Animal agriculture has recently received much attention,1, 2 for its role in producing gases that contribute to global warming. Prominent among those gases so produced is methane, which cattle emit as a consequence of their digestion.
Based on the estimate that the typical grass-fed cow produces 600–700 liters of methane per day,3 we can calculate the annual amount of this gas produced by cattle grazing on 260 million acres of federal public lands managed by the U.S. Forest Service and the Bureau of Land Management in the forty-eight contiguous states.4 In the interest of producing a conservative estimate, I will perform the calculation using the lower limit (i.e., 600 liters) of a cow’s daily methane production.
The BLM5 and U.S. Forest Service6 report recent annual forage utilization from their lands by cattle of 7,862,879 and 6,025,788 AUMs7 respectively, with the combined forage utilization being 13,888,667 AUMs.
As each AUM represents thirty-one days of a cow’s forage consumption, it similarly represents thirty-one days of that same cow’s methane production. In other words, each AUM represents (31 days × 600 liters/day) of methane production, i.e., each AUM represents the production of 18,600 liters of methane.
Consequently, the annual methane production by cattle on U.S. federal public lands is equal to (18,600 liters/AUM) x (13,888,667 AUMs/year), or 2.583 x 1011 liters/year.
4. The U.S. Forest Service manages 97 million acres for livestock production; the Bureau of Land Management manages 163 million acres for this purpose. George Wuerthner and Mollie Matteson, eds. 2002. Welfare Ranching: The Subsidized Destruction of the American West. Washington, DC: Island Press, 5.
5. Bureau of Land Management, Department of the Interior, “Public Land Statistics 2007,” Table 3-8c (Summary of Authorized Use of Grazing District Lands and Grazing Lease Lands, Fiscal Year 2007), (accessed October 31, 2008).
6. United States Department of Agriculture, Forest Service, Range Management, “Grazing Statistical Summary FY 2005,” April 2006, p. 4, (accessed October 31, 2008).
7. AUM: acronym for “animal unit month”—the amount of forage consumed by a cow over a period of 31 days.
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As 1,000 liters is equivalent in volume to 1 cubic meter (m3), the annual public lands cattle production of methane is 2.583 x 108 m3/year.
Based on methane’s density of 0.68 kg/m3 (under conditions of 1.013 bar and 15 °C (59 °F)),8 the above volume of methane weighs 1.757 x 108 kg. (Note that the density of methane gas will increase down to its boiling point (1.013 bar) at -161.6 °C (i.e., 1.819 kg/m3), but 15 °C should reasonably reflect the temperatures at which the gas is produced by cows.)
What does the annual production of 1.757 x 108 kg of methane represent in terms of CO2 emissions? Well, I plugged the weight of the methane into the U.S. Environmental Protection Agency’s online “Greenhouse Gas Equivalencies Calculator.”9 And based on the U.S. government’s figures, I can state that the methane annually produced by cattle grazing on U.S. federal public lands has the global warming potential equivalent to 3.689 x 109 kg of CO2. This is then equivalent to any of the following:
• annual greenhouse gas emissions from 675,630 passenger vehicles
• CO2 emissions from 418,722,027 gallons of gasoline consumed
• CO2 emissions from 8,578,933 barrels of oil consumed
• CO2 emissions from the electricity use of 488,601 homes for one year
• CO2 emissions from the energy use of 325,591 homes for one year
• CO2 emissions from burning 19,263 railcars’ worth of coal
• CO2 emissions of 0.79 coal-fired power plants for one year
• carbon annually sequestered by 838,396 acres of pine or fir forests
• carbon annually sequestered by 25,730 acres of forest preserved from deforestation.
“Wait a minute,” I can hear critics from the livestock industry protesting. “What about ways in which cattle grazing on public lands actually reduces greenhouse gas emissions? What about all the diseases that meat consumption contributes to, like stroke, heart disease, and diabetes that shorten people’s lives, and thereby reduces the amount of greenhouse gases those people will indirectly generate through their use of fossil fuels?” While there may be some merit to that objection, I will leave it as an exercise for ranching advocates to perform the calculations.
9. U.S. Environmental Protection Agency, “Greenhouse Gas Equivalencies Calculator,” (accessed October 8, 2009). (Assumptions underlying the greenhouse gas equivalencies can be found on the website.)