Total methane emissions in the United States may be 50 percent higher than the amounts previously estimated by the U.S. Environmental Protection Agency (EPA), U.S. researchers said Monday.

The results cast doubt on a recent decision by the EPA to downscale its emissions estimate, the researchers reported in the U.S. journal the Proceedings of the National Academy of Sciences.

Methane, a potent greenhouse gas, is produced through natural gas production and distribution, cattle farming, landfills, coal mining, manure management, and many other anthropogenic and natural sources, though human activities are thought to contribute approximately 60 percent of the total.

Researchers from Harvard University, Carnegie Institute for Science and several other U.S. institutions 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 U.S., including 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 previous estimates, it said.

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

"This paper provides the most solid and the most detailed estimate to date of total U.S. methane emissions," coauthor Anna Michalak of the Carnegie Institution for Science, said in a statement. "It really offers an opportunity for governments to reexamine the inventories in light of what we now know." Endi

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