British researchers found more than 250 plumes of methane bubbling up in the sea northwest of Svalbard. Credit: D. VAUGHAN/SCIENCE PHOTO LIBRARY

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.

The risk is real, but there’s no reason to panic. Hans-Wolfgang Hubberten , Alfred Wegener Institute of Polar and Marine Research

"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."

Semiletov says that the scientists did measure higher concentrations of dissolved methane this summer compared to summer sampling in 2003 and 2004 (N. Shakhova and I. Semiletov J. Mar. Sys. 66, 227–243; 2007). At one ice-covered site in the mere 50-metre shelf water, they detected methane bubbling at the surface, indicating that at least some of the gas released at the seabed is escaping into the atmosphere before being consumed by bacteria in the water column.

Geologists think that billions of tonnes of methane lie beneath the sub-sea permafrost in some parts of the shallow Siberian shelf, although estimates vary widely. The hydrocarbon — trapped there either as a gas, or bound in solid ice-like structures called methane hydrates — is a remnant from the last ice age when the sea level was about 100 metres lower. The big fear is that the methane could escape as a result of the permafrost becoming porous, possibly from an increased influx of freshwater from the relatively warm Lena River.

"The risk is real," says Hans-Wolfgang Hubberten, a permafrost expert at the Alfred Wegener Institute of Polar and Marine Research in Potsdam, Germany. "But there's no reason to panic. Claims that gas hydrates are on the brink of dissociating in a big way should be taken with a large pinch of salt."

Thermal modelling suggests that the marine permafrost in the region is relatively stable. However, drillings conducted in 2005 revealed that the permafrost may have slightly warmed and thinned (V. Rachold et al. Eos 88, 149–156; 2007). Even so, says Hubberten, it is likely that the observed emissions come from 'new' methane produced by increased bacterial activity in thawing soil, rather than from degradation of ancient gas hydrates.

Methane, air and water samples taken by both teams will now be sent to isotope labs in the Netherlands and the United Kingdom to help determine the source of the methane. Geochemical analysis should also show how much of the gas escapes to the atmosphere, says Westbrook. "The new findings will be useful in helping us assess the history of climate change in the region, and how the methane reservoirs responded to past temperature changes."

Globally, atmospheric methane concentrations increased by 7.5 parts per billion to nearly 1,800 parts per billion during 2007 after almost zero growth since 1999. The upward trend is likely to continue this year, says Ed Dlugokencky, who oversees the methane database run by the National Oceanographic and Atmospheric Administration (NOAA) in Boulder, Colorado. "Our data suggest increased emissions in the Arctic and the tropics," he says. "Both regions were apparently warmer and wetter than average."

Data collected by NOAA at remote sites are usually at least 6 weeks out of date. And NOAA's measurement network in the Arctic is not dense enough to tell if increased methane emissions come from wetlands, permafrost or from gas hydrates on the continental shelves.