Many ice sheets that sit like caps over rock crevices trap natural seeps of methane; when they melt, the gas can quickly be released into the atmosphere in "burps". Geologists have long suspected that iced-over geological structures might entrap vast stores of ancient methane that seep from coal and gas deposits, although no one knows exactly how much is there.
These stores, along with deep-water stores and shallow (more recent) deposits of decaying plant material in frozen soil, might open as the Arctic warms, releasing vast amounts of methane. Then, as the climate warms, more methane seeps could open and warming could accelerate.
During the winter, when Alaska was covered with ice, Katey Walter Anthony of the University of Alaska in Fairbanks and her colleagues flew planes over 6700 lakes in the state, looking for holes in the ice that might indicate lakes with methane seeps. They analysed gas from 50 holes and carbon dated the methane leaking from them to determine the age of the gas. They did a similar survey of 25 lakes in Greenland.
Ancient gas
At boundaries where permafrost cover is melting or glaciers are retreating, the researchers found old methane, indicating that it came from deep in the Earth and is only now being released. The team then created a model that extrapolated where these natural pockets would be located in lakes throughout Alaska. They found the likeliest locations at the edges of ice sheets.
The team estimate that Alaska is emitting 50 to 70 per cent more methane into the atmosphere than previously thought. Geological records indicate that the model would also apply to deep methane stores in Canada and Siberia, currently covered by ice.
Walter Anthony says that the presence of oil and gas in the Antarctic indicates it may hold ancient methane as well. "This is a far more nuanced study than has been done," says Carolyn Ruppel of the US Geological Survey in Woods Hole, Massachusetts. Although she praises Walter Anthony's modelling work, she points out that extrapolating the findings to draw conclusions about methane seeps on other continents is very difficult.
Any release of methane could accelerate warming at the poles and speed the entire process, Walter Anthony says, but it's hard to predict exactly how soon this could happen. To answer this, her group plans to look at how methane is captured and stored in permafrost, and the pattern in which it melts.