The impact of human activities on the atmosphere and the accompanying risks of long-term global climate change are by now familiar topics to many people. Although most of the increase in greenhouse gas (GHG) concentrations is due to carbon dioxide (CO2) emissions from fossil fuels, globally about one-third of the total human-induced warming effect due to GHGs comes from agriculture and land-use change. U.S. agricultural emissions account for approximately 8 percent of total U.S. GHG emissions when weighted by their relative contribution to global warming. The agricultural sector has the potential not only to reduce these emissions but also to significantly reduce net U.S. GHG emissions from other sectors. The sector's contribution to achieving GHG reduction goals will depend on economics as well as available technology and the biological and physical capacity of soils to sequester carbon. The level of reductions achieved will, consequently, strongly depend on the policies adopted. In particular, policies are needed to provide incentives that make it profitable for farmers to adopt GHG-mitigation practices and to support needed research. The agricultural sector can reduce its own emissions, offset emissions from other sectors by removing CO2 from the atmosphere (via photosynthesis) and storing the carbon in soils, and reduce emissions in other sectors by displacing fossil fuels with biofuels. Through adoption of agricultural best management practices, U.S. farmers can reduce emissions of nitrous oxide from agricultural soils, methane from livestock production and manure, and CO2 from on-farm energy use. Improved management practices can also increase the uptake and storage of carbon in plants and soil. Every tonne of carbon added to, and stored in, plants or soils removes 3.6 tonnes of CO2 from the atmosphere. Furthermore, biomass from the agricultural sector can be used to produce biofuels, which can substitute for a portion of the fossil fuels currently used for energy. Carbon stocks in agricultural soils are currently increasing by 12 million metric tonnes (MMT) of carbon annually. If farmers widely adopt the best management techniques now available, an estimated 70 to 220 MMT of carbon could be stored in U.S. agricultural soils annually. Together with attainable nitrous oxide and methane reductions, these mitigation options represent 5 to 14 percent of total U.S. GHG emissions. The relevant management technologies and practices can be deployed quickly and at costs that are low relative to many other GHG-reduction options. To achieve maximum results, however, policies must be put in place to promote, and make attractive to farmers, practices that increase soil carbon and efficiently use fertilizers, pesticides, irrigation, and animal feeds. It is also important to ensure funding to improve the measurement and assessment methods for agricultural GHG emissions and reductions, including expansion of the U.S. Department of Agriculture's National Resource Inventory. In particular, this inventory needs to include a network of permanent sites where key management activities and soil attributes are monitored over time. Such sites would provide information vital to helping farmers select the most promising management practices in specific locations. Profitability of management practices varies widely by region, as does the amount of carbon storage attainable. Initial national-level studies suggest that, with moderate incentives (up to $50/tonne of carbon, or $13 per tonne of CO2), up to 70 MMT of carbon per year might be stored on agricultural lands and up to 270 MMT of carbon per year might be stored through converting agricultural land to forests. Mitigation options based on storage of carbon in soils would predominate in the Midwest and Great Plains regions; whereas in the Southeast, agricultural land would tend to be converted to forestland. Information on the costs and supply of GHG reductions from reducing nitrous oxide and methane emissions are very limited, and more studies in these areas are needed. Agriculture can also reduce GHG emissions by providing biofuels - fuels derived from biomass sources such as corn, soybeans, crop residues, trees, and grasses. Substitution of biofuels for fossil fuels has the potential to reduce U.S. GHG emissions significantly and to provide a major portion of transportation fuels. The contribution of biofuels to GHG reductions will be highly dependent on policies, fossil fuel prices, the specific fossil fuels replaced, the technologies used to convert biomass into energy, and per acre yields of energy crops. In a "best-case" scenario, where energy crops are produced on 15 percent of current U.S. agricultural land at four-times current yields, bioenergy could supply a total of 20 exajoules (EJ) - almost one-fifth of the total U.S. year-2004 demand for energy. This corresponds to a 14 to 24 percent reduction of year-2004 U.S. GHG emissions, depending on how the biomass is used. If advanced conversion technologies are not widely deployed, or if yield gains are more modest, GHG reductions would be on the order of 9 to 20 percent. For biofuels to reach their full potential in reducing GHG emissions, long-term, greatly enhanced support for fundamental research is needed. Application of best management practices in agriculture and use of biofuels for GHG mitigation can have substantial co-benefits. Increasing the organic matter content of soils (which accompanies soil carbon storage) improves soil quality and fertility, increases water retention, and reduces erosion. More efficient use of nitrogen can reduce nutrient runoff and improve water quality in both surface and ground waters. Similarly, improving manure management to reduce methane and nitrous oxide emissions is beneficial to water and air quality and reduces odors. Biofuel use, particularly substituting energy crops for imported petroleum for transportation, has important energy security benefits. However, as biofuel use expands, it will be important to ensure that biomass is produced responsibly, taking both environmental and socio-economic impacts into consideration. Although challenges remain, agriculture has much to offer in helping to reduce net GHG emissions to the atmosphere, while at the same time improving the environment and the sustainability of the agricultural sector. Further research and development will result in improved assessments of GHG contributions from agriculture, increases in agriculture's contribution to renewable energy for the nation, better ways to manage lands, and design of more efficient policies. Government policy plays an important role in making best management practices and biofuel production economically attractive, and farmers will adopt best management practices for GHG reduction only if they seem profitable. Perceived risks and availability of information and capital play important roles in perceptions of profitability. Thus, risk reduction, availability of information, and access to capital are some of the key issues that must be addressed through policies. With the right policy framework, U.S. farmers will be important partners in efforts to reduce GHG emissions while reaping multiple co-benefits.
