By Michael Marshall / NewScientist
As Arctic sea ice breaks apart, massive amounts of methane could be released into the atmosphere from the cold waters beneath.
High concentrations of the greenhouse gas have been recorded in the air above cracks in the ice. This could be evidence of yet another positive feedback on the warming climate – leading to even faster Arctic warming.
The Arctic is home to vast stores of methane – there are billions of tonnes of methane in permafrost alone. It is a potent greenhouse gas, so a major methane release would greatly accelerate climate change. The gas is found in icy crystals called hydrates beneath the shallow seas that flood some areas of the continental crust, as well as in permafrost. It is also being released from Arctic wetlands.
But this doesn’t explain why Eric Kort of the Jet Propulsion Laboratory in Pasadena, California, and his colleagues found patches of methane in remote regions of the Arctic Ocean, far from any of these known methane sources.
The team found the patches during five flights over the Arctic Ocean between 2009 and 2010, as part of a project to systematically map greenhouse gas levels in the atmosphere.
Kort estimates that, in the methane-rich regions, about 2 milligrams of the gas were being released per square metre of ocean every day. Some of the patches were close to the oil and gas plants in Prudhoe Bay, Alaska, but prevailing wind directions make these plants an unlikely source of the release.
So where does the gas come from? Since the 1970s, scientists have known that ocean surface waters are rich in methane. It seems to be made by marine bacteria trying to survive in waters that don’t have many nutrients in the form of nitrates. “This source appears to be a likely candidate to explain what we observed,” Kort says.
Water in the Arctic Ocean doesn’t mix well, so the water near the surface tends to remain there. Consequently, the methane ends up trapped near the surface. In other oceans, it would get broken down through reaction with oxygen or consumed by methanotrophic bacteria, but the cold weather helps to preserve it.
Kort saw methane releases close to cracks in the sea ice, or in places where the ice had broken up. This could be because methane only escapes from agitated water, says Ellen Damm of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. This agitation is most likely to occur when autumn sets in and ice crystals start forming in the water, creating turbulence.
Hotter and hotter
The findings will need to be replicated, says Euan Nisbet, an earth scientist at Royal Holloway, University of London. But if the leak is widespread across the Arctic, this mechanism could prove to be a significant source of greenhouse gas.
“We know the Arctic is warming very fast indeed,” Nisbet says. And as the warming climate leads to more breaks in the sea ice, more ice-surrounded patches of open water will be able to release their methane, further accelerating global warming.
The question now is: how significant will this new effect on warming be? “It might be small,” Nisbet says, “or it could be another serious problem.”