A large chunk of the Arctic seabed that sits on a methane reservoir has become unstable and is releasing some of the heat-trapping gas into the atmosphere, new research suggests.

It has long been believed that the frozen seafloor of the East Siberian Arctic Shelf formed an impermeable seal against the vast stores of methane trapped beneath it, but the latest findings suggest about 7.7 million tons of the gas are being released there each year.

It isn't clear whether this is an old phenomenon or a new one linked to global warming.

Of the roughly 500 million tons of methane emitted annually world-wide, an estimated 40% has a natural origin—such as wetlands and the digestive processes of termites—while the rest results from human activities, including cattle farming and fossil-fuel production.

"This particular source has never been taken into account" in tallying methane emissions, said Natalia Shakhova, researcher at the University of Alaska Fairbanks and lead author of the study, which appears in the journal Science. She said the thawing of the seabed could result from an increase in warmer runoff from the rivers that feed the Arctic Ocean.

A key worry is that the East Siberian Arctic Shelf is shallow, with a depth of about 50 meters (165 feet) or less. Methane oxidizes in deep water, becoming carbon dioxide before it reaches the surface. But in shallow water, methane doesn't have enough time to oxidize and more of it ends up in the atmosphere.

Concerns about global warming have centered on rising levels of carbon dioxide in the atmosphere, but scientists note that methane can be 20 times more effective at trapping heat than carbon dioxide.

Methane concentration in the atmosphere was as high as 0.7 parts per million in pre-industrial times, but reached about 1.7 ppm in the 1980s. The level has been stable for the past 15 years, but no one knows exactly why