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U.S. methane emissions may be 50% more than EPA measure


CAMBRIDGE, Mass. -- U.S. emissions of methane, a potent heat-trapping greenhouse gas, may be 50% higher than federal estimates, reported a team of Harvard and other researchers today.

Nationwide, emissions from cows and livestock operations may be twice as high as previously thought, and in the south-central region, those from fossil fuel extraction and refining may be almost five times higher than calculated by the U.S. Environmental Protection Agency, according to a study in the Proceedings of the National Academy of Sciences.

"It' s really a very clear signal" of how much methane U.S. industry and other sources emit, says co-author Anna Michalak of the Carnegie Institution of Science' s Department of Global Ecology. She says the study of the continental U.S. combines an unprecedented amount of data, taken by federal agencies from the tops of telecommunication towers, with newer statistical tools and meteorological models to calculate how much methane is actually in the atmosphere, and where it probably came from.

This top-down approach is notably different from the EPA' s bottom-up estimates, which calculate emissions based on the amount of methane typically released per cow or per unit of coal or natural gas sold.

"The main result is significant," said Colm Sweeney of the University of Colorado-Boulder, who leads the aircraft group that does flyovers to measure methane for NOAA' s Earth System Research Laboratory Global Monitoring System. Sweeney, who was not involved in the new study, said it provides a helpful overall picture, quantifies the discrepancy between the two approaches and shows the need for more observing stations.

"We don' t know why" there' s such a discrepancy, said study co-author Steven Wofsy, an atmospheric and environmental professor at Harvard University' s School of Engineering and Applied Science. Wofsy said it may be that the EPA is not measuring every possible source, such as broken natural gas pipes that are leaking methane.

He says this top-down analysis echoes the overall findings of a study he did in 2006, but offers much finer detail that will be used to create a national methane database. Yet he says that, although it may indicate what role the natural gas industry plays, it can' t distinguish whether emissions come from drilling, processing or refining.

"This study seems very plausible," said David Archer, a climate scientist at the University of Chicago' s Department of Geophysical Sciences who was not involved in the research. He says if there are more methane emissions than previously thought, "that means there' s more we can cut."

He said, unlike carbon dioxide, which accumulates and lingers in the atmosphere for hundreds of years, methane tends to degrade in a decade or so. Also unlike CO2, he notes, U.S. methane concentrations have held fairly steady in the past 20 years.



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