U.S. methane emissions probably exceed estimates, study finds

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WASHINGTON — U.S. emissions of the greenhouse gas methane are probably 50 percent higher than current estimates show, according to a study published Monday in the Proceedings of the National Academy of Science.

The study estimated emissions in 2007 and 2008, using measurements on the ground, in telecommunications towers and from aircraft for a comprehensive inventory of the second most abundant greenhouse gas after carbon dioxide. It found that the U.S. now underestimates methane releases from the raising of livestock and the extraction of oil and natural gas.

That may mean methane has a bigger role in climate change than now thought, as state officials and the EPA consider new rules designed to limit emissions that may lead to global warming.

"Methane is a powerful climate change pollutant, and the study gives greater impetus to the EPA and states to establish stronger standards to reduce leaks from the oil and gas system," Dan Lashof, who directs the climate and clear air program at the Natural Resources Defense Council, said in an interview.

Texas, Oklahoma and Kansas represent a significant source of domestic methane emissions, Scot Miller, a co-author of the study, said in an interview.

"The fossil fuel industry are probably an important source of methane gas emissions in that part of the country," said Miller, who's pursuing a doctorate in Earth and Planetary Sciences at Harvard University in Cambridge, Mass.

Emissions of methane, the main component of natural gas, have newfound prominence with the rise of hydraulic fracturing and the sharp growth of oil and natural gas production in the U.S. In the fracking process, water, sand and chemicals are shot underground to break apart rocks and free trapped natural gas or oil.

Methane accounted for 9 percent of the greenhouse gases emitted in the U.S. in 2011, according to the EPA. While methane is 21 times more potent at trapping heat in the atmosphere, it stays in the atmosphere for less time than carbon dioxide.

Burning natural gas emits about half of the carbon that coal does when burned, but its climate benefits can be undercut by methane leaks at well sites and in transporting the fuel. Carbon dioxide accounted for 84 percent of U.S. greenhouse gas emissions in 2011, according to the EPA.

Miller said the study's authors are now examining more recent data, which would account for the sharp increase in oil and gas drilling in the U.S. in recent years.

In a statement, the EPA said it was "encouraged that more methane emissions measurement data are now available to the public."

Colorado said last week that it would become the first state to limit methane emissions from oil and gas production.