Comparison of Methodologies Used to Estimate Enteric Methane Emissions and Warming Impact from 1920 to 2020 for U.S. Beef Production

Estimates of historical enteric methane emissions by US beef cattle using various methodologies recommended by IPCC were compared, then translated using two expressions of carbon dioxide equivalence. Three existing methodologies (Tier 1, Tier 2, and Tier 3 as used by FAO, EPA, and NASEM, respectively) were compared using a common time series (1920 to 2020) for each sector of the US beef cattle production system. Annual enteric methane emissions were converted to annual CO2 equivalents for global warming potential on a 100-year horizon (GWP100) and CO2 warming equivalents (GWP*) to compare two expressions of methane equivalence to carbon dioxide. While the ranking of estimates among methods was stable, the magnitude of difference between the methods increased over time. In 1920, the Tier 3 method estimated emissions 16% greater than Tier 1; this difference increased to 60% greater in 2020. Cumulative GWP* ranged from 8.9% below to 29.4% below cumulative GWP100 in 2020, depending upon method; differences in annual emissions metrics were larger, with GWP* metrics ranging from 261% below to 123% above GWP100 expression. While several methods exist to generate emissions inventories, method choice results in substantial differences in direct emissions estimates and carbon dioxide equivalence.