Warmth from a distant fire

Science News, Feb 24, 2001

While it's obvious that climatic conditions can affect the extent and frequency of wildfire, the converse is true as well. As major wildfires burn, they send huge amounts of carbon dioxide and other planet-warming greenhouse gases into the atmosphere. Fires can rapidly oxidize the carbon slowly sequestered by vegetation from atmospheric carbon dioxide and send it right back into the air.

Trees in the United States stockpile about 288 million tons of carbon dioxide each year. However, the U.S. Forest Service estimates that last year's wildfires emitted about 100 million tons of the gas, more than a third of a typical year's sequestration (SN: 12/16/00, p. 396).

Although media attention last year focused on conflagrations in the western states, far beyond their northern horizon even larger fires raged almost unnoticed.

In a typical year, the boreal forests that stretch across Alaska and Canada lose about three times the acreage that's lost in the lower 48 states. For example, the 351 fires that raced across portions of Alaska last year amounted to less than one-third of 1 percent of the nation's total number of fires. But they blackened in excess of 750,000 acres, accounting for more than 10 percent of the nation's fire-stricken land.

In a fire-heavy year, fires in the boreal forests of North America, Russia, and Mongolia contribute up to 10 percent of the carbon dioxide returned to the atmosphere from fires worldwide, says fire ecologist Eric S. Kasischke of the University of Maryland in College Park. This is no small amount, he told an audience at the American Geophysical Union annual meeting in San Francisco last December. In North America, from May to August in a typical year, the quantity of firespawned carbon dioxide equals fully 50 percent of the amount of the gas spewing from vehicle tailpipes, he says. The full scope of the fire problem in Russia is unknown.

Kasischke notes that about a third of the boreal forest in Siberia isn't protected against fire or monitored because the region is so sparsely populated.

Boreal forest fires can affect Earth's climate in other ways, too. Many of these ecosystems grow in a thin, seasonally thawed surface atop permanently frozen soil. Layers of lichens grow among the needles, leaves, and branches that have fallen from the trees to the ground. This thick mat of organic matter, or duff, acts just like a layer of thermal insulation, says Kasischke.

During intense blazes, about half the carbon burned comes from the duff. After this layer burns away, the exposed soil--no longer shaded by trees, because they've burned, too--can absorb sunlight, warm up, and dry out. This increases the oxygen in the soil, which oxidizes the carbon sequestered there and returns it to the atmosphere.

In areas where the climate has warmed considerably since the last fire, the permafrost may never recover, Kasischke says. In such cases, a different set of species can move in to replace those burned out. These species often sequester carbon less effectively than the earlier residents did.

The long-term effects of such ecosystem shifts may aggravate global warming, says Kasischke.

--S.P.