Large amounts methane are bubbling up from a long-frozen seabed north of Siberia, raising fears of far bigger leaks, say scientists.
But it is unclear if the emissions are new or have been going on unnoticed for centuries - since before the Industrial Revolution of the 18th century.
The study, which appears today in the journal Science, says about 8 million tonnes 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 north of Russia.
"Subsea permafrost is losing its ability to be an impermeable cap," says study co-author Dr Natalia Shakhova, a scientist at the University of Fairbanks, Alaska.
The experts measured levels of methane, a gas that can be released by rotting vegetation, in water and air at 5000 sites on the East Siberian Arctic Shelf from 2003 to 2008. In some places, methane was bubbling up from the seabed.
Previously, the sea floor had been considered an impermeable barrier sealing methane, says Shakhova. Current methane concentrations in the Arctic are the highest in 400,000 years.
Whether the venting was caused by global warming or by natural factors, Shakhova says "No one can answer this question." 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," says Professor Martin Heimann, an expert at the Max Planck Institute for Biogeochemistry in Germany who wrote a separate article on methane in Science.
"These leaks could have been occurring all the time" since the last Ice Age 10,000 years ago, he says. He writes that the release of 8 million tonnes of methane a year was "negligible" compared to global emissions of about 440 million tonnes.
Shakhova's study says 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 US, Russian and Swedish research institutions.
The release of just a "small fraction of the methane held in (the) East Siberian Arctic Shelf sediments could trigger abrupt climate warming," they write.
The shelf has sometimes been above sea level during the earth's history. When submerged, temperatures rise by 12°C to 17°C since water is warmer than air. Over thousands of years, that may thaw submerged permafrost.
About 60% 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.