setbacks, which could both raise emissions and reduce carbon uptake. Agriculture is a both a source and a sink of greenhouse gases, generating emissions that enter the atmosphere and removing carbon dioxide (CO 2 ) from the atmosphere through photosynthesis and storing it in vegetation and soils (a process known as sequestration). Sequestration in farmland soils partially offsets agricultural emissions. Despite this offset, however, the U.S. agriculture sector remains a net source of GHG emissions. Estimates of GHG emissions and sinks for the U.S. agriculture sector presented in this report are the official U.S. estimates of national GHG emissions and carbon uptake, as published annually by the U.S. Environmental Protection Agency (EPA) in its Inventory of U.S. Greenhouse Gas Emissions and Sinks . EPA's Inventory data reflect annual national emissions by sector and fuel, including estimates for the agriculture and forestry sectors. EPA's estimates rely on data and information from the U.S. Department of Agriculture (USDA), the Department of Energy, the Department of Transportation, the Department of Defense, and other federal departments. The EPA-published data are rigorously and openly peer reviewed through formal interagency and public reviews involving federal, state, and local government agencies, as well as private and international organizations. For the agriculture and forestry sectors, USDA publishes a supplement to EPA's Inventory , which builds on much of the same data and information, but in some cases provides a more detailed breakout by individual states and sources. In this CRS report, emissions from agricultural activities are aggregated in terms of carbon dioxide or CO 2 -equivalents, and expressed as million metric tons (MMTCO 2 -Eq.). This aggregation is intended to illustrate agriculture's contribution to national GHG emissions and to contrast emissions against estimates of sequestered carbon. Total GHG emissions from U.S. agricultural activities have averaged 514 MMTCO 2 -Eq. in the past few years ( Table 1 ). As a share of total U.S. GHG emissions, the agriculture sector represents about 7% of all estimated annual emissions. Data dating back to 1990 indicate that emissions associated with the U.S. agriculture activities have been increasing, rising from estimated total emissions of 460 MMTCO 2 -Eq. in 1990. EPA's reported emissions are expressed in terms of CO 2 -equivalent units, and cover both estimated direct emissions and indirect emissions related to electricity use in the sector. These estimates do not cover other types of emissions associated with some agricultural activities, such as carbon monoxide, nitrogen oxides, and volatile organic compounds. Although the agriculture sector is a leading economic sector contributing to national GHG emissions, its share of total emissions is a distant second compared to that of the energy sector. Fossil fuel combustion is the leading source of GHG emissions in the United States (about 80%), with the energy sector generating 85% of annual emissions across all sectors. The types of direct GHG emissions associated with agricultural activities are methane (CH 4 ) and nitrous oxide (N 2 O), which are among the key gases that contribute to GHG emissions. These gases are significant contributors to atmospheric warming and have a greater effect warming than the same mass of CO 2. Agricultural sources of CH 4 emissions mostly occur as part of the natural digestive process of animals and manure management in U.S. livestock operations. Sources of N 2 O emissions are mostly associated with soil management and commercial fertilizer and manure use on U.S. croplands, as well as production of nitrogen-fixing crops. Emissions of N 2 O from agricultural sources account for about two-thirds of all reported agricultural emissions; emissions of CH 4 account for about one-third of all reported emissions. Across all economic sectors, the U.S. agriculture sector is the leading source of N 2 O emissions (about 70%) and a major source of CH 4 emissions (about 25%). These direct emissions account for the bulk (more than 90%) of estimated emissions associated with U.S. agriculture activities, totaling 530 MMTCO 2 -Eq. in 2007. Estimates dating back to 1990 indicate that direct emissions from the U.S. agriculture sector have increased steadily, up from about 430 MMTCO 2 -Eq. in 1990. These estimates do not include emissions associated with on-farm energy use and forestry activities. Sources of CH 4 and N 2 O emissions from agricultural activities are measured across five categories. Agriculture soil management: Nitrous oxide emissions from farmland soils are associated with cropping practices that disturb soils and increase oxidation, which can release emissions into the atmosphere. The types of practices that contribute to emissions releases are fertilization; irrigation; drainage; cultivation/tillage; shifts in land use; application and/or deposition of livestock manure and other organic materials on cropland, pastures, and rangelands; production of nitrogen-fixing crops and forages; retention of crop residues; and cultivation of soils with high organic content. Enteric fermentation: Methane emissions from livestock operations occur as part of the normal digestive process in ruminant animals and are produced by rumen fermentation in metabolism and digestion. The extent of such emissions is often associated with the nutritional content and efficiency of feed utilized by the animal. Higher feed effectiveness is associated with lower emissions. Manure management: Methane and nitrous oxide emissions associated with manure management occur when livestock or poultry manure is stored or treated in systems that promote anaerobic decomposition, such as lagoons, ponds, tanks, or pits. Rice cultivation: Methane emissions from rice fields occur when fields are flooded and aerobic decomposition of organic material gradually depletes the oxygen in the soil and floodwater, causing anaerobic conditions to develop in the soil, which releases methane. Agricultural residue burning: Methane and nitrous oxide emissions are released by burning residues or biomass. The share of GHG emissions for each of these categories is as follows: agriculture soil management (68% of emissions), enteric fermentation (21%), manure management (10%), rice cultivation (1%), and field burning of agricultural residues (less than 1%).