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
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
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.