May 21, 2012 -- These cracks in sea ice north of Alaska reveal a surprising and potentially important source of methane: the Arctic Ocean.
Methane's ability to exacerbate global warming means that realistic climate projections depend on understanding all of its sources. Arctic ecologists have known for years that huge reservoirs of methane are frozen in permafrost soils, and strong evidence has revealed the gas escaping into the atmosphere from thawing permafrost and bubbling Arctic lakes. A recent study documented hundreds of thousands of methane seeps under glaciers in Alaska and Greenland.
Oceanographers have also measured high concentrations of methane dissolved in upper layers of Arctic seawater. (The source is not certain but could be methane-producing microbes in seawater and/or thawing marine sediments). Until recently, however, no one was sure whether that seawater-bound methane actually entered the atmosphere.
Now a multi-institutional team has reported the first clear evidence that methane escapes from the ocean surface, in places where there were cracks and openings in the sea ice. Eric Kort, now at NASA's Jet Propulsion Laboratory, led the research while at Harvard University.
During five research flights in 2009 and 2010, Kort and colleagues measured enhanced methane levels while flying at low altitudes north of the Chukchi and Beaufort Seas. Over open water the team detected methane levels about one-half percent higher than normal background levels; over solid ice, they did not enhanced levels of the gas.
They inferred that the cracks were allowing methane in the top layers of the sea to escape into the atmosphere, in part by ruling out two other potential sources. First, if the methane had been coming from burning of fossil fuels, the team would have detected excess carbon monoxide as well (but they did not). And second, the time of year, location and nature of the emissions made high-latitude wetlands or geologic reservoirs unlikely.
"While the methane levels we detected weren't particularly large, the potential source region, the Arctic Ocean, is vast," Kort said in a summary posted on the NASA Earth Observatory. "Our finding could represent a noticeable new global source of methane."