Greenhouse Gas Emissions and Climate Change: Animal Agriculture's Carbon Footprint
What it is
Animal agriculture is a significant contributor to global greenhouse gas emissions — a fact that is well established in the scientific literature but frequently absent from public discourse about climate change, in part because it implicates dietary choices in a way that most climate communication avoids.
The numbers: The United Nations Food and Agriculture Organization (FAO) published a landmark 2006 report, Livestock's Long Shadow, which estimated that animal agriculture was responsible for 18% of global greenhouse gas emissions when assessed on a lifecycle basis — more than the entire transportation sector. A 2013 FAO follow-up study, using updated methodology, placed the figure at approximately 14.5% of global anthropogenic greenhouse gas emissions. This figure includes direct emissions (methane from enteric fermentation in ruminants — essentially, methane from cow digestion and belching — and nitrous oxide from manure) as well as indirect emissions from feed crop production, land use change, and transport.
The composition of livestock's climate impact is distinct from most other emission sources in its mix of gases. Carbon dioxide — the primary climate gas from fossil fuel combustion — is significant in livestock production through land use change (deforestation for pasture and feed crops), but methane and nitrous oxide are proportionally more important than in other sectors. Methane, produced in large quantities by enteric fermentation in cattle, sheep, and other ruminants, has a global warming potential approximately 28 times that of CO2 over a 100-year horizon (and approximately 86 times over 20 years — a more urgent timeframe for near-term climate impacts). Nitrous oxide, produced primarily by the application and breakdown of manure and synthetic nitrogen fertilizer, has a global warming potential approximately 265 times that of CO2.
The breakdown by livestock category is important. Beef and dairy cattle are responsible for the majority of livestock's climate impact — approximately 65% of the total, according to the FAO — due to the high methane output of ruminant digestion and the land requirements of both animals and their feed. Poultry and pigs, as monogastrics (single-stomach animals that do not produce significant enteric methane), have substantially lower greenhouse gas intensities per unit of protein than ruminants. The climate case for dietary shift is therefore asymmetric: reducing beef and dairy consumption has far more climate impact than reducing poultry or pork.
Land use: The land footprint of industrial animal agriculture is staggering. According to FAO data, approximately 77% of global agricultural land is used for livestock grazing and the production of livestock feed crops, producing approximately 18% of global food calories and 33% of global food protein. The inverse — 23% of agricultural land producing 82% of calories — reflects the thermodynamic losses inherent in converting plant protein and energy into animal protein and energy. This land footprint is the primary driver of habitat destruction globally: the expansion of agricultural land into tropical forests, savannas, and wetlands is responsible for the majority of global terrestrial biodiversity loss, and the majority of that agricultural expansion is for livestock and feed crops, primarily soybeans.
The Amazon basin, the Congo basin, and the Cerrado (Brazilian savanna) have seen the most dramatic agricultural encroachment. The Brazilian Amazon has lost approximately 17% to 20% of its original forest cover; the Cerrado, less internationally recognized but ecologically critical, has lost more than half of its native vegetation. The primary driver in both cases is the expansion of cattle ranching and soybean production for export (the majority of Brazil's soybean production goes to feed livestock in China and the EU). The links between consumer demand for cheap beef and animal feed in wealthy countries and deforestation in tropical regions are traceable and quantifiable.
Water use: Animal agriculture is a major consumer of freshwater globally. The water footprint of food production — the total volume of freshwater used directly and indirectly in the production of a food product — shows dramatic differences between animal and plant proteins. Beef production has a water footprint of approximately 15,000 liters per kilogram of product; pork approximately 6,000 liters per kilogram; chicken approximately 4,300 liters per kilogram; soybeans approximately 2,100 liters per kilogram; corn approximately 900 liters per kilogram. These figures vary by production system and geography, but the ranking is consistent: animal proteins require substantially more water per unit of nutrition than plant proteins.
In water-stressed regions — including the American Southwest, California's Central Valley, Northern India, North Africa, and parts of Sub-Saharan Africa — the water demands of animal agriculture compete directly with human water needs and ecosystem water requirements. The Ogallala Aquifer, which underlies approximately 174,000 square miles of the Great Plains and provides irrigation water for approximately one-fifth of all U.S. irrigated land (much of it for feed crops), is being depleted at rates estimated at ten to forty times natural recharge rates in some areas. The feedlot and feed crop complex of the Great Plains is, in effect, mining a prehistoric water reserve that formed in the last Ice Age and cannot be replenished on any human timescale.
Agricultural Runoff and Dead Zones: The concentrated manure production of CAFOs — and the synthetic nitrogen and phosphorus runoff from the feed crops that supply them — is the primary driver of one of the most significant ongoing ecological disasters in North America: the hypoxic dead zone in the Gulf of Mexico. Each summer, an area of the Gulf — averaging approximately 5,500 square miles, roughly the size of Connecticut, though varying dramatically by year — becomes so depleted of oxygen that it cannot support marine life. Fish, shrimp, crabs, and other species that can move flee; those that cannot, die.
The cause is eutrophication: the enrichment of water with nutrients (primarily nitrogen and phosphorus) that stimulates explosive growth of algae, which then dies, decomposes, and consumes the oxygen dissolved in the water. The nutrients come primarily from agricultural runoff — fertilizer applied to feed crops (corn and soybeans) throughout the Mississippi River basin, and manure from livestock operations — carried south by the Mississippi and deposited in the Gulf. The scale of the problem is directly tied to the scale of industrial corn, soybean, and livestock production in the Midwest; years with heavy rainfall that carries more runoff show larger dead zones. The shrimping and fishing industries of the Louisiana Gulf Coast — themselves significant cultural and economic institutions — have been significantly affected.
Reference notes
Cross-link to: The Carbon Footprint of Food, Plant-Based Diets and Climate, Amazon Deforestation and Beef, The Mississippi River Watershed, Gulf of Mexico Dead Zone, Water Footprint of Food.
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