The Earth's temperature is rising as a result of increased atmospheric concentrations of greenhouse gases (see Basic Information on Climate Change from EPA). According to NOAA and NASA data, the Earth's average surface temperature has increased by 1.2-1.4º F over the last 100 years. If greenhouse gases continue to increase, climate models predict that the average temperature at the Earth's surface could increase 3.2-7.2º F above 1990 levels by the end of this century.
The major part of the observed increase in global average temperatures since the mid-20th century is likely due to an increase in greenhouse gas concentrations resulting from human activity, according to the Intergovernmental Panel on Climate Change's Fourth Assessment Report. Human activities across the globe-including fossil fuel use, land cover conversion (deforestation), and agricultural practices-are contributing to the buildup of atmospheric carbon dioxide and other greenhouse gases. Over the past 1 to 2 centuries, land use and land-use change were responsible for roughly 40 percent of human emissions of carbon dioxide ( IPCC Third Assessment Report, Box 3.2).
Within the United States, agriculture accounts for a relatively small share of greenhouse gas emissions, about 7 percent of U.S. greenhouse gas emissions in 2008.
However, agriculture is a major source of two greenhouse gases, methane and nitrous oxide. Methane and nitrous oxide emissions come from livestock production, rice cultivation, and nutrient management practices. Agriculture contributed 34.5 percent of U.S. methane emissions in 2008, and 79.3 percent of nitrous oxide emissions. Direct carbon dioxide emissions from agriculture are small and are not shown as a separate figure, although the contribution is included in the total greenhouse gas chart.
Agriculture and forestry also act as a "sink" by removing carbon dioxide from the atmosphere, thus offsetting carbon dioxide emissions from the burning of fossil fuels in other sectors of the U.S. economy. EPA estimates that agriculture and forest land uses in the U.S. offset close to 13.4 percent of overall national emissions in 2008.
Changes in agricultural practices and land uses can alter the magnitudes of greenhouse gas emissions and mitigation. Shifts to reduced-tillage or no-till practices, changes in crop rotations to include more hay or small grains, and conversion of cropland to pasture or forest may increase carbon uptake and storage by soils and vegetative matter. Reductions in nitrogen fertilizer application, less application in the Fall, use of injection as a method of application, and the use of inhibitors can all reduce emissions of nitrous oxide from cropping. Installation of anaerobic digesters can reduce both methane and nitrous oxide emissions from livestock operations. These and many other procedures are available to farm enterprise operators to reduce agriculture's contribution to net emissions.