Discrepancies in calculations linked to fossil fuel production, cattle farming
The United States emits far more methane than government estimates indicate, according to new research published November 25 in the Proceedings of the National Academy of Sciences.
In the study, researchers calculate that the United States released 44.5 billion kilograms of methane in 2008, which is more than 1.5 times higher than estimates from the U.S. Environmental Protection Agency and the European Union. That gap is the result of errors in how EPA and the EU estimate methane emissions from oil and gas facilities and cows, the team says.
The agencies compute emissions by counting methane sources such as oil rigs and cows, and then multiplying by estimates of how much gas each emitter gives off. Using this technique, EPA found that the United States put out around 29.4 billion kilograms of methane in 2008; the EU figure for the United States is 26.1 billion. Although that’s a tiny fraction of the roughly 5.5 trillion kilograms of carbon dioxide the United States releases annually, methane is a much stronger greenhouse gas. According to EPA, methane accounts for around 9 percent of U.S. greenhouse gas emissions.
In the new study, a team of scientists led by Harvard earth scientist Scot Miller studied methane measurements from communications towers and airplanes, gathered by several U.S. agencies from 2007 to 2008. The researchers combined those data — more than 10,000 observations — with weather simulations and a sophisticated statistical method to estimate national methane emissions.
After finding higher methane emissions than the government estimates, Miller and his colleagues wanted to know what accounted for the discrepancy. The researchers observed that methane measured over Texas and Oklahoma, states with plenty of fossil fuel production and cattle ranches, was more than twice as high as the EU estimates for these regions. (EPA does not provide regional estimates.) Pinpointing the source of the difference further, the scientists found that the gas propane accompanied methane emissions over Texas and Oklahoma but not over other areas. Propane often escapes into the atmosphere where fossil fuels are produced but does not come from cows. So its presence suggests that EPA and the EU are underestimating how much methane is leaking from oil and gas facilities in this region, Miller says. The finding “gives us a pretty good idea that fossil fuel industries play at least one large piece in the discrepancy.”
The team also found that methane emissions over the upper Midwest were up to twice EU estimates. Because this region is mostly agricultural, the researchers say this indicates livestock also account for a substantial part of the nationwide discrepancy.
These results will help scientists narrow the gap between emission estimates from different methods, says Steven Hamburg, chief scientist at the Environmental Defense Fund, which partially funded the study. “It’s an important piece of the puzzle.”
EPA press secretary Alisha Johnson says the agency has not reviewed the new study. But EPA "continually seeks opportunities to update and improve our estimates" of methane emissions, she adds. "Research studies like these will add to our knowledge base of greenhouse gas emissions and will help us refine our estimates going forward," Johnson says.
S.M. Miller et al. Anthropogenic emissions of methane in the United States. Proceedings of the National Academy of Sciences. Published online November 25, 2013. doi: 10.1073/pnas.1314392110.
E. Wayman. Cuts in some greenhouse gases could slow sea level rise. Science News. Vol. 183, May 4, 2013, p. 20.
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