Livestock Use on Public Lands in the Western USA Exacerbates Climate Change: Implications for Climate Change Mitigation and Adaptation

Abstract

Public lands of the USA can play an important role in addressing the climate crisis. About 85% of public lands in the western USA are grazed by domestic livestock, and they influence climate change in three profound ways: (1) they are significant sources of greenhouse gases through enteric fermentation and manure deposition; (2) they defoliate native plants, trample vegetation and soils, and accelerate the spread of exotic species resulting in a shift in landscape function from carbon sinks to sources of greenhouse gases; and (3) they exacerbate the effects of climate change on ecosystems by creating warmer and drier conditions. On public lands one cow-calf pair grazing for one month (an "animal unit month" or "AUM") produces 875 kg CO₂e through enteric fermentation and manure deposition with a social carbon cost of nearly $36 per AUM. Over 14 million AUMs of cattle graze public lands of the western USA each year resulting in greenhouse gas emissions of 12.4 Tg CO₂e year^-1. The social costs of carbon are > $500 million year^-1 or approximately 26 times greater than annual grazing fees collected by managing federal agencies. These emissions and social costs do not include the likely greater ecosystems costs from grazing impacts and associated livestock management activities that reduce biodiversity, carbon stocks and rates of carbon sequestration. Cessation of grazing would decrease greenhouse gas emissions, improve soil and water resources, and would enhance/sustain native species biodiversity thus representing an important and cost-effective adaptive approach to climate change.

Fig. 1

Vegetation change of a riparian ecosystem following cessation of grazing. The left photos are riparian zones on the Hart Mountain National Antelope Refuge, Oregon in 1990 which was the last year of grazing on these public lands. The right photos are the same sites about 24 years after cattle were removed. Wetland vegetation now predominates where there was mostly bare ground and exotic dry grasses. (Photos by W. Pyle and S.Ries)

Fig. 2

The interacting effects of livestock grazing and climate change on western rangelands. There are four primary immediate effects of livestock: herbage removal, trailing trampling effects, dispersal of exotics, and creation of metabolic and nonmetabolic waste products. Through time, these effects on native rangelands affect fire regimes, increase erosion, compact soils affecting ecosystem hydrology, and alter competitive relationships between plant species. These actions decrease the net ecosystem productivity (NEP) such that the rangelands shift from carbon sinks to net sources of greenhouse gases. Products of animal metabolism are significant additional sources of greenhouse gases, especially CH₄ and N₂O. Ultimately the results of grazing have led to a simplification of vegetation structure typified by increases in exotic, ruderal, and less palatable species, that are more adapted to the drier conditions created by lower water holding capacities of compacted soils. The shifts in species composition further decrease the capacity of rangeland ecosystems to function as carbon sinks. Other impacts of grazing include a decline in riparian vegetation structure, shifts to drier species dominance, and degraded stream channels which increase stream temperatures, ground surface temperatures and alter stream flows. The consequent shifts in the net ecosystem productivity of the landscape, coupled with GHG additions from livestock, results in additional contributions to the greenhouse gases causing climate change. The effects of livestock accentuate the effects of climate change such as increased stream and air temperatures, loss in biological diversity, and an overall decline in the productivity of rangelands (desertification). There are also strong feedbacks associated with climate change. The warmer and drier temperatures, and reduced snow pack associated with climate change interacts with livestock grazing to negatively affect stream flows, water quality and biological diversity. These factors result in further degradation and a lower capacity for carbon storage, hence higher greenhouse gas emissions

Fig. 3

Left photo: A long-term grazed site dominated by the annual exotic Cheatgrass (Bromus tectorum), Prineville District, BLM, Oregon. In addition to a dominance by exotic species, there is an absence of biotic soil crusts. The site had been burned about three years prior to the time this photo was taken. Right photo: An ungrazed site dominated by native species, Prineville District, BLM, Oregon. The dominant grasses are Bluebunch Wheatgrass (Pseudoregnaria spicata). The interspaces are dominated by native forbs, Sandberg’s Blue grass (Poa sandbergii) and biological soil crusts. Exotic annuals are <1% cover that this site. This site had also burned ≈3 years prior to the taking of this photo (Photos by J.B. Kauffman)

Fig. 4

Total aboveground carbon stocks for non-forested ecosystems occupying public lands of the intermountain West. Total aboveground carbon stocks range from 2.69 (Mg C ha⁻¹) for Wyoming Big sagebrush (A.t wyomingensis) communities to 7.8 Mg C ha⁻¹ for Basin big sagebrush (A. t. tridentata) stands. The aboveground carbon stocks of intermountain woodlands are 18.3 Mg C ha⁻¹ and for coniferous forests is about 97 Mg C ha⁻¹ (Law et al. 2018). In contrast, mean aboveground carbon stocks for converted stands were 0.5 Mg C ha⁻¹ for crested wheatgrass (Agropyron cristatum) and 0.2 Mg C ha⁻¹ for cheatgrass (Bromus tectorum) stands. There is an 84% decline in aboveground biomass when Wyoming Big sagebrush stands are converted to crested wheatgrass and an 88% decline when they are converted to cheatgrass

Fig. 5

Forage allocation for domestic livestock, feral horses, and wildlife on the Bureau of Land Management (BLM) Lakeview District, Oregon (USDI BLM 2003a)

Fig. 6

A The average number of animal unit months (AUMs) for cattle that utilized Bureau of Land Management (BLM) lands (2009–2018) and US Forest Service (USFS) lands in the western US (2007–2016). The totals (BLM + USFS) are means from the years 2009–2016. B The annual total emissions (Tg CO₂e) from enteric fermentation and manure deposition on western public lands for the same time periods as above. C The annual total methane emissions (Mg) from cattle grazing public lands. D The annual social cost of carbon from livestock on public lands (millions of US dollars). The standard errors are not included as they were less than 2% of the mean (Supplementary Information, Table S3)