Underexposed

SAN FRANCISCO--Researchers have linked global climate change to a tripling in the frequency of large fires in major forests of Alaska and Canada.

Black spruce forests cover about 2.7 million square kilometers in Canada and Alaska--about a third of the area of the lower 48 states of the U.S., and fire records date back to the 1950s. Beginning around 1987, the rate that large wildfires struck the forest jumped from about once every 10 years to once every 3 years, said Eric Kasischke of the University of Maryland at College Park, speaking at the American Geophysical Union conference here Monday.

"We've seen an increase in the number of very large arboreal fires," Kasischke said. (He defines a large fire as one that burns more than 1 percent of the land area in a particular region, such as Alaska's interior.)

There are two links to the gradually warming climate, he said. First, the fires increasingly show up in the fall, when soils are driest and fires therefore are more severe, he said.

Second, the fires are burning deeper into the soil, a significant change given that these northern forests have a thick layer of biological material, typically about 10 inches deep.

"The fires that change the ecosystem the most are occurring more frequently," he said. He predicted that the gradual warming will mean black spruce forests gradually will be replaced by aspen, birch, lodgepole pine, and jack pine forests. In addition, when the organic layer burns, permafrost below no longer is as well insulated.

The deep-burning fires are something of a vicious cycle, too, from a global warming perspective. With the thick layer of biomass, the black spruce forests typically have about 50 tons of carbon per hectare--or about 20 tons per acre--where ordinary U.S. forests have only about a fifth that. When burned, that carbon becomes carbon dioxide, the dominant greenhouse gas culprit in global warming.

The deep burn releases enough carbon dioxide that "you can detect the signal in trace gas emissions," Kasischke said.