By LEE BOWMAN
Scripps Howard News Service
November 17, 2006
Not necessarily when the fires occur in the northern forests of Alaska, Canada and Siberia, according to a new study published online Friday by the journal Science.
Although the forest fires "release greenhouse gases that contribute to climate warming, inseparable changes in the forest canopy cause more sunlight to be reflected back into space during spring and summer for many decades after a fire," said James Randerson, an associate professor of earth science at the University of California-Irvine and lead author of the study.
"This cooling effect cancels the impact of the greenhouse gases, so the net effect of fire is close to neutral when averaged globally, and in northern regions may lead to slightly colder temperatures."
Randerson and his colleagues focused their research on the Donnelly Flats fire, which burned about 16,500 acres in central Alaska in June 1999. Soon after the fire, the scientists took field measurements of incoming and outgoing radiation, carbon dioxide being emitted by plants, wind speed and other conditions around the site. They took similar measurements on land nearby that had burned in 1987 and in the 1920s.
They found that right after the fire, large amounts of greenhouse gases entered the atmosphere and caused warming. Ozone levels increased in the atmosphere of the region and ash from the fire fell on remote sea ice and the Greenland ice sheets, darkening the surfaces and causing more radiation to be absorbed and more melting.
The following spring, though, the landscape within the fire zone was brighter than before the fire because fewer evergreen trees shaded the ground. That meant that snow on the ground was more exposed and better able to reflect sunlight back into space, causing more cooling.
Over the years, lighter-colored deciduous trees like aspen and birch grew in the more open space to replace the dark pine forest. When they lost leaves for the winter, snow-covered ground was still more exposed.
Younger trees also take in carbon dioxide at a faster rate than older trees, so the area's net contribution to warming was even less in the first decades after a fire. But as the oldest blackened plot showed, after about 80 years, enough conifers had grown back to darken the landscape and make the ecosystem more climate-neutral.
The scientists modeled the amount of radiation entering and leaving the climate system as a result of fire, which closely tracks global air temperatures. Usually, fires in northern forests occur in the same area every 80 to 150 years.
The researchers found that when the fires occur more frequently, however, more radiation is reflected away from Earth and net cooling results. So, they calculated that if an area is burned 20 years earlier than it would be expected to burn, on average, 0.5 watts more energy per square meter of area burned is absorbed by Earth, but with more snow exposure, 0.9 watts of energy per square meter is reflected back into space, for net cooling effect of 0.4 watts.
Randerson said the findings have particular implications for reforestation projects that have been proposed for the northern woods with a goal of sequestering carbon from the atmosphere and slowing climate warming.
"We need to explore all possible ways to reduce accumulation of greenhouse gases to the atmosphere," he said, primarily through increased efficiency of fossil fuels. "Storing carbon in terrestrial ecosystems can help, but we have to consider all the different ways that ecosystems can influence climate.
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