Cattle Grazing on Federal Public Lands
Contributes to Global Climate Change
10 November 2008
Revised 7 February 2017
Animal agriculture has recently received much attention for its role in producing gases that contribute to global climate change.1, 2 Prominent among those gases so produced is methane which cattle emit as a consequence of their digestion.3
Based on the measurement that a typical grass-fed cow emits 600–700 liters (L) of methane per day,4 we can estimate the mass of this gas annually produced by cattle that graze 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.5 In the interest of producing a conservative estimate, I will perform the calculation using the lower limit (i.e., 600 L) of a cow’s daily methane production.
The BLM6 and U.S. Forest Service7 report annual forage utilization from their lands by cattle of 7,574,183 and 6,070,229 animal unit months (AUMs) respectively, with the combined forage utilization being 13,644,412 AUMs.
As each AUM represents thirty-one days of a cow’s forage consumption, it likewise represents thirty-one days of that animal’s methane production. In other words, each AUM consumed produces 18,600 L of methane (i.e., 31 days × 600 L day-1).
Consequently, the annual volume of methane produced by public lands cattle is equal to 253,786,063,200 L, i.e., 18,600 L AUM-1 × 13,644,412 AUMs year-1.
Copyright © Mike Hudak 2008–, All Rights Reserved
Since 1,000 L are equivalent in volume to 1 cubic meter (m3), public lands cattle produce 253,786,063 m3 of methane per year.
The mass of this volume is 172,574,522 kg based on the density of methane being 0.68 kg/m3 (i.e., under assumed conditions of 1.013 bar (one atmosphere) and 15 °C (59 °F)).8
Greenhouse gasses, such as methane, have a property called Global Warming Potential (GWP) that denotes the ability of the gas relative to carbon dioxide (CO2) to trap heat in the global climate system over a given time frame. Current studies peg the GWP of methane at “34” over a 100-year interval (GWP100) and at “86” over a 20-year interval (GWP20).9 Stated otherwise, over a 20-year interval, a given mass of methane would have the same effect in the global climate system as a mass of CO2 that is 86 times greater than that mass of methane.
Authors of climate-related articles have often chosen to consider methane’s impact over a 100-year period. But in 2013, the IPCC noted that “there is no scientific argument for selecting 100 years compared with other choices.”10 Moreover, the IPCC found that at the 20-year timescale, total global emissions of methane are equivalent to over 80% of global CO2 emissions.11 In that light, Howarth (2014) argued for focusing on the 20-year rather than the 100-year period based on “the urgent need to reduce methane emissions over the coming 15–35 years.”12
Therefore the environmental impact of the mass of methane produced by public lands cattle is equivalent to 14,841,408,964 kg of CO2 (i.e., (GWP20: 86) × 172,574,522 kg of methane).
What does this mass of CO2 represent in terms of other forms of greenhouse gas emission or sequestration? The U.S. Environmental Protection Agency’s online Greenhouse Gas Equivalencies Calculator13 reports that it is equivalent to any of the following:
• annual greenhouse gas emissions from 3,124,507 passenger vehicles
• CO2 emissions from 1,670,013,278 gallons of gasoline consumed
• CO2 emissions from 34,514,902 barrels of oil consumed
• CO2 emissions from 196,471 tanker trucks’ worth of gasoline
• CO2 emissions from the electricity use of 2,041,459 homes for one year
• CO2 emissions from the energy use of 1,354,143 homes for one year
• CO2 emissions from burning 15,941,361,976 pounds of coal
• CO2 emissions from burning 79,579 railcars’ worth of coal
• CO2 emissions from 618,392,000 propane cylinders used for home barbecues
• CO2 emissions of 3.9 coal-fired power plants for one year
• carbon sequestered by 380,548,923 tree seedlings grown for 10 years
• carbon annually sequestered by 12,165,089 acres of U.S. forests
• carbon annually sequestered by 114,597 acres of forest preserved from conversion to cropland.
Having determined the quantity of methane produced by cattle that graze on public lands, one might ask whether removing these cattle would reduce the greenhouse gas contribution of these public lands by that amount. The answer to that question is beyond the scope of this essay. But I will mention a few of the factors that must be considered in seeking the answer.
Removal of cattle from public lands would allow several ecosystem components to begin their recovery from more than a century of harmful impacts. In particular, vegetation that had been consumed by cattle would now be available for wildlife. Consequently, we would expect wildlife populations to increase. And among that wildlife would be native ruminant mammals, such as pronghorn and deer, which, like cattle, emit methane as a by-product of their digestion. But they produce the gas in much smaller quantities than cattle. For example, an individual deer produces on average only 22 grams of methane per day14—approximately 5% the amount produced by a cow.
Following the exclusion of cattle, research shows that land-based sources of atmospheric carbon sequestration may increase. For example, a Chinese temperate grassland after 20 years of grazing exclusion had increased its carbon storage in the top 40 cm of soil by 35.7%.15 Other research performed on a semiarid, 17-year grazer-excluded grassland in northwest China found similar benefits to sequestration of carbon (C) and nitrogen (N). The researchers state: “Our results showed that the aboveground biomass, root biomass and plant litter were 70–92%, 56–151% and 59–141% higher, respectively, in grazer excluded grassland than in grazed grassland. Grazing exclusion significantly increased C and N stored in plant biomass and litter and increased the concentrations and stocks of C and N in soils. Grazing exclusion thus significantly increased the C and N stored in grassland ecosystems. The
increase in C and N stored in soil contributed to more than 95% and 97% of the increases in ecosystem C and N storage.”16
Then there are the microbiotic crusts17 whose prevalence across deserts of the West has been greatly reduced by the trampling of cattle. These crusts “can be dominant sources of productivity and carbon sequestration in extremely dry environments.”18 But since damaged crusts may require from 40 to 250 years to fully recover,19 depending on environmental conditions, significant carbon sequestration by the crusts may not be achieved for many years.
Quantifying the biological and chemical processes of these and other greenhouse gas sources and sinks following the cessation of cattle grazing would be a daunting task—one made even more difficult by the need to account for impacts on vegetation and wildlife from future global climate change.
1. Henning Steinfeld, Pierre Gerber, Tom Wassenaar, Vincent Castel, Mauricio Rosales, and Cees de Haan. Livestock’s Long Shadow: Environmental Issues and Options
, Food and Agriculture Organization of the United Nations, 2006, http://www.fao.org/docrep/010/a0701e/a0701e00.htm
(accessed 18 July 2015).
2. European Vegetarian Union, “Less Meat, Less Heat - IPCC Chairman Insists on Eating Less Meat” (press release, 31 August 2008), European Vegetarian
2 (2008): 14, http://www.euroveg.eu/lang/en/news/magazine/pdf/2008
(accessed 18 July 2015).
, s.v. “Enteric fermentation,” https://en.wikipedia.org/wiki/
(accessed 19 July 2015).