U.S. Agriculture and Forestry Greenhouse Gas Inventory: 1990–2008

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 38%, 143%, and 18%, respectively, since 1750. In the U.S., agriculture accounted for approximately 6% of total GHG emissions (6,957 Tg CO2 eq. [teragrams of carbon dioxide equivalent]) in 2008. Livestock, grasslands, crop production, and energy use contributed a total of 462 Tg CO2 eq. to the atmosphere in 2008. This total includes an offset from agricultural soil carbon sequestration of roughly 40 Tg CO2 eq. The primary agricultural sources are N2O emissions from cropped and grazed soils (214 Tg CO2 eq.), CH4 emissions from enteric fermentation (141 Tg CO2 eq.), CO2 emissions from energy use (72 Tg CO2 eq.), and CH4 emissions from managed livestock waste (45 Tg CO2 eq.). Forests in the United States contributed a net reduction in atmospheric GHG of approximately 886 Tg CO2 eq. in 2008, which offset total U.S. GHG emissions by approximately 13%. In aggregate, the U.S. agricultural sector (including GHG sources from crop and livestock production, grasslands, energy use and GHG sinks for forests and urban trees) was estimated to be a net sink of 424 Tg CO2 eq. in 2008.