Arctic freeze triggers big squeeze: methane release linked to wetlands covering permafrost.

The annual freeze of wetland soils lying atop permafrost in many high arctic regions may trigger the long-noted, yet mysterious rise of atmospheric methane concentrations over those 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. But for decades scientists have detected an unexplained autumn uptick in atmospheric methane at arctic latitudes, says Torben Christensen, a biogeochemist at Lund University in Sweden.

In the Dec. 4 Nature, he and his colleagues speculate that as winter approaches, the freezing of the soil overlying permafrost boosts the autumn methane emissions. "Most of the methane is produced during the warm summer months, but not all of it is emitted then," Christensen says.

Monitoring wetlands in northeastern Greenland, he and his collaborators have found that summer emissions of methane roughly track soil temperatures, peaking in July and then dropping off into early September. Observations in 2007 showed that methane emissions began to rise again in mid-September and remained high for several weeks.

At the Greenland site, only the upper 30 to 50 centimeters of soil thaws each summer. In fall, the top layer of soil freezes and expands, pressurizing the soil beneath, Christensen contends. Because the underlying permafrost is impermeable, methane that accumulated in the thawed soil during the summer is squeezed out and forced to the surface.