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Methane emissions surpass previous estimates

Nov. 28, 2013 – Government calculations of total U.S. methane emissions may underestimate the true values by 50 percent, a new study finds.

The results were published in the early online edition of the Proceedings of the National Academy of Sciences, and cast doubt on a recent Environmental Protection Agency decision to downscale its emissions estimate. Carnegie’s Anna Michalak, Harvard’s Scot Miller and Steven Wofsy, and colleagues used atmospheric methane observations from across North America in 2007 and 2008 to improve estimates of methane gas emissions from a variety of human sources, including agriculture and fossil fuel drilling and refining.

The study found large discrepancies with government estimates in some regions of the United States, particularly the south-central US, specifically Texas, Oklahoma and Kansas, where total methane emissions were 2.7 times greater than those reported in most inventories. Emissions from oil and gas drilling and processing in this region could account for 50 percent of that total, representing a source of methane almost five times higher than in the most commonly used global emissions database. Other sources of the methane are likely from livestock, including manure and flatulence, the study says.

Methane is the second-most important greenhouse gas after carbon dioxide, and the study findings may help inform national and state greenhouse gas reduction strategies, according to the authors.

The team used modeling tools developed by Michalak’s lab that make it possible to trace variations in atmospheric methane measurements back to emissions throughout the U.S., and to relate the emissions to known economic sectors. The methods thereby provide a direct constraint on total emissions, as well as provide insight into what is behind them.