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

Abstract

The objective of this project was to dem- onstrate differences in methane (CH4) emission estimates between 2 reporting entities and to illustrate how the con- tribution to climate warming of enteric CH4 emissions is dependent on accounting methodologies. 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 ob- tained from the USDA-National Agricultural Statistics Service (NASS) website and EPA spreadsheet. Enteric CH4 emission estimates from both sources were then ex- pressed as CO2 equivalence (CO2-e) and warming equiva- lence (CO2-we) using both the 100-yr global warming potential (GWP100) and the newer global warming po- tential* (GWP*) methodologies. 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 inven- tory for the EPA data set (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 data sets. This work highlight- ed 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 con- tribution 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.