20 Methane Accounting and Its Importance to the Beef industry’s Implied Contribution to Climate Warming

Abstract

Abstract At 27% of U.S. methane (CH4) emissions (3% of U.S. total emissions), enteric CH4 emissions represent the largest source of U.S. CH4 emissions, surpassing natural gas systems according to the latest report from the Environmental Protection Agency (EPA). Total manure CH4 emissions represent 1% of total U.S. emissions. Enteric and manure CH4 emissions from the beef industry represent 2.1% and 0.03%, respectively, of U.S. total emissions. These contributions of CH4 emissions to total U.S. greenhouse gas emissions are determined by relating CH4 emissions to a carbon dioxide (CO2) equivalent basis (CO2-e) by multiplying the amount of CH4 emissions by its global warming potential (EPA uses 25 for CH4 on a 100-y basis; GWP100). The GWP100 method was adopted following the Kyoto Protocol in 1997, and ever since this time its appropriateness for short-lived climate forcers (SLCF; e.g., CH4 has a 12-y atmospheric lifespan) has been debated. This debate exists because GWP100 is a misnomer, as it is known to have no relationship with the contribution of a gas to warming. Since CH4 is a SLCF, when emission rates are decreasing there is a decreasing concentration of CH4 in the atmosphere. To account for SLCF, an alternative accounting methodology, termed GWP-star (GWP*), has been proposed to convert SLCF to a CO2-warming equivalence (CO2-we). The CO2-we values from GWP* methodology has been demonstrated to closely relate to warming contributions compared with the CO2-e value from the GWP100 method. Applying the GWP* metric to enteric and manure CH4 emissions from the beef industry reduces the implied contribution to climate warming of these emission sources by 92% and 62%, on average from 2010-2020, respectively, compared with the GWP100 methodology. There are currently promising mitigation technologies emerging for mitigating enteric CH4 emission. For example 3-nitrooxypropanol may reduce enteric CH4 by 30% and Asparagopsis seaweed may reduce it by 80%. We explored the effects of these technologies on CO2-we if they had 100% adoption by the beef industry and if these mitigation potentials were consistent across all production systems (admittedly a big assumption). Both technologies would result in negative CO2-we for 20-y, after which a new baseline would be achieved and the CH4 emissions would begin contributing to warming again (i.e., accumulating CH4 in the atmosphere). During the 20-y of net-cooling, a 30% and 80% reduction in enteric CH4 would result in -143 (3% of U.S. total CO2 emissions) and -461 (10% of U.S. total CO2 emissions) million metric tons of CO2-we/y, respectively. Ultimately, adopting GWP* not only provides estimates that actually relate to contributions to climate warming, therefore providing a more appropriate metric than what is currently used, but also provides a means for the beef industry to leverage mitigation strategies to be a solution for climate change.