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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 2007.
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
36 percent of U.S. methane emissions in 2007, and 73 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 12.5 percent of overall national emissions in
2007.
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.
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