The East Siberian Sea is bubbling with methane, a powerful greenhouse gas, being released from underwater reserves, according to a recent expedition.

This could be a sign that global warming is thawing underwater permafrost, which is releasing methane that has been locked away for many thousands of years.

If these methane emissions from the Arctic speed up, it could cause "really serious climate consequences," said expedition member Igor Semiletov of the University of Alaska, Fairbanks.

(Related: "Global Warming Feedback Loop Caused by Methane, Scientists Say" [August 29, 2006].)

Semiletov and colleagues have traveled along the Siberian coast—this year they covered 13,000 miles (22,000 kilometers)—while monitoring methane concentrations in the air and observing the seas.

"According to our data, more than 50 percent of the Arctic Siberian shelf is serving as a source of methane to the atmosphere," Semiletov said.

This vast shelf is about 750,000 square miles (2 million square kilometers)—about the same size as Greenland or Mexico—and about 80 percent of it is covered with permafrost, Semiletov said.

He presented the findings from his group at an American Geophysical Union meeting in San Francisco this week.