As the world gets ever warmer, some of the most pronounced changes are happening at the high latitudes. Now a study of the Swedish subarctic has shown that the tundra has become much wetter during the last 3 decades. Permafrost is thawing, vegetation is becoming much more marshlike, and emissions of methane--a potent greenhouse gas--appear to have risen by up to 66% in one bog. "We're seeing dramatic changes," says biogeochemist Torben Christensen of Lund University in Sweden.
The finding comes from the Abisko region of northern Sweden. As temperatures have risen since the early 1980s, permafrost has completely disappeared from some bogs. To find out the extent of the change, Christensen and colleagues compared aerial photos of the Stordalen mire taken in 1970 and 2000. They tabulated four types of vegetation and checked the accuracy of the 2000 photo with a detailed field survey. The extent of drier plant communities, mainly mosses and shrubs, had declined from 9.2 to 5.9 hectares, the team reported 20 February in Geophysical Research Letters. Meanwhile, sedges and other marshy plants increased more than 50%. "The thing that surprised me is the rate of change," says Christensen. "Almost from year to year, we can see the vegetation changing."
The team also made detailed measurements of methane emissions for each of the plant communities. Wetter conditions lead to anaerobic conditions and more methane from bacteria decomposing organic matter. Extrapolating from the vegetation patterns of 1970 and 2000, they calculated that the shift in plants has caused a rise in methane production between 22% and 66%. The evidence for an increase is backed up by methane measurements made in the early 1970s at Stordalen by team member Bo Svensson of Linköping University in Sweden.
"It's a pretty massive change," comments William Reeburgh of the University of California, Irvine. The same kind of shifts have been seen in central Alaska. Part of the reason is that the average regional temperature had hovered near freezing--making the permafrost vulnerable to thawing and leaving ecosystems vulnerable. Colder tundra to the north may be more resistant to thawing, Christensen notes.
The article abstract