Implied climate warming contributions of enteric methane emissions are dependent on the estimate source and accounting methodology

Authors: Beck, Matthew; THOMPSON, LOGAN - Kansas State University; Thompson, Terra; STACKHOUSE-LAWSON, KIM - Colorado State University; ARCHIBUQUE, SHAWN - Colorado State University

Submitted to: Applied Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2022
Publication Date: 12/8/2022
Citation: Beck, M.R., Thompson, L., Thompson, T.N., Stackhouse-Lawson, K., Archibuque, S. 2022. Implied climate warming contributions of enteric methane emissions are dependent on the estimate source and accounting methodology. Applied Animal Science. 38(6):639-647. https://doi.org/10.15232/aas.2022-02344.
DOI: https://doi.org/10.15232/aas.2022-02344

Interpretive Summary: Beef and dairy production has received increasing societal scrutiny due to their contribution to greenhouse gas emissions. Methane is the primary greenhouse gas resulting from beef and dairy production. Typically, reporting entities such as the FAO and the EPA relate all greenhouse gases to carbon dioxide equivalence by multiplying the emissions of that gas by its global warming potential (GWP). This accounting methodology likely works well for gases that have a long lifespan in the atmosphere, such as nitrous oxide; however, it does not adequately capture the contribution of short-lived greenhouse gases to actual climate warming. For example, methane only lasts 9-12 years in the atmosphere. This means that if emissions are held near constant, then there is actually a decreasing concentration of methane in the atmosphere. Further, in periods where methane emissions are decreasing there can actually be a cooling effect. As such, a new technique termed GWP* has been developed. There is also great confusion in how much estimated enteric methane emissions because there is considerable differences between reporting body sources. To date there has been no investigation into how implementing GWP* can influence enteric methane’s implied contribution to climate warming. Accordingly, scientist with ARS (Bushland, TX), Kansas State University, and Colorado State University’s AgNext initiative aimed to 1) highlight how FAO compared with EPA derived enteric methane emissions differ and 2) how these estimates’ implied contribution to climate warming is influenced by accounting methodology, i.e., traditional GWP compared with the new GWP*. This work demonstrated that there are large differences in US enteric methane emission estimates from the FAO and EPA. These differences are largely due to methodological practices, where 99% of year-to-year variability is accounted for by cattle inventory for FAO, compared with only 45% for the EPA data. Despite these differences enteric methane’s implied contribution to warming was greatly reduced for both data sources when accounted for by the GWP* method. In the FAO data, the years 2013-2016 were negative, indicating a cooling effect, while this only occurred in 2015 for the EPA data. Importantly, reducing methane emissions could be viewed as a short-term solution to reduce total anthropogenic contributions to warming, as our society transitions to lower emission technologies.

Technical Abstract: Objective: The objective of this project was to demonstrate differences in methane (CH4) emission estimates between 2 reporting entities and to illustrate how the contribution to climate warming of enteric CH4 emissions is dependent on accounting methodologies. Materials and Methods: United States enteric CH4 emissions were accessed from the Food and Agriculture Organization (FAO) and US Environmental Protection Agency (EPA), and cattle inventory numbers were obtained from the USDA-National Agricultural Statistics Service (NASS) website and EPA spreadsheet. Enteric CH4 emission estimates from both sources were then expressed as CO2 equivalence (CO2-e) and warming equivalence (CO2-we) using both the 100-year global warming potential (GWP100) and the newer global warming potential (GWP*) methodologies. Results and Discussion: Almost all of the year-to-year variability in the FAO data set was explained by NASS cow and calves inventory (R2 = 0.99), whereas much less variability was explained by EPA cattle inventory for the EPA dataset (R2 = 0.45). The EPA estimates were consistently greater than FAO estimates, and only a small amount of variation was accounted for (R2 = 0.41). Despite these differences, GWP* methodologies produced much smaller CO2-e values compared with GWP100 for both datasets. Implications and Applications: This work highlighted several important concepts to understand. First, it is important to understand the methodology for estimating enteric CH4 emissions used by different reporting bodies. Second, with both the FAO and EPA data sources, GWP* methodology provides a smaller estimate of warming contribution than the GWP100 method. Finally, this evaluation also highlights how this greenhouse gas accounting method is not a panacea for this source of greenhouse gas emissions.