New study finds US anthropogenic methane emissions 1.5 times higher than previously estimated by EPA
Using a new methodology, a study published in Proceedings of the National Academy of Sciences (PNAS) has concluded that anthropogenic emissions of methane from agriculture and fossil fuel extraction and processing (i.e., oil and/or natural gas) are likely significantly greater than cited in existing studies.
The researchers quantitatively estimated the spatial distribution of anthropogenic methane sources in the United States by combining comprehensive atmospheric methane observations, extensive spatial datasets, and a high-resolution atmospheric transport model.
The authors are from Harvard University; Carnegie Institution for Science; University of Michigan; National Oceanic and Atmospheric Administration; Lawrence Berkeley National Laboratory; Atmospheric and Environmental Research; European Commission Joint Research Centre; and University of Colorado Boulder.
Results indicated that emissions due to ruminants and manure are up to twice the magnitude of existing inventories, and that regional methane emissions due to fossil fuel extraction and processing could be 4.9 ± 2.6 times larger than in EDGAR, the most comprehensive global methane inventory.
… the discrepancy in methane source estimates is particularly pronounced in the south-central United States, where we find total emissions are ∼2.7 times greater than in most inventories and account for 24 ± 3% of national emissions. The spatial patterns of our emission fluxes and observed methane–propane correlations indicate that fossil fuel extraction and refining are major contributors (45 ± 13%) in the south-central United States.
… These results cast doubt on the US EPA’s recent decision to downscale its estimate of national natural gas emissions by 25–30%. Overall, we conclude that methane emissions associated with both the animal husbandry and fossil fuel industries have larger greenhouse gas impacts than indicated by existing inventories.—Miller et al.
Overall, results show that current inventories from the US Environmental Protection Agency (EPA) and the Emissions Database for Global Atmospheric Research underestimate US methane emissions nationally by a factor of ∼1.5 and ∼1.7, respectively.
The difference lies in the methodology. The EPA and EDGAR use a bottom-up approach, calculating total emissions based on emissions factors—the amount of methane typically released per cow or per unit of coal or natural gas sold, for example. The new study takes a top-down approach, measuring what is actually present in the atmosphere and then using meteorological data and statistical analysis to trace it back to regional sources.
Scot M. Miller, Steven C. Wofsy, Anna M. Michalak, Eric A. Kort, Arlyn E. Andrews, Sebastien C. Biraud, Edward J. Dlugokencky, Janusz Eluszkiewicz, Marc L. Fischer, Greet Janssens-Maenhout, Ben R. Miller, John B. Miller, Stephen A. Montzka, Thomas Nehrkorn, and Colm Sweeney (2013) “Anthropogenic emissions of methane in the United States” PNAS 2013 doi: 10.1073/pnas.1314392110