Agriculture: Contribution to Climate Change

Introduction

Agriculture is the growing of plants (crops) and animals (livestock) for food and other purposes. In 2007, lands used for crop growing and animal grazing took up 40–50% of Earth's land surface, a 10% increase since 1961. Agriculture contributes to global climate change by releasing carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), the three gases presently causing the most greenhouse warming. Agriculture emitted 5.1–6.1 billion tons of CO₂ equivalent in 2005, that is, 10–12% of all human-caused (anthropogenic) releases of greenhouse gases in that year.

Its share of CH₄ and N₂O emissions was much greater: agriculture contributes about 47% of global CH₄ and 58% of N₂O. These figures do not count greenhouse emissions from electricity and fuel used in agriculture for machinery, buildings, processing, and transport. Also, emissions vary widely among countries, with more industrialized countries deriving much less of their greenhouse emissions from agriculture: for example, in the United States in 2006, direct emissions from agriculture accounted for only 6% of total emissions.

Historical Background and Scientific Foundations

For most of the human race's existence, it did not practice agriculture. For many hundreds of thousands of years, humans and their near-human ancestors practiced various forms of hunting and gathering, finding edible plants and animals in the environment rather than raising them. The first known tools, made about 2.5 million years ago, were food processors, chipped stones devised to help butcher antelopes and horses in the part of Africa that is now Ethiopia. Cropping and livestock-raising began in the Middle East about 9500 BC and gradually spread to most peoples of the world, though there are still a few communities that live primarily through hunting and gathering.

Agriculture both releases and absorbs greenhouse gases. Plants absorb CO₂ from the atmosphere, extracting its carbon to build their tissues. Dead roots and other plant parts remaining in the soil after harvest increase the soil's carbon content, though depending on environmental conditions this carbon may be re-released to the atmosphere by bacterial decay. Not counting CO₂ releases from fuel and electricity usage associated with plowing, spraying, harvesting, transport, processing, and storage, agriculture's CO₂ release and uptake are about balanced. Almost all of agriculture's direct impact on climate comes from its releases of CH₄ and N₂O.

CH₄ and N₂O have a greater greenhouse impact, ton for ton, than does carbon dioxide. Methane is over 20 times as effective, by weight, at causing climate change than is CO₂, and nitrous oxide is about 296 times as effective. However, because much greater quantities of CO₂ are being released into the atmosphere, CO₂ accounts for most of the greenhouse warming now occurring.

Both CH₄ and N₂O are emitted, for the most part, by biological processes. These are as follows (where U.S. figures are for 2006 and global figures are for 2005):

• Soil management. When soils are disturbed, oxygen combines more readily with nitrogen in the soil and N₂O is released. Nitrogen is an essential plant nutrient that is added to soils as fertilizer in much of the world. Practices that increase formation of N₂O from soils include fertilization with nitrogen or manure, irrigation, draining, and the growing of nitrogen-fixing plants such as alfalfa. In the United States, 60% of direct agricultural emissions— in terms of greenhouse-warming efficacy, not by raw tonnage—consist of N₂O from soils. Globally, N₂O from soils comprises 38% of total non-CO₂ direct emissions from agriculture.
• Enteric fermentation. Bacteria that live in oxygen-free environments digest organic materials such as dead plants using a chemical process that produces methane. Oxygen-free (anaerobic) environments are found at the bottoms of bodies of water and in the intestines of ruminant animals such as sheep, goats, and cows. Methane generated by bacteria in ruminant animals is emitted as flatulence. Continued anaerobic digestion of animal manure after it has left the animal produces both CH₄ and N₂O (mostly methane). In the United States, 25% of direct agricultural emissions consist of methane from enteric fermentation. Globally, CH₄ from enteric fermentation constitutes 32% of non-CO₂ agricultural emissions.
• Manure management. Manure from livestock is often collected in ponds, lagoons, pits, or tanks. Anaerobic digestion of the waste in these systems releases methane and nitrous oxide. In the United States, 13% of direct agricultural emissions consists of CH₄ and N₂O emissions from enteric fermentation (9% CH₄, 4% N₂O). Globally, CH₄ from manure management constitutes 7% of non-CO₂ agricultural emissions.
• Rice farming. Anaerobic digestion of plant matter at the bottom of rice paddies (rice fields), which are flooded with water for much or all of the year, produces significant methane. In the United States, only 2% of direct agricultural emissions consist of methane from rice paddies, but globally, rice is a much more important food than it is in the United States and most other developed countries. Thus, CH₄ from rice paddies constitutes 11% of non-CO₂ agricultural emissions. Ninety-seven percent of world rice emissions occur in developing countries.
• Biomass residue burning. Burning crop residues such as cornstalks releases CO₂,CH₄, and N₂O. In the United States, less than 1% of direct agricultural emissions consist of greenhouse gases from residue burning; globally, however, it constitutes 12% of non-CO₂ agriculture emissions. Ninety-two percent of world biomass burning emissions occurs in developing countries.

WORDS TO KNOW

ANAEROBIC: Lacking free molecular oxygen (O₂). Anaerobic environments lack O₂; anaerobic bacteria digest organic matter such as dead plants in anaerobic environments such as deep water and the digestive systems of cattle. Anaerobic digestion releases methane, a greenhouse gas.

BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosyn-thesis are no longer present and contributing to carbon storage.

ENTERIC FERMENTATION: Digestion of organic matter by bacteria in an animal's digestive system. Enteric fermentation produces methane, a greenhouse gas.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

KYOTO PROTOCOL: Extension in 1997 of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by almost all countries with the goal of mitigating climate change. The United States, as of early 2008, was the only industrialized country to have not ratified the Kyoto Protocol, which is due to be replaced by an improved and updated agreement starting in 2012.

PER CAPITA: Latin phrase meaning “for each person.” The per capita greenhouse emissions of a country, for example, are the country's total emissions divided by the number of people in the country.

RUMINANT ANIMALS: Animals such as cows, deer, sheep, and buffalo, and many others, also called simply ruminants, that digest their food partly by bringing it from the stomach to the mouth (where it is called a “cud”) and chewing it there before re-swallowing. The digestive systems of ruminant animals contain bacteria that produce methane: methane from domesticated ruminant animals contributes significantly to global warming.

Emissions of CH₄ and N₂O from agriculture are increasing with human population growth, which is mostly occurring in developing countries and entails corresponding growth in agriculture, and with rising per capita demand for meat in some developing countries. From 1990 to 2005, world agricultural emissions of CH₄ and N₂O increased by almost 17%. Direct agricultural emissions in developing countries (i.e., those not on the Annex I list attached to the Kyoto Protocol climate treaty) increased by 32% during this time period, while emissions from developed (Annex I) countries fell by 12% overall. By 2005, developing countries were accounting for about 75% of all agricultural direct emissions. Indirect agricultural emissions were higher in developed countries, where agriculture is more mechanized and therefore energy-intensive, and where food is far more intensively processed, packaged, marketed, and refrigerated. CO₂ emissions from on-farm fossil fuel use alone were equal to 12.7% (14 million metric tons CO₂ equivalent, MMTCE) of direct greenhouse emissions in the United States in 2006 (120 MMTCE).

Deforestation in the tropics is one of the main non-fossil-fuel contributors to global climate change, producing almost a third of global CO₂ emissions. Expansion of agricultural land area is one of the main drivers of such deforestation. In Brazil, where a third of the world's rain-forest is found, the rainforest was being cleared in the early 2000s at a rate of about 4,000 mi²(10,000 km²) per year, almost entirely for cattle ranching, soy farming, and small-scale subsistence farming.

Such deforestation is often referred to as slash-and-burn agriculture, because land is cleared by the simple expedient of cutting down all the trees, piling them up, and burning them. This immediately releases their stored carbon into the air—750 tons of CO₂ per acre (0.4 hectare) for an old-growth Indonesian rain forest. As of 2005, slash-and-burn deforestation to clear land for agriculture made Indonesia the world's third largest greenhouse-gas emitter and Brazil the world's fourth largest.

In some cases, the burning of cleared trees is only the beginning of the greenhouse emissions from slash-and-burn agriculture. In Indonesia in the 1990s, a government plan to expand rice production led to the clearance and drainage of almost 4,000 mi²(about 1 million hectares) of peat-swamp forests. The plan was a disaster: rice was never successfully produced, but much of the peat—a dense mat of partly decomposed plants rich in carbon, essentially young coal—dried out and caught fire, contributing to catastrophic fires in the late 1990s.

In 2007, the Australian and Indonesian governments announced a joint plan to preserve 270 mi²(70,000 hectares) of remaining Indonesian peat forests, re-flood 770 mi²(200,000 hectares) of dried peatland, and replant 100 million trees on the rehabilitated land. These measures were predicted to prevent 700 million tons of greenhouse emissions over 30 years. Annually, this would be more than the entire annual output of Australia. However, the joint project would still restore only a fraction of the land ravaged by the Mega Rice project of the 1990s.

Impacts and Issues

Is Meat the Number One Cause of Global Warming?

In 2007, vegan and animal-rights organizations ran ads in the U.S. media stating that emissions from meat-raising contribute more to global warming than cars do. For example, an ad by the group People for the Ethical Treatment of Animals (PETA) read: “Too Chicken to Go Vegetarian? Meat Is the Number One Cause of Global Warming.”

According to the United Nations' Intergovernmental Panel on Climate Change's (IPCC's) 2007 Assessment Report, road transport (cars and trucks) contribute 17.8% of greenhouse emissions, with 10.2% coming from automobiles. In comparison, the IPCC report stated that agriculture contributed 10–12% of emissions. Some other U.N. sources attributed a much higher percentage of greenhouse emissions to agriculture. For example, a 2006 report from the Food and Agriculture Organization of the United Nations stated that the livestock sector of agriculture alone is responsible for 18% of greenhouse-gas emissions, “a higher share than transport.” Although 18% is higher than 17.8%, the 0.2% difference between the two numbers is not statistically meaningful. It is therefore not clear whether, globally, agriculture or the livestock sub-sector of agriculture contribute more to climate change than does transport.

Moreover, research does not indicate that meat-eating by people in developed countries such as the United States contributes more to global warming than does driving. As noted earlier, U.S. direct agricultural emissions comprise only 6% of U.S. greenhouse emissions. Gases are also emitted to create food imported by the United States, but only 15% of U.S. food consumption was imported as of 2007. Therefore, assuming proportional emissions for imported food, all U.S. food consumption is responsible for less than 7% of U.S. emissions. Non-diesel cars and trucks, on the other hand, accounted for almost 20% of U.S. greenhouse emissions. The average U.S. citizen thus contributes about three times more to greenhouse warming by driving than by eating, and the meat portion of the American diet is proportionally even smaller.

Finally, the energy sector, not agriculture, is the largest contributor to global warming worldwide. Burning fossil fuels for heat and to generate electricity causes more than half of all global warming.

Nevertheless, it is true that, ounce for ounce, meat production contributes significantly more to global climate change than does the production of vegetable foods, and that agriculture is a major contributor to global warming. Reducing per-capita meat consumption in the developed world, medical and climate experts have argued, would have both health and climate benefits.

Mitigation of Agricultural Emissions

Altered agricultural practices can reduce agriculture's contribution to global warming. More efficient delivery of nitrogen to crops would reduce N₂O emissions and other ecological harms; livestock management for more efficient digestion of feeds would save money and reduce CH₄ emissions; crop residues and manures can be used as fuel sources (e.g., methane from decaying manure can be burned to generate electricity, rather than released to the atmosphere, and the fermentation residue can still be used as fertilizer). Many other measures are described by the IPCC. Cost-effective greenhouse mitigation measures are often friendly to sustainable-development goals, enhancing profitability and reducing soil losses.

Primary Source Connection

Methane (CH₄) and nitrous oxide (N₂O) are major greenhouse gases. Agriculture is a common, and often overlooked, contributor of methane and nitrous oxide. Both methane and nitrous oxide are produced naturally by livestock and soil management. Some human-controlled agricultural management techniques, however, increase methane and nitrous oxide production. This article examines methane and nitrous oxide production by various United States agricultural processes.

The Environmental Protection Agency (EPA) is an agency of the United States government that is devoted to protecting human health and working toward a cleaner environment. The EPA was founded in 1970 and currently employs over 17,000 people.

INVENTORY OF U.S. GREENHOUSE GAS EMISSIONS AND SINKS: 1990–2005

Agricultural activities contribute directly to emissions of greenhouse gases through a variety of processes, including the following source categories: enteric fermentation in domestic livestock, livestock manure management, rice cultivation, agricultural soil management, and field burning of agricultural residues. CH₄ and N₂O were the primary greenhouse gases emitted by agricultural activities. CH₄ emissions from enteric fermentation and manure management represented about 21 percent and 8 percent of total CH₄ emissions from anthropogenic activities, respectively, in 2005. Agricultural soil management activities such as fertilizer application and other cropping practices were the largest source of U.S. N₂O emissions in 2005, accounting for 78 percent. In 2005, emission sources accounted for in the Agricultural chapters were responsible for 7.4 percent of total U.S. greenhouse gas emissions….

Agricultural Soil Management

N₂O is produced naturally in soils through microbial nitrification and denitrification processes. A number of anthropogenic activities add to the amount of nitrogen available to be emitted as N₂O by microbial processes. These activities may add nitrogen to soils either directly or indirectly. Direct additions occur through the application of synthetic and organic fertilizers; production of nitrogen-fixing crops and forages; the application of livestock manure, crop residues, and sewage sludge; cultivation of high-organic-content soils; and direct excretion by animals onto soil. Indirect additions result from volatilization and subsequent atmospheric deposition, and from leaching and surface run-off of some of the nitrogen applied to or deposited on soils as fertilizer, livestock manure, and sewage sludge. In 2005, agricultural soil management accounted for 78 percent of U.S. N₂O emissions. From 1990 to 2005, emissions from this source decreased by 1.8 Tg CO₂ Eq. (0.5 percent); year-to-year fluctuations are largely a reflection of annual variations in weather, synthetic fertilizer consumption, and crop production.

Enteric Fermentation

During animal digestion, CH₄ is produced through the process of enteric fermentation, in which microbes residing in animal digestive systems break down food. Ruminants, which include cattle, buffalo, sheep, and goats, have the highest CH₄ emissions among all animal types because they have a rumen, or large fore-stomach, in which CH₄-producing fermentation occurs. Non-ruminant domestic animals, such as pigs and horses, have much lower CH₄ emissions. In 2005, enteric fermentation was the source of about 21 percent of U.S. CH₄ emissions, and about 70 percent of the CH₄ emissions from agriculture. From 1990 to 2005, emissions from this source decreased by 3.6 Tg CO₂ Eq. (3 percent). Generally, emissions have been decreasing since 1995, mainly due to decreasing populations of both beef and dairy cattle and improved feed quality for feedlot cattle.

Manure Management

Both CH₄ and N₂O result from manure management. The decomposition of organic animal waste in an anaerobic environment produces CH₄. The most important factor affecting the amount of CH₄ produced is how the manure is managed, because certain types of storage and treatment systems promote an oxygen-free environment. In particular, liquid systems tend to encourage anaerobic conditions and produce significant quantities of CH₄, whereas solid waste management approaches produce little or no CH₄. Higher temperatures and moist climatic conditions also promote CH₄ production.

CH₄ emissions from manure management were 41.3 Tg CO₂ Eq., or about 8 percent of U.S. CH₄ emissions in 2005 and 26 percent of the CH₄ emissions from agriculture. From 1990 to 2005, emissions from this source increased by 10.4 Tg CO₂ Eq. (34 percent). The bulk of this increase was from swine and dairy cow manure, and is attributed to the shift of the swine and dairy industries towards larger facilities. Larger swine and dairy farms tend to use liquid management systems.

N₂O is also produced as part of microbial nitrification and denitrification processes in managed and unmanaged manure. Emissions from unmanaged manure are accounted for within the agricultural soil management source category. Total N₂O emissions from managed manure systems in 2005 accounted for 9.5 Tg CO₂ Eq., or 2 percent of U.S. N₂O emissions. From 1990 to 2005, emissions from this source category increased by 0.9 Tg CO₂ Eq. (10 percent), primarily due to increases in swine and poultry populations over the same period.

Rice Cultivation

Most of the world's rice, and all of the rice in the United States, is grown on flooded fields. When fields are flooded, anaerobic conditions develop and the organic matter in the soil decomposes, releasing CH₄ to the atmosphere, primarily through the rice plants. In 2005, rice cultivation was the source of 1 percent of U.S. CH₄ emissions, and about 4 percent of U.S. CH₄ emissions from agriculture. Emission estimates from this source have decreased about 3 percent since 1990.

Field Burning of Agricultural Residues

Burning crop residues releases N₂O and CH₄. Because field burning is not a common debris clearing method in the United States, it was responsible for only 0.2 percent of U.S. CH₄(0.9 Tg CO₂ Eq.) and 0.1 percent of U.S. N₂O (0.5 Tg CO₂ Eq.) emissions in 2005. Since 1990, emissions from this source have increased by approximately 28 percent.

epa. “inventory of u.s. greenhouse gas emissions and sinks: 1990–2005. #x201d; < http://www.epa.gov/climatechange/emissions/downloads06/07cr.pdf> (accessed november29, 2007).

See Also Agriculture: Vulnerability to Climate Change; Biofuel Impacts; Cow Power; Sustainability.

BIBLIOGRAPHY

Books

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Asner, Gregory P. “Grazing Systems, Ecosystem Responses, and Global Change.” Annual Review of Environment and Resources 29 (2004): 261–299.

Deutsch, Claudia H. “Trying to Connect the Dinner Plate to Climate Change.” The New York Times (August 29, 2007).

Izaurralde, R. César, et al. “Carbon Cost of Applying Nitrogen Fertilizer.” Science 288 (2000): 809.

Web Sites

Johnson, Renée. “Climate Change: The Role of the U.S. Agriculture Sector.” Congressional Research Service, March 6, 2007. < http://fpc.state.gov/documents/organization/81931.pdf> (accessed November 5, 2007).

Steinfeld, Henning, et al. “Livestock's Long Shadow: Environmental Issues and Options.” Food and Agriculture Organization of the United Nations, 2006. < http://www.virtualcentre.org/en/library/key_pub/longshad/A0701E00.pdf> (accessed November 5, 2007).

Larry Gilman