Methane, a potent global warming gas, is bubbling out of the frozen Arctic faster than had been expected.

Methane had become trapped in the permafrost over time and a warming climate is now resulting in its release, researchers report in Friday's edition of the journal Science.

"The amount of methane currently coming out of the East Siberian Arctic Shelf is comparable to the amount coming out of the entire world's oceans," said Natalia Shakhova, of the University of Alaska Fairbanks International Arctic Research Center and the co-author.

Concerns about global warming have centered on rising levels of carbon dioxide in the atmosphere, but scientists note that methane can be 30 times more effective at trapping heat than carbon dioxide.

Historically, methane concentrations in the world's atmosphere have ranged between 0.3 and 0.4 parts per million in cool periods to 0.6 to 0.7 in warm periods. Current methane concentrations in the Arctic average about 1.85 parts per million, the scientists said, the highest in 400,000 years.