In this essay I will estimate the annual contribution to atmospheric green-house gases from methane (CH4) that results from enteric fermentation1 in cattle that graze on U.S. federal public lands. I’ll also compare the CH4 con-tribution of public lands cattle to that of several other sources and sinks of CH4, including emission of CH4 due to oil and gas production on federal lands.

The amount by which the public lands contribution to atmospheric CH4 would change as a result of cattle removal is a more complex matter than are the above-mentioned topics. In the absence of ranching operations, the re-covery of native flora and fauna would provide many new sources and sinks of atmospheric CH4. Although a detailed greenhouse gas analysis of such re-covered ecosystems is beyond the scope of this essay, I will note a few factors that such an analysis should include.

Based on the measurement that a typical grass-fed cow emits 600–700 liters (L) of CH4 per day,2 the mass of this gas annually produced by cattle that graze on 250 million acres of federal public lands managed by the U.S. Forest Service and the Bureau of Land Management (BLM) can be estimated.3 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 CH4 production.

The BLM4 and U.S. Forest Service5 report annual forage utilization from their lands by cattle of 7,920,576 and 6,380,872 animal unit months (AUMs) respectively, with the combined forage utilization being 14,301,448 AUMs.

As each AUM represents 31 days of forage consumption by a cow and her suckling calf, it likewise represents 31 days of the cow’s CH4 production, and therefore each AUM consumed produces 18,600 L of CH4.6

Based on the total number of AUMs used per year and the volume of CH4 emitted per AUM, the annual volume of CH4 produced by public lands cattle is equal to 266,006,932,800 L.7 Since 1,000 L are equivalent in volume to 1 cubic meter (m3), public lands cattle produce 266,006,932 m3 of CH4 per year.

Based on the density of CH4 being 0.68 kg/m3, under assumed conditions of 1.013 bar (one atmosphere) and 15°C (59°F),8 the mass of this volume is 180,884,714.3 kg.9

Gases, such as CH4, contribute to global warming. The relative ability of CH4 to trap heat in the global climate system over a given time frame (com-pared to CO2) is expressed by CH4’s “global warming potential” (GWP).10 Internationally accepted values for CH4’s GWP (with climate-carbon feedback) are “34” over a 100-year interval (GWP100) and “86” over a 20-year interval (GWP20).11 Stated otherwise, over a 20-year interval, a given mass of CH4 would have the same effect in the global climate system as a mass of CO2 that is 86 times greater than that mass of CH4.12

Authors of climate-related articles have often chosen to consider CH4’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.”13 Moreover, the IPCC found that at the 20-year timescale, total global emissions of CH4 are equivalent to over 80% of global CO2 emissions.14 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.”15

Applying GWP20 for CH4 of 86, the environmental impact of the mass of CH4 produced by public lands cattle is equivalent to 15,556,085,430.14 kg of CO2 (over a 20-year interval).16

The U.S. Environmental Protection Agency’s online Greenhouse Gas Equivalencies Calculator17 reports that the 15,556,085,430.14 kg of CO2-equivalent annually cow-emitted CH4 is equivalent to any of the following:

In addition to providing forage for livestock, federal lands produce sig-nificant quantities of oil and natural gas. In FY2014 these lands produced 148,802.95 thousand barrels of oil and 2,499,845.86 million cubic feet of natural gas.18

In the course of their installation and operation, these oil and gas wells waste natural gas (primarily consisting of CH4) through venting, flaring, and leaks. An Environmental Defense Fund report dated September 201519 cites an analysis by ICF International20 that estimates these losses for 2013. Based on a graphic produced from the ICF report,21 I compute that these lands accounted for CH4 emissions of approximately 47.2 billion ft3, or equivalently 1,336,555 m3. The 266,006,932 m3 year-1 of CH4 emitted by the cattle on these lands therefore amounts to 19.9% of the CH4 emissions from current oil and gas production.22

On 15 November 2016, the BLM issued a rule that the agency has stated would annually reduce by between 175,000 and 180,000 tons the CH4 that is wasted during the construction and operation of gas and oil wells on public and tribal lands.23 Assuming the less optimistic, lower, value of the range, the CH4 annually emitted by the public lands cattle represents 113.9% of this antici-pated annual reduction.24

Having determined the quantity of CH4 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. Although the answer to that question is beyond the scope of this essay, I will indicate a few of the factors that must be considered in seeking the answer.

Removing cattle from public lands would allow several ecosystem com-ponents 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 elk, pronghorn and deer, which, like cattle, emit CH4 as a by-product of their digestion. But such animals produce the gas in much smaller quantities than cattle. For example, an individual deer produces on average only 31.5 grams of CH4 per day25—approximately 7.7% of the amount produced by a cow.26

Perhaps the removal of ranching from federal public lands would reduce much of the public opposition to restoration of predator populations, especially wolves. If predator populations were to increase, they would tend to limit the populations of native ungulate CH4 producers.27

Following the exclusion of cattle, research shows that land-based sources of atmospheric C sequestration may increase. For example, a Chinese temperate grassland after 20 years of grazing exclusion had increased its C storage in the top 40 cm of soil by 35.7%.28 Other research performed on a semiarid, 17-year grazer-excluded grassland in northwest China found similar benefits to sequestration of 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 grass-land 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.”29

Microbiotic crusts,30 which were once prevalent across deserts of the American West, “can be dominant sources of productivity and C sequestration in extremely dry environments.”31 But more than a hundred years of trampling by cattle has markedly reduced the presence of these crusts. And even when cattle impacts are removed, crusts may require from 40 to 250 years to fully recover,32 depending on environmental conditions. Consequently, significant C sequestration by the crusts may not be achieved until many years after the removal of cattle.

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 anticipate impacts on vegetation and wildlife from global climate change.

The author thanks T. Shuman for his comments on previous drafts of this essay.