Methane even escapes from freezing permafrost

The annual freeze of wetland soils lying atop permafrost in many high arctic regions may trigger a long-noted yet mysterious rise of atmospheric methane concentrations over such areas each fall, a new study suggests.

The bacteria-aided decomposition of organic material in high-latitude wetlands in large part depends on soil being warm. During the summer, the breakdown process generates prodigious amounts of methane.

As autumn slides toward winter, methane emissions should wane. Nevertheless, scientists have for decades detected an unexplained uptick in atmospheric methane at arctic latitudes during autumn, says Torben Christensen, a biogeochemist at Lund University in Sweden. Now, in the Dec. 4 Nature, he and his colleagues describe a phenomenon that could explain this anomaly in atmospheric chemistry.

Christensen and his colleagues have been monitoring methane emissions from wetlands in northeastern Greenland for several summers, but in 2007 the team’s field season was extended by two months as part of the International Polar Year. As the researchers noted in previous years, summertime emissions of methane roughly tracked soil temperatures at the site, peaking in early July and then dropping off gradually into early September. The team estimates that over the course of the summer, each 1-meter-square patch of wetland studied emitted about 4.5 grams of methane.  

Then, unexpectedly, methane emissions from the wetland patches began to rise in mid-September, and remained high for several weeks. During two of those weeks, emissions outpaced those tallied at the height of the summer, says Christensen. At their peak in early October, methane outputs measured on average about 112 milligrams per hour from each 1-meter-square patch of wetland. Although one study has identified a higher rate of methane emission from small spots in some Siberian lakes (SN: 9/9/06, p. 165), no land-based studies have identified rates this high, the researchers note.

Christensen and his colleagues speculate that as winter approaches, the freezing of the soil overlying permafrost — a physical process, not a biological one — is what boosts the autumn methane emissions. “Most of the methane is produced during the warm summer months, but not all of it is emitted then,” says Christensen.

At the Greenland site, for example, only the uppermost 30 to 50 centimeters of soil thaws each summer. In autumn, the ground freezes from above. The top layer of soil freezes and expands, pressurizing the soil beneath, Christensen contends. Because the underlying permafrost is impermeable, any methane that has accumulated in the thawed soil during the summer is squeezed out and forced to the surface.

The freezing process “not only builds pressure, it opens fractures and pores in the soil to let the methane out,” says Nigel Roulet, a geographer at McGill University in Montreal. Even a moderate amount of methane stored beneath soil could add up to substantial emissions worldwide, considering the large amount of arctic wetlands that overlie permafrost, he notes.

Christensen and his colleagues estimate that about 880,000 square kilometers of arctic wetlands exist that are similar to the site in northeastern Greenland. “There’s no reason to believe that the site we studied is unusual in any way,” Christensen says. If that’s true, such wetlands would emit about 4 million metric tons of methane each autumn during what previously had been considered an inactive time of year. The new finding doesn’t significantly boost the estimated amount of yearly methane emissions but does substantially change scientists’ understanding of when that methane reaches the atmosphere, says Christensen.

“This goes to show that we cannot explain the dynamics of such arctic ecosystems by just observing them in the summer,” he adds.

The team’s measurement of autumnal methane emissions “is a surprising finding,” says Zicheng Yu, a paleoecologist at Lehigh University in Bethlehem, Pa. He notes that the newly proposed freeze-and-squeeze mechanism could also explain anomalous emissions of carbon dioxide from high-arctic soils — emissions also assumed to be the result of bacterial decomposition — that occur long after the region’s summertime warmth has faded.