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

Objective: The objective of this project was to demonstrate differences in methane (CH 4 ) emission estimates between 2 reporting entities and to illustrate how the contribution to climate warming of enteric CH 4 emissions is dependent on accounting methodologies. Materials and Methods: United States enteric CH 4 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 CH 4 emission estimates from both sources were then expressed as CO 2 equivalence (CO 2 -e) and warming equivalence (CO 2 -we) using both the 100-yr 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 (R 2 = 0.99), whereas much less variability was explained by EPA cattle inventory for the EPA data set (R 2 = 0.45). The EPA estimates were consistently greater than FAO estimates, and only a small amount of variation was accounted for (R 2 = 0.41). Despite these differences, GWP* methodologies produced much smaller CO 2 -e values compared with GWP100 for both data sets. Implications and Applications: This work highlighted several important concepts to understand. First, it is important to understand the methodology for estimating enteric CH 4 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 evalu-ation also highlights how this greenhouse gas accounting method is not a panacea for this source of greenhouse gas emissions.