Preliminary data from two Arctic cruises suggest that rising temperatures are already causing substantial amounts of methane to be released from beneath the ocean floor. But catastrophic gas leaks, like those believed to have occurred 55 million years ago, are unlikely, scientists say.

In the past few weeks, scientists aboard the British research ship James Clark Ross have discovered more than 250 plumes of methane bubbling up along the continental margin northwest of Svalbard. The findings add to a similar discovery by a Russian team in August, that reported elevated methane concentrations near the Lena River delta, as part of the International Siberian Shelf Study (ISSS).

The findings have provoked alarmist media reports predicting massive methane bursts that could accelerate global warming. Methane is a far more powerful greenhouse gas than carbon dioxide, although it is present in much lower concentrations in the atmosphere.

But the phenomenon is probably not new. The scientists believe that methane has been released in the region for at least 15,000 years. "What we're now seeing certainly did not start in the last year or so," says geophysicist Graham Westbrook of the University of Birmingham, UK, who led the British team.

"We have observed increased methane concentrations in the Laptev Sea during several expeditions since the mid-1990s," says Igor Semiletov, who oversees the ISSS methane programme aboard the Russian research ship Jacob Smirnitskyi. "But the data set is extremely limited. Whether what we're seeing in the region is of any relevance for the global climate is mere speculation."

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