Greenhouse Gas Emissions and Sinks in U.S. Agriculture

Greenhouse Gas Emissions and Sinks in U.S. Agriculture

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September 17, 2018
Greenhouse Gas Emissions and Sinks in U.S. Agriculture
Agricultural and land use activities have played a central
tracked emissions and sinks by source and economic sector
role in the broader debate regarding energy and climate
since 1990. These estimates are reported annually in EPA’s
policy options in the United States and abroad. Most U.S.
Inventory of U.S. Greenhouse Gas Emissions and Sinks
climate-related proposals would not require emission
(available at https://www.epa.gov/ghgemissions). For the
reductions in the agricultural sectors but rather would
U.S. agricultural sector, EPA’s Inventory estimates are
incentivize reductions through voluntary measures. For
compiled, in part, using information from other federal
example, previous energy and climate legislation
agencies, including the U.S. Department of Agriculture
considered by Congress would have established a carbon
(USDA). These data are expressed in terms of CO2-
offset program for domestic farm- and land-based carbon
equivalents and aggregated to millions of metric tons
storage activities and provided for tradeable allowances
(MMTCO2-Eq.). CO2-equivalents equate an amount of a
from certain agricultural and land use activities.
GHG to the amount of CO2 that could have a similar impact
on global temperature over a specific time period (typically
Agriculture is both a source and a sink of greenhouse
25 or 100 years). This aggregation helps illustrate
gases: sources generate emissions to the atmosphere
agriculture’s contribution to national GHG emissions and
and sinks remove carbon dioxide from the atmosphere
contrast emissions against estimates of sequestered carbon.
and sequester it in plants and soil.
Agricultural GHG Emissions
Agricultural activities are a source of greenhouse gas
EPA reports that GHG emissions from the U.S. agricultural
(GHG) emissions, but some activities can remove carbon
sectors totaled 651 MMTCO2-Eq. in 2016 (Table 1). These
dioxide (CO2) from the atmosphere (sinks or emissions
estimates are based on certain assumptions and cover both
removals). Figure 1 illustrates both GHG emissions from
direct emissions and indirect emissions related to electricity
farming activities and carbon sequestration in agricultural
use (see text box below). These estimates do not include
soils. Agricultural sources generate GHG emissions that
emissions associated with food processing or distribution,
accumulate in the atmosphere and may contribute to global
which are typically aggregated with other transportation
climate change. These sources account for many of the
and industrial emissions. These estimates also do not
primary GHGs of interest to policymakers, including CO2,
include other types of emissions associated with some
methane (CH4), and nitrous oxide (N2O) (Figure 1). At the
agricultural activities, such as carbon monoxide, nitrogen
same time, some agricultural activities can remove CO2
oxides, and volatile organic compounds. They also do not
from the atmosphere (sink) through photosynthesis and
account for potential sequestered carbon by agricultural or
storing (or sequestering) the carbon in vegetation and soils,
land use activities.
either temporarily or more long term. Carbon
sequestration—or the process of capturing and storing
Agriculture accounted for 10% of total U.S. GHG
carbon—in farmland soils partially offsets agricultural
emissions in 2016, a smaller proportion of the total than
emissions. Despite this offset potential, U.S. agriculture
other end-use sectors—transportation, industry, commercial
remains a net source of GHG emissions.
and residential (Figure 2). Compared to 1990, when EPA
estimates of GHG emissions associated with the U.S.
Official estimates of GHG emissions and sinks, as reported
agricultural sectors totaled 557 MMTCO2-Eq., emissions
by the U.S. Environmental Protection Agency (EPA), have
from agricultural activities have been increasing (Table 1).
Figure 1. GHG Emissions from Farming Activities and Carbon Sequestration in Agricultural Soils

Source: CRS.
https://crsreports.congress.gov

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Greenhouse Gas Emissions and Sinks in U.S. Agriculture
GHG Emissions from Agricultural Activities
Figure 2. U.S. GHG Emissions, by End-Use Sector
Direct Emissions
 Soil management. N2O emissions from farmland soils
are associated with cropping practices that disturb soils
and increase oxidation, which can release emissions into
the atmosphere. These practices include fertilization,
irrigation, drainage, cultivation and tillage, shifts in land
use, application and/or deposition of livestock manure
and other organic materials on cropland and other
farmland, and other types of practices.
 Enteric fermentation. CH4 emissions from livestock
operations occur as part of the normal digestive process

in ruminant animals and are produced by rumen
Source: CRS from EPA’s Inventory (Table 2-12). 2016 data.
fermentation in metabolism and digestion. Emissions are
associated with the nutritional content and efficiency of
Table 1. Emissions and Net Total Sequestration
feed utilization by the animal.
1990
2016
Avg. 2012-16
 Manure management. CH
Source
4 and N2O emissions are
(MMTCO
associated with livestock or poultry manure that is
2-Eq)
stored and treated in systems that promote anaerobic
GHG Emissions

decomposition, such as lagoons, ponds, tanks, or pits.
U.S. total, all sources
6,355.6
6,511.3
6,630.1

Agricultural activities
556.9
650.7
638.6
Other production methods. CH4 and N2O
emissions are also associated with rice cultivation, urea
Direct emissions
522.0
611.8
596.9
fertilization, liming, and biomass burning, as well as CO2
Electricity-related
34.8
38.9
41.8
emissions from fossil fuel combustion by motorized farm
% agriculture, total
8.8%
10.0%
9.6%
equipment, such as tractors.
Carbon Uptake

Electricity-Related Emissions. Mostly CO2 emissions
U.S. net total, LULUCF
(819.6)
(716.8)
(728.3)
are associated with on-farm electricity use (e.g., other farm
Cropland remaining cropland
(40.9)
(9.9)
(12.2)
machinery and buildings). Estimated emissions do not
include energy emissions from value-added food processing
% Cropland, net total
5.0%
1.4%
1.7%
or fuel combustion from food distribution/transportation.
Source: CRS from EPA’s Inventory (Tables 2-12, 5-1, and 6-4). May
not add due to rounding.

In 2016, the majority of U.S. emissions from agricultural
Carbon uptake from agricultural lands accounts for a small
sources were attributable to soil management (52%), enteric
share of net LULUCF estimates. Among EPA’s estimate
fermentation (30%), and manure management (15%), with
categories, removals associated with “cropland remaining
the remaining emissions associated with other production
cropland” accounts for less than 2% of net removals and
methods. The text box describes both direct and indirect
has been decreasing since 1990 (Table 1). This estimate
GHG emissions associated with U.S. agricultural activities.
does not reflect possible offsetting higher emissions from
Agricultural Carbon Sequestration
the conversion of farmland to other uses.
On agricultural lands, carbon can enter the soil through
Practices That Offset GHG Emissions
roots, litter, cover crops, harvest residues, and animal
Farming practices that either sequester carbon or destroy
manure and may be stored primarily as soil organic matter
GHG emissions could play a role in future legislation
(Figure 1). EPA’s Inventory expresses such estimates as
involving establishing a carbon offset or carbon banking
Land Use, Land Use Change, and Forestry (LULUCF).
program. In general, converting industrial land to
LULUCF estimates measure additional removals from a
agricultural use, or keeping agricultural land in farming,
range of land use and land management activities, including
will sequester carbon in soil compared to other types of
forestry and land use conversion—for example, from less-
industrial, commercial, or residential uses. Other types of
resource conserving to forestland, grassland, wetlands, and
practices that could improve carbon capture and storage in
croplands. These estimates measure both removals and any
agricultural soils include land retirement or restoration and
offsetting changes in emissions—for example, higher
farmland conversion to more resource-conserving uses.
emissions from the conversion of forested lands to less
Carbon uptake may also be increased through practices that
resource-conserving uses—thus reflecting net changes in
raise biomass retention in soils or reduce soil disturbance,
carbon stocks. EPA’s LULUCF estimates rely on data from
such as conservation tillage practices. Installing windbreaks
USDA and the Departments of Energy, Transportation, and
and buffers also promote sequestration.
Defense, among other federal agencies.
EPA’s LULUCF
Among the types of farming practices that reduce GHG
estimate for 2016 indicates that U.S.
emissions are improved soil management and improved
forestry and land use changes contributed to a net carbon
feed and manure management. Manure management
reduction of 730 MMTCO2-Eq. in 2016 (Table 1). About
systems that collect manure in an uncovered lagoon release
98% of all estimated uptake is associated with maintaining
CH4 into the atmosphere. Installing an anaerobic digester to
and conserving forests, wetlands, and grasslands, as well as
manage manure destroys CH4 emissions and is usually part
converting land to more resource conserving uses. Land
of most voluntary carbon offset programs.
improvements through practices such as reforestation,
forest management, and tree-planting activities would also
Renée Johnson, Specialist in Agricultural Policy
contribute to carbon uptake on forested and also crop land.
IF10979
https://crsreports.congress.gov

Greenhouse Gas Emissions and Sinks in U.S. Agriculture

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https://crsreports.congress.gov | IF10979 · VERSION 2 · NEW