Emissions of the three most important long-lived greenhouse gases (GHG) have increased measurably over the past two centuries. Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) concentrations in the atmosphere have increased by approximately 35%, 155%, and 18%, respectively, since 1750. In the U.S., agriculture accounted for close to 7% of total GHG emissions (7260 Tg CO2 eq.) in 2005. Livestock, poultry, and crop production contributed a total of 481 Tg CO2 eq. to the atmosphere in 2005. This total includes an offset from agricultural soil carbon sequestration of roughly 32 Tg CO2 eq. The primary agricultural sources are N2O emissions from cropped and grazed soils (263 Tg CO2 eq.), CH4 emissions from enteric fermentation (112 Tg CO2 eq.), and CH4 emissions from managed livestock waste (41 Tg CO2 eq.). Forests in the United States contributed a net reduction in atmospheric GHG of approximately 787 Tg CO2 eq. in 2005, which offset total U.S. GHG emissions by approximately 11%. In aggregate, the U.S. agricultural sector (including GHG sources for crop, poultry, and livestock production and GHG removal from the atmosphere via sinks for in) was estimated to be a net sink of 306 Tg CO2 eq. in 2005.
