Agriculture and Greenhouse Gas Emissions
Ray Massey and Ann Ulmer
Commercial Agriculture Program
Although the impact of human activity on global warming is unclear, national and international organizations are tracking the amount of greenhouse gases (GHGs) released by humans. Agriculture is a key sector being monitored. A United Nations study credits animal agriculture with 18 percent of all GHG emissions. As a result of the focus on agriculture's emissions, crop and livestock farmers in the United States are showing interest in the opportunity to be paid for sequestering carbon in the soil or capturing GHGs from manure storage.
This guide presents basic information on agriculture's role in GHG emissions. It draws heavily from the U.S. Environmental Protection Agency (EPA) report issued in 2010 titled Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2008.
Greenhouse gases in the atmosphere capture the earth's infrared radiation and warm the surface of the earth. Without GHGs, the average temperature of the earth would be about minus 2 degrees F rather than its current temperature of about 59 degrees F. The concern among many scientists is that the concentration of GHGs has increased significantly since the beginning of the industrial revolution in the 1800s. Some scientists hypothesize that these increased concentrations of GHGs can warm the surface of the earth and cause changes in climatic conditions.
Naturally occurring GHGs, in order of relative abundance, include water vapor, carbon dioxide, methane, nitrous oxide and ozone. Greenhouse gases that are not naturally occurring include substances containing fluorine, chlorine and bromine.
This guide focuses on the GHG concentrations affected by U.S. agricultural activity: carbon dioxide, methane and nitrous oxide. Carbon dioxide is the most well known GHG because it is the most prevalent (making up more than 80 percent of GHG emissions related to human activity) and is associated with burning of fossil fuels. However, methane and nitrous oxide actually trap more heat in the atmosphere than carbon dioxide. To compare the effect of each gas on global warming, scientists have developed the global warming potential (GWP) concept, which uses carbon dioxide as the reference with a value of 1. Methane and nitrous oxide have values of 23 and 296, respectively. This means that, pound for pound, methane contributes 23 times the impact of carbon dioxide to global warming. Similarly, a given amount of nitrous oxide in the atmosphere exerts 296 times the effect on global warming as the same amount of carbon dioxide. All GHG emissions are reported as carbon dioxide equivalents (abbreviated CO2 Eq.).
In 2008, U.S. GHG emissions totaled 6,901 million metric tons (MMT) CO2 Eq. Total U.S. emissions increased 13 percent from 1990 to 2008. During this same period, the U.S. population increased by 21 percent and the country's real gross domestic product increased by 66 percent (Figure 1). Changes in emissions frequently are associated with changes in population, economic growth, energy price, seasonal temperatures and technology.
A portion of GHG emissions can be offset when carbon in the atmosphere is taken up and stored, or sequestered, by plants. In 2008, the EPA estimated that carbon sequestration by forests, trees in urban areas, agricultural soils and other sources offset 940 MMT CO2 Eq. (13 percent of total GHG emissions). Carbon storage occurs in the soil as roots and other organic matter increase or in tree growth that is not destroyed.
U.S. greenhouse gas emissions per capita and per dollar of gross domestic product.
U.S. Environmental Protection Agency. 2010. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2008.
Sources of greenhouse gases
The EPA distributes GHG emissions among five economic end-user sectors: industry, transportation, residential, commercial and agriculture. Agriculture is estimated to be responsible for 8 percent of the GHGs emitted in the United States in 2008 (Figure 2).
The EPA's estimate of agricultural activities contributing 8 percent of human-induced GHG emissions is considerably lower than the often-quoted U.N. estimate of 18 percent attributable to livestock alone.
In 2010, one of the authors of this U.N. report admitted that the comparison of meat to transportation sources of GHGs was flawed because it used different methods for the two sectors.
The U.N. estimate is based on all GHG emissions associated with livestock and meat production. In addition to emissions from crop production associated with livestock, livestock feeding and manure management, the U.N.'s estimate includes emissions from transport of livestock to market, livestock processing and transport of meat and products to retail. The EPA estimate for agriculture includes all crop production and livestock production but does not include transportation and processing of livestock and meat products. According to the U.N. report, processing and transport account for less than 1 percent of GHG emissions in their accounting procedure, so it would probably result in little change from the U.S. estimate.
The U.N. report divides the impact between intensive livestock systems (common in the United States) and extensive systems (pastoral-type systems). The intensive systems produce the most food (meat, milk or eggs) with the least amount of GHG emissions. Extensive systems are responsible for two-thirds of the GHG emissions, due mainly to deforestation to obtain grazing land.
The largest contributors to GHG emissions are deforestation (34 percent) and enteric fermentation (25 percent). Both of these categories are predominately a problem in extensive systems where land is being converted from forests to grazing land and where poor-quality feed increases enteric fermentation per unit of meat produced. The intensive livestock production common in the United States is production that is reducing its carbon footprint per unit of meat produced.
The U.N. report admits that many estimates are relatively imprecise because of lack of data in many countries. Of the estimated 7 billion metric tons of CO2 Eq. emitted in livestock production, 52 percent is designated as "imprecise estimates." The 7,000 MMT GHG estimate could be viewed as an upper bound rather than an accurate estimate, as would be provided by the U.S. EPA Inventory of U.S. Greenhouse Gas Emissions and Sinks.
Greenhouse gas emissions by economic sector.
Agricultural GHG emissions
Agriculture contributes to GHG through crop and soil management, livestock manure management, and methane production during animal digestion, a process called enteric fermentation. Greenhouse gas emissions associated with the production and use of electricity occur within each of these activities. Agriculture is estimated to have directly released 504 MMT of CO2 Eq. in 2008. When electricity-related emissions are distributed to the economic sectors, agriculture released an additional 28 MMT CO2 Eq., for a total of 532 MMT of CO2 Eq. in 2008.
Nitrous oxide (N2O) and methane (CH4) are the two major GHGs emitted by agricultural activities. Carbon dioxide accounts for about 10 percent of direct agriculture-related GHG emissions.
Agricultural crop and soil management was the largest source of N2O emissions, contributing 216 MMT CO2 Eq., or 41 percent of total agricultural CO2 Eq. The N2O emissions for agriculture have decreased over the last 15 years.
In 2008, 26 percent of CO2 Eq. released in agriculture came from enteric fermentation release of CH4; another 8 percent of CO2 Eq. in the form of CH4 was from manure management.
The U.N. Intergovernmental Panel on Climate Change (IPCC) 2007 report Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories recommends setting priorities among GHG sources and sinks within the national inventory. A source is designated a key category when it has a "significant influence on the country's total inventory of GHG in terms of the absolute level of emissions, the trend in emissions, or both." The EPA listed 18 key categories based on 2008 emission levels and trends. Categories 5, 7, 14, 15 and 19 on the list were direct N2O emissions from agriculture soil management, direct CH4 emissions from enteric fermentation, indirect N2O emissions from agriculture soil management, CH4 emissions from manure management, and N2O from manure, respectively (Figure 3).
The U.S. Environmental Protection Agency listed 20 categories of greenhouse gas emissions in 2008. Agricultural emissions (categories 5, 7, 14, 15 and 19) totaled an estimated 419 million metric tons of CO2 equivalent in 2008.
U.S. Environmental Protection Agency. 2010. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2008.
Crop and soil management
Major GHG emissions associated with crop and soil management come mainly from soil management (97 percent), while much less is attributed to field burning and rice cultivation. The EPA estimated that crop and soil management was responsible for 225 MMT of CO2 Eq. in 2008.
The bulk of the CO2 Eq. emitted through crop and soil management is in the form of N2O. Nitrous oxide is produced naturally in the soil but is influenced by human activities that increase soil mineral nitrogen. The EPA considers the following activities to directly increase the amount of N2O emissions: fertilization, application of manure or other organic materials, retention of crop residues, production of nitrogen-fixing crops and forages, and cultivation of soils with high organic matter content. Other practices that directly affect N2O emissions are irrigation, drainage, tillage practices and fallowing of land. Practices that indirectly increase N2O emissions are volatilization and subsequent atmospheric deposition of applied nitrogen, and surface runoff and leaching of applied nitrogen.
Cropland emits more N2O per acre than grassland, but the United States has more acres of grassland than cropland. Cropland accounts for about 69 percent of direct N2O emissions even though it constitutes only 49 percent of agricultural land. Grassland constitutes 51 percent of agricultural land but accounts for only 31 percent of direct N2O emissions.
Direct N2O emissions tend to be high in the Corn Belt (because of nitrogen fertilization on corn and nitrogen fixation in soybean cropping), where irrigation is prevalent, and where land is intensively cropped (such as portions of California and states along the Mississippi River Valley).
Rice production contributes 7.2 MMT CO2 Eq. because rice is grown on flooded fields where microbes under anaerobic soil conditions produce methane. This methane escapes to the atmosphere, where it captures heat 23 times more effectively than CO2.
Field burning of crop residues is not considered a net source of actual CO2 emissions because it releases CO2 that was captured from the atmosphere during that growing season. However, other gases released during the burning — CH4, carbon monoxide (CO), N2O and other nitrogen oxides (NOx) — are considered a release of GHGs. Although residue burning is perhaps a visible release of gas into the atmosphere, it is a minor contributor. The EPA estimated that 1.5 MMT of CO2 Eq., or about 0.35 percent of total agriculture-related emissions, resulted from field burning in 2008.
Manure management is a source of CH4 and N2O emissions. The EPA estimated that manure management was responsible for 62 MMT of CO2 Eq. emissions in 2008. The manure application emissions counted as crop and soil management are not double-counted here; this estimate includes only emissions from manure storage.
Methane is produced by the anaerobic decomposition of manure. Methane production occurs when manure is handled under anaerobic conditions such as in liquids and slurries. When manure is handled as a solid, little or no CH4 is produced. The amount of CH4 produced is affected by temperature, moisture, time in storage, manure composition and storage system.
Nitrous oxide is produced from organic nitrogen in both manure and urine. Solid manure management systems produce N2O because they have both aerobic and anaerobic decomposition that nitrifies and then denitrifies the nitrogen in the manure and urine.
Most GHG emissions from manure management are in the form of CH4 and come from dairy and swine operations, which tend to use liquid manure management systems (Figure 4). Swine and dairy manure emissions increased during the 1990s when the industries moved toward confinement systems with liquid manure storage systems, but emissions have been holding steady during the 2000s.
Carbon dioxide equivalent emissions from manure management.
Methane production by enteric fermentation is a part of normal digestive processes in animals, especially ruminants such as cattle, sheep and goats. The amount of CH4 produced by enteric fermentation is affected by the number of livestock in the United States and by the amount and type of feed they consume. Livestock fed higher-quality feed produce less CH4 than those fed low-quality feed.
The EPA estimated that enteric fermentation was responsible for 141 MMT of CO2 Eq. emissions in 2008. Beef and dairy cattle were responsible for the overwhelming majority of CH4 emissions (Figure 5). All other classes of livestock contributed 5 percent of CH4 emissions.
Methane emissions by livestock type.
Agricultural carbon sequestration
Land use and forestry activities resulted in a net carbon sequestration of about 940 MMT of CO2 Eq., roughly 14 percent of total U.S. CO2 emissions, in 2008. This carbon sequestration associated with land use and forestry increased by more than 13 percent from 1990 to 2008. Although most of this sequestration occurs in trees and forests, U.S. crop and livestock farmers have expressed considerable interest in sequestration opportunities in crop and rangeland management.
Land use and carbon sequestration
Land activities cause both emission and sequestration of carbon. The emission and removal of carbon from the atmosphere is called a GHG flux. Following the IPCC Good Practice Guidance report, the EPA reports agricultural fluxes in the following land use change categories: cropland remaining cropland, land converted to cropland, grassland remaining grassland, and land converted to grassland. Net U.S. carbon sequestration from these four categories was 45 MMT of CO2 Eq. in 2008 (Figure 6).
Cropland remaining cropland refers to land that has been in crops for the past 20 years. Activities that increased carbon stocks in the soil were reduction of bare-summer fallow in semiarid regions, increased hay production and adoption of conservation tillage. Across the United States, cropland remaining cropland was estimated to sequester a net 18 MMT of CO2 Eq. Soils containing 1 to 6 percent soil organic matter sequestered 46 MMT of CO2 Eq., whereas soils containing 12 to 20 percent soil organic matter released 28 MMT of CO2 Eq. Organic soils constitute less than 1 percent of U.S. cropland but emit significant amounts of GHG when cropped. Limestone added to acidic soils generated 4 MMT CO2 Eq.
Land converted to cropland includes all land designated as cropland that had a different designation in an earlier USDA National Resources Inventory (NRI) land use survey. Lands are kept in this category for 20 years, after which they are considered cropland remaining cropland. These areas release CO2 as they equilibrate to a new, lower soil organic carbon level. In 2008, these soils were estimated to have released 6 MMT of CO2 Eq.
Grassland remaining grassland includes all areas that have been designated as grassland for the past 20 years. Grassland is normally considered to sequester carbon. Grassland remaining grassland released about 16 MMT CO2 Eq. each year from 2000 to 2008, largely due to droughts causing small losses of carbon per acre over large geographic areas.
Land converted to grassland includes all land designated as grassland that had a different designation in an earlier USDA NRI land use survey. Lands are kept in this category for 20 years, after which they are considered grassland remaining grassland. Carbon sequestration was primarily due to conversion of cropland to continuous pasture in the southeast and northwest United States.
Carbon dioxide equivalent emissions from land use, 2008.
Trees and carbon sequestration
The largest sources of carbon sequestration in the United States are attributed to forestland and urban trees. Forestlands are estimated to have sequestered 792 MMT of CO2 Eq. in 2008, while urban trees sequestered 94 MMT. Both estimates take into account GHGs other than CO2 released from burning. Carbon dioxide released during burning is not considered an addition to GHGs emitted because that CO2 is part of the natural carbon cycle.
Ethanol and greenhouse gases
Ethanol, when burned as a fuel, releases CO2 into the environment. However, the EPA does not consider ethanol to increase atmospheric CO2. The CO2 released during use is assumed to be sequestered in the same year by the growth of corn or other biomass used to make the ethanol. Because ethanol is a substitute for gasoline produced from fossil fuels, the use of ethanol actually reduces CO2 emissions associated with the transportation industry by 52 MMT CO2 Eq. Ethanol further reduces GHG emissions because it is lower in NOx and hydrocarbon emissions than gasoline. Countering this net reduction, ethanol production emits 27 MMT CO2 Eq. in its wastewater treatment.
Agriculture directly released 428 MMT of CO2 Eq. and sequestered 45 MMT in 2008. Land use changes and forests sequestered 837 MMT CO2 Eq. Land management for agriculture and forest resulted in a net sequestration of 410 MMT CO2 Eq.
||MMT CO2 Eq
|Total U.S. (all sectors)
|Total direct agriculture1
| Crop and soil management
| Enteric fermentation
| Manure management
| Land use changes
| Forest land
1Total agriculture does not include sequestration in forestland and does not include electricity used in agriculture.
Many farmers have been optimistic about the opportunities to be compensated for sequestering carbon by reducing tillage. Currently, the Chicago Climate Exchange (CCX) credits conservation tillage in most of the nation east of the Missouri and Mississippi rivers with 0.6 ton CO2 Eq. per acre and conservation tillage in other areas with 0.2 ton CO2 Eq. per acre. The CCX currently credits Conservation Reserve Program (CRP) land with 0.6 ton CO2 Eq. per acre for carbon sequestration and rangeland with 1.0 ton CO2 Eq.
While 2010 CCX market prices for CO2 Eq. are a fraction of those paid in Europe, many think that the price will increase if carbon caps are implemented. One noticeable difference between the CCX and the European Climate Exchange (ECX) is that the ECX does not permit agriculture to be paid for carbon sequestration.
Farmers need to be aware of what is happening in the debate over GHG emissions. Compensation for sequestering carbon offers an opportunity for farmers to benefit from their stewardship of resources. It also presents a potential opportunity for the government to limit farmers' production activities to reduce GHG emissions.
The EPA recognizes agriculture as a net emitter of GHGs and lists five agricultural sources of GHGs as key categories that have a significant influence on the country's total inventory of GHGs. While regulating the use of nitrogen fertilizer, because of its ubiquity over the landscape, may be difficult, it is not impossible. The EPA has expressed a desire to have nutrient management plans on land receiving fertilizer. A logical next step would be to target emissions from livestock manure management because they are already associated with larger businesses that are subject to permitting requirements.
The EPA study Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2008 used the 1996 GWP (methane = 21; nitrous oxide = 310) to maintain consistency of national reports, per the instructions of the U.N. Intergovernmental Panel on Climate Change (IPCC).
For further information
- Le Treut, H., et al. 2007. Historical Overview of Climate Changes Science, in S. Solomon, et al. (eds.), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press.
- Houghton, J.T., et al. 2001. Climate Change 2001: The Scientific Basis. Contribution of Working Group III to the Third Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press.
- U.S. Environmental Protection Agency. 2010. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2008. Washington, D.C.: EPA.
- Steinfeld, H., et al. 2006. Livestock's Long Shadow: Environmental Issues and Options. New York: Food and Agriculture Organization of the United Nations.
G310, revised August 2010