Methane, a potent global warming gas, is bubbling out of the frozen Arctic faster than had been expected.
Researchers report in Friday's edition of the journal Science that methane had become trapped in the permafrost over time and a warming climate is now resulting in its release.
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.
The researchers focused on a long-frozen seabed north of Siberia.
It was unclear, however, if the emissions were new or had been going on unnoticed for centuries — since before the Industrial Revolution of the 18th century led to wide use of fossil fuels that are blamed for climate change.
8 million tons a year
The study said about 8 million tons of methane a year, equivalent to the annual total previously estimated from all of the world's oceans, were seeping from vast stores long trapped under permafrost below the seabed.
"Subsea permafrost is losing its ability to be an impermeable cap," Natalia Shakhova, a scientist at the University of Fairbanks, Alaska, said in a statement. She co-led the study.
The experts measured levels of methane, a gas that can be released by rotting vegetation, in water and air at 5,000 sites on the East Siberian Arctic Shelf from 2003-08. In some places, methane was bubbling up from the seabed.
Previously, the sea floor had been considered an impermeable barrier sealing methane, Shakhova said.
"No one can answer this question," she said of whether the venting was caused by global warming or by natural factors. But a projected rise in temperatures could quicken the thaw.
"It's good that these emissions are documented. But you cannot say they're increasing," Martin Heimann, an expert at the Max Planck Institute for Biogeochemistry in Germany who wrote a separate article on methane in Science, told Reuters.
"These leaks could have been occurring all the time" since the last Ice Age 10,000 years ago, he said. He wrote that the release of 8 million tons of methane a year was "negligible" compared to global emissions of about 440 million tons.
Shakhova's study said there was an "urgent need" to monitor the region for possible future changes since permafrost traps vast amounts of methane, the second most common greenhouse gas from human activities after carbon dioxide.
Monitoring could resolve if the venting was "a steadily ongoing phenomenon or signals the start of a more massive release period," according to the scientists, based at U.S., Russian and Swedish research institutions.
'Abrupt climate warming' possible
The release of just a "small fraction of the methane held in (the) East Siberian Arctic Shelf sediments could trigger abrupt climate warming," they wrote.
The shelf has sometimes been above sea level during the earth's history. When submerged, temperatures rise by 22-31 degrees F since water is warmer than air. Over thousands of years, that may thaw submerged permafrost.
About 60 percent of methane now comes from human activities such as landfills, cattle rearing or rice paddies. Natural sources such as wetlands make up the rest, along with poorly understood sources such as the oceans, wildfires or termites.
Most studies about methane focus on permafrost on land. But the shelf below the Laptev, East Siberian and Russian part of the Chuckchi sea is three times the size of Siberia's wetlands.
The researchers said the East Siberian Arctic Shelf is of particular concern because it is very shallow and that doesn't give the methane enough time to oxidize into less powerful carbon dioxide before it reaches the surface.
That and the volume of methane there could add a previously uncalculated variable to climate models, the experts said.
"The release to the atmosphere of only one percent of the methane assumed to be stored in shallow hydrate deposits might alter the current atmospheric burden of methane up to 3 to 4 times," Shakhova said. "The climatic consequences of this are hard to predict."