The gassy rumblings of ruminating cattle, along with the hisses and sloshes as natural gas is extracted, refined, and transported to communities across the United States, may release 50 percent more methane into the atmosphere than the government had estimated, according to a report published Monday by Harvard University scientists. Because methane (CH4) is a potent greenhouse gas, the new findings could tilt the international debate on the safety of natural gas, which the White House has promoted as a "clean energy" source responsible for a domestic manufacturing boom.

The nearly 90 million cattle who spend their days in US feedlots are the country's largest source of methane from anthropogenic (human-caused) emissions, the Environmental Protection Agency estimates. But the new report finds that ruminant animals generate twice as much methane as the EPA supposed. In Texas, Oklahoma, and Kansas, anthropogenic methane emissions from all sources were 2.7 times greater than believed, making up 24 percent of the nation's emissions, found the study, which was published in the Proceedings of the National Academy of Sciences.

The report comes on the heels of a decision by the EPA to reduce its estimates – by 25 percent to 30 percent – of the atmospheric carbon released by the natural-gas industry.

"These results cast doubt on the EPA's recent decision," wrote the researchers for the report. "Overall, we conclude that methane emissions associated with both the animal husbandry and fossil fuel industries have larger greenhouse gas impacts than indicated by existing inventories."

The MIT Technology Review described how the study might change the perceptions of different fossil fuels:

"At stake is whether switching from coal to natural gas can provide a net benefit in reducing greenhouse-gas emissions. Burning natural gas releases about half as much carbon dioxide as burning coal. But that benefit could be offset by leaks of methane, the primary component of natural gas."

President Obama remains a strong supporter of the natural-gas industry, but he has begun discussing it in more measured terms. In his 2012 State of the Union address, he praised the industry's cleanliness and economic promise, which he said was "proving that we don't have to choose between our environment and our economy." But his 2013 Economic Report included some reservations:

"Measuring fugitive methane emissions from the U.S. natural gas supply chain and, more generally, understanding the potential impacts of natural gas development on water quality, air quality, ecosystems, and induced seismicity, are critical to understanding the impact on the environment of the increasing use of natural gas."

The report also called anthropogenic greenhouse gases "the most significant long-term pollution challenge facing America."

After carbon dioxide, methane is the second most prevalent greenhouse gas, and it affects the atmosphere's ability to oxidize other pollutants. According to the report, anthropogenic methane emissions account for 50 to 65 percent of the global methane "budget," the largest portion of which comes from cattle. The natural-gas industry is the next largest source, followed by fermentation in landfills and then coal mining.

So, why are the results of this report so different from government data?

These scientists took a somewhat more on-the-ground (and, it turns out, up-in-the-air) approach. Whereas the EPA bases its estimates on assumed emissions per animal, or per unit of coal or gas sold, these researchers gathered nearly 13,000 measurements of airborne methane from points on the ground, on telecommunications towers, and on airplanes. They also used known information about population density and economic activity to try to determine the relative responsibilities of different methane-generating sectors, in different regions.

But, the MIT Technology Review wrote, the study does not provide "the last word" on methane pollution. "For one thing, it doesn’t directly measure emissions from specific sources, so it doesn’t pinpoint causes of leaks. As more data is gathered, steps can be taken to reduce methane leaks; for example, natural-gas producers and [distributors] could be required to follow best practices."

The EPA said Monday that it is reviewing the Harvard study. "EPA is committed to using the best available data for our inventory and continually seeks opportunities to update and improve our estimates," the agency said in a statement.

Natural-gas industries have been less welcoming of the news.

"Australia's coal seam gas industry has rejected" the study, saying it conflicts with existing information on natural-gas extraction, reports Australia's Sydney Morning Herald.

A source in the US natural gas industry said officials there were not ready to comment on the report.

Just one day before the Harvard report was published, a group of US and Russian scientists published a study in Nature Geoscience, which found that bubbles of ancient methane are surfacing at increasing rates in the Arctic region, as warming oceans thaw underwater permafrost and increasingly fierce storms break up structures that kept them underwater.

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"Increasing storminess and rapid sea-ice retreat causing increased CH4 fluxes from the [East Siberian Arctic Shelf] are possible new climate-change-driven processes," the scientists wrote. "Continuing warming of the Arctic Ocean will strengthen these processes."

On Friday, US Defense Secretary Chuck Hagel announced the Pentagon's first-ever Arctic strategy to maintain peace and cleanliness in the rapidly fracturing region, as global warming makes it increasingly vulnerable to drillers and shipping companies.