As people watch the decline of Arctic sea ice, the most obvious sign of climate warming in that region, scientists are noting other signs of change, like methane seeping out of the ground as permafrost thaws and glaciers melt across the Arctic. Scientists suspected these methane seeps existed, but no one had measured how much methane was escaping—until recently.
After working for nearly ten years on the ground studying Siberian lakes, Katey Walter Anthony, an aquatic ecosystem ecologist at the University of Alaska, was flying over the Alaskan tundra in 2008 when she spotted something odd in the lakes there. She said, “There were large open areas in some lakes, which at that time of year should have been frozen solid. When we got to these sites on the ground, we saw large plumes of bubbling gas—it looked like these parts of the lake were boiling.” These upwellings were plumes of methane, seeping out of the ground and up through the water. The convection associated with the bubbling prevented the ice from freezing. Where does this methane come from? And does its escape mean more warming in the Arctic?
From whence the gas?
Methane is trapped in and beneath the permafrost overlying the Arctic’s sedimentary basins, and is common in the organic material deposited by glaciers, and in marshy lakes and ponds. The frozen soil acts like a bathtub, holding water in the lake basins and preventing methane beneath the permafrost from percolating to the surface. When the permafrost thaws beneath lakes, gas-permeable chimneys open up, and the methane seeps out.
Walter Anthony and her team started to investigate these large methane seeps, which are thought to occur all across the Arctic. Having surveyed Alaska and Greenland using airplanes and field expeditions on the ground, they discovered more than 150,000 seeps. There are likely many more across the vast reaches of Arctic Russia and Canada, and the team hopes to use remote sensing to confirm this.
During ground surveys, they examined the chemical and isotope composition of the bubbling methane to determine where it was coming from. In many of the smaller bubbling seeps methane was newer, formed when plants and other organic material decayed in the lakes. However, they found that the largest seeps were outgassing fossil methane from ancient sources, such as natural gas and coal beds. Much of the seeping geologic methane had been trapped underground for tens of thousands of years, meaning that permafrost was thawing to such an extent that it was finally releasing those long-stored gases.
Why worry about methane?
Seeping methane is worrisome because it is a potent greenhouse gas. Melanie Engram, a researcher at the University of Alaska and a colleague of Walter Anthony’s said, “Methane is twenty-five to twenty-eight times more effective at retaining heat as carbon dioxide.” Engram is one of the researchers working with Walter Anthony to figure out how to measure the amount of methane now seeping out. When scientists model the effects of greenhouse gases, they need to account for as many sources of a gas as possible, now including these seeps. “Currently there is no quantification for these lakes in the methane budget,” Engram said. “One of the most exciting aspects of this project is investigating the use of synthetic aperture radar (SAR) satellite imagery, provided by NASA through the Alaska Satellite Facility, to quantify methane bubbles trapped by lake ice.” SAR remote sensing, which can image through clouds and at night, is a tool well-suited for monitoring northern landscapes during dark Arctic winters. Satellite remote sensing is valuable for providing images of remote Arctic regions that would otherwise be too logistically difficult or expensive to observe.
And as permafrost warms and glaciers recede across the Arctic, the frozen cap locking methane underground will continue to thaw. Scientists are still trying to understand the extent of seeping methane. If thawing continues, Walter Anthony estimates that more than ten times the amount of methane currently in the atmosphere may bubble up out of the lakes. More methane could fuel the feedback loop that further warms the Arctic and the global atmosphere.
Walter Anthony, K. 2009. Methane: A menace surfaces. Scientific American 301: 68-75