Far more methane — a powerful greenhouse gas — is emitted into the atmosphere during the Arctic region's coldest months than was previously believed, according to a just-released study that includes contributions by a pair of San Diego State University researchers.
The upshot of the research, published Monday in the Proceedings of the National Academy of Sciences, is that scientists might have to change the assumptions they use in their global climate models.
"Virtually all the climate models assume there's no or very little emission of methane when the ground is frozen," SDSU ecologist Walter Oechel said. "That assumption is incorrect."
Oechel took part in the study along with another SDSU ecologist, Donatella Zona.
Methane trapped in the Arctic tundra comes primarily from microbial decomposition of organic matter in soil that thaws in the region's few warmer months, and it seeps out naturally. Arctic methane emission measurements have generally taken place when it's less cold, according to SDSU.
Oechel, Zona and colleagues from NASA's Jet Propulsion Laboratory, Harvard University, the National Oceanic and Atmospheric Administration, University of Montana and other institutions took measurements during several months when the region is at its coldest.
They found that even when it gets really cold in the Arctic, the ground doesn't entirely freeze over, but becomes a sort of sandwich in which the unfrozen middle layer continues to break down organic matter and emit methane.
Gas seepage is most pronounced when the temperature is zero degrees Celsius, known as the "zero curtain," according to the researchers.
"This is extremely relevant for the Arctic ecosystem, as the zero curtain period continues from September until the end of December, lasting as long or longer than the entire summer season," said Zona, the study's first author.
"These results are opposite of what modelers have been assuming, which is that the majority of the methane emissions occur during the warm summer months while the cold-season methane contribution is nearly zero," Zona said.
The researchers recorded methane emissions over two summer-fall-winter cycles between June 2013 and January 2015, using specialized instruments that had to operate continuously and autonomously through extreme cold for months at a time.
Pound for pound, methane is 25 times more potent as a greenhouse gas than carbon dioxide, though it doesn't stick around in the atmosphere as long, according to the U.S. Environmental Protection Agency. The EPA says about 40 percent of methane emissions occur naturally around the world, mostly from wetlands.
The researchers said they also found that more emissions during the coldest months took place in drier upland tundra than in Arctic wetlands.
