Enteric methane emissions
Where we currently stand, why we should care and future mitigation scenarios?
By Matthew R. Beck and Logan R. Thompson
The U.S. beef industry has received much attention from society at large and researchers for its contribution to climate change. Much of this attention is due to the relatively large contribution of enteric methane, that is methane derived from the digestive process in all ruminant species, such as cattle, bison, goats, deer, etc. At 26% of total U.S. methane emissions, enteric methane is the largest source. The next largest source of U.S. methane emissions result from natural gas systems (23% of total; EPA, 2022).
Greenhouse gas reporting and accounting agencies relate warming potential of different greenhouse gases to an equivalence basis. This is commonly achieved by both national (U.S. Environmental Protection Agency; EPA) and international (Intergovernmental Panel on Climate Change; IPCC) reporting bodies by multiplying a given gases’ emission rate (mass per year) by its global warming potential (GWP). The GWP value is, in essence, a time-integrated radiative forcing, or simply an energy flow, from a pulse emission of a specific gas over a specific period, most commonly a 100-year basis (GWP100).
For all greenhouse gases, this value is always compared against carbon dioxide, and thus carbon dioxide has a GWP of 1. The GWP100 value for methane is 28, such that methane emission rate multiplied by 28 will equal the carbon dioxide equivalence (CO2-eq.) of methane.The GWP approach has been the means of relating non-carbon dioxide emissions to a CO2-eq. since the IPCC released its first assessment report in 1990. The GWP100 method has become an international convention since the Kyoto Protocol to the United Nations Framework Convention on Climate Change occurred in December of 1997.
Since GWP was initially adopted, its ability to relate greenhouse gases with a short atmospheric lifespan has been debated (Smith and Wigley, 2000; Shine et al., 2005). In fact, it has been well established that the value derived from the GWP100 method does not relate to a short-lived greenhouse gas emission, such as methane, actual contribution to warming (Allen et al., 2018; Cain et al., 2019). It does, however, do an adequate job of relating directional changes and its long-standing use has made it the standard for policy setting around global warming issues.
Methane is a short-lived greenhouse gas that only lasts in the atmosphere for around 12 years. In time periods where total methane emission rates are measurably reduced, the concurrent conversion of existing methane to carbon dioxide and water vapor will result in a net reduction in atmospheric methane levels.
To account for this nuanced atmospheric behavior of short-lived greenhouse gases, an alternative method to relate these gases to a CO2-eq. basis has been developed by atmospheric scientists at the University of Oxford (Allen et al., 2018; Cain et al., 2019). This relatively new metric is termed GWP-star (GWP*). This metric relates current methane emission rates to emission rates from 20-years ago, to account for changes in emission rates over time. It has been demonstrated that the GWP* method relates well to actual effects on climate change (Cain et al., 2019; Smith et al., 2021).So, where do the enteric methane emissions from beef production currently stand? Well, it depends on which emissions accounting lens you are looking through, either GWP100 or GWP*. Total enteric methane emissions account for 3.1% of total U.S. greenhouse gas emissions but 32.6% of total agriculture greenhouse gas emissions, when employing the GWP100 method (EPA, 2022). This puts enteric methane as the second largest agriculture emission source, behind agriculture soil management.
Of the total U.S. enteric methane emissions, 71.4% are from beef and 25.2% are from dairy. This means that enteric methane emissions from beef represent 2.2% of U.S. total emissions. By sector of the beef industry, the cow-calf sector represents 72.6%, the stocker sector represents 14.5%, and the feedlot sector represents 12.8% of beef enteric methane. This means that enteric methane from the cow-calf sector represents 1.6%, the stocker sector represents 0.3%, and the feedlot sector represents 0.27% of total U.S. greenhouse gas emissions (EPA, 2022).
Now, let’s determine how much of U.S. total emissions that enteric methane emissions from the beef industry represent when calculated using the GWP* methodology. To do this, we recalculated total U.S. emissions in CO2-eq using GWP* for all methane emissions and for enteric methane emissions. Implementing GWP* reduced the implied climate warming contribution of enteric methane by 92% for all sectors on average from 2010 to 2020 relative to the GWP100 method (Beck et al., 2023). Using the GWP* metric, enteric methane represents only 0.76% of total U.S. emissions, with enteric methane from beef only representing 0.17% of total U.S. emissions. This implies that the beef industry’s implied contribution to climate change is influenced by the metric of choice.
No matter how enteric methane is related to a CO2-eq., by GWP100 or GWP*, it represents a relatively small contribution to total U.S. greenhouse gas emissions. For example, enteric methane emissions represent 3.1% of total U.S. greenhouse gas emissions when calculated using GWP100 and 0.76% of total U.S. greenhouse gas emissions when calculated by GWP*. However, it is important to mention that regardless of the accounting method, it is clear that there are still opportunities to reduce the impacts of beef enteric methane emissions on climate change.
So, why should we care about enteric methane emissions? When implementing GWP* it can be demonstrated that a reduction of 0.32% of methane per year results in a CO2-eq. of 0 (Figure 1; Beck et al., 2022). This means that if methane emissions are reduced further than a 0.32% annual reduction, then the calculations would yield a negative value. This finding agrees with efforts that explored methane mitigation scenarios and their subsequent effects on climate warming (Cain et al., 2019; Ocko et al., 2021). Based on this relationship, mitigating methane emissions represents a very promising means to reduce climate warming in the short term (<20-years; Ocko et al., 2021).
It is important to highlight beef cattle production plays an important role in numerous aspects of society and ecosystem function, especially related to food security, grassland and rangeland ecosystem health, fighting wildfires, and rural economy sustainability (White and Hall, 2017; Manzano and White, 2019; Manzano et al., 2023).
There are promising opportunities to reduce enteric methane from cattle as a means to contribute to broader societal climate stability goals.
To demonstrate the ability of enteric methane reduction practices to provide negative CO2-eq. (i.e., provide emissions offsets) we ran through 4 scenarios, which are displayed in Figure 2 (Beck et al., 2023). The scenarios considered are either no-change, a 30% reduction, an 80% reduction, and a 100% reduction with GWP100 or GWP* being applied to the emissions.
One potential road-block to implementing enteric methane mitigation strategies is a general lack of research in pasture systems. This is immensely important because a reduction in emissions from the cow-calf sector would have the largest potential to reduce emissions compared to any other sector of the beef industry.
For example, a 30% reduction in methane emissions in the cow-calf sector results in an average offset of -93.3 million metric tons of CO2-eq. per year for 20-years (Figure 2, when using the GWP* calculation). This is a larger offset in emissions than if the stocker and feedlot industry reduced their emissions by 100%, which would be an offset of -74.1 and -67.8 million metric tons of CO2-eq. per year, respectively.
This demonstrates that for the beef industry to make significant strides in reducing enteric methane, a reduction in emissions from the cow-calf sector will need to be first and foremost.
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Beck is a research animal scientist at the USDA-ARS Livestock Nutrient Management Research Unit. Thompson is an assistant professor in the Department of Animal Sciences and Industry, Kansas State University.
