WASHINGTON – Climatologists have worried for years that forest fires would worsen global warming by adding carbon dioxide to the atmosphere. Now, there is an indication that the fires could have a regional cooling effect.
Fires in northern forests do release greenhouse gases that contribute to climate warming. But they also cause changes in the forest canopy that result in more sunlight reflected back into space during spring and summer for many decades after the fire, said James T. Randerson, associate professor of earth system science at the University of California, Irvine.
"This cooling effect cancels the impact of the greenhouse gases," he said. "The net effect of fire is close to neutral when averaged globally, and in northern regions may lead to slightly colder temperatures," said Randerson, lead author of a study appearing in Friday's issue of the journal Science.
Brian Stocks, an expert on fires and climate change who recently retired from Canada's forest service, was cautious about the finding.
"I wouldn't want readers to get the impression that we don't have to worry about this so much anymore, and I'm sure that was not their intention," Stocks said.
The study focused on a single fire, and Stocks said he would feel more comfortable with the conclusion if the report included 10 or more fires under a variety of conditions.
"This is still a huge problem and is going to continue to be a huge problem," said Stocks, who was not part of the research team.
Climatologist Jonathan Overpeck of the University of Arizona said the study is unique in trying to assess effects of fire on climate over 80 years, the time it would take the forest to grow back.
The common wisdom is that there would be a net warming, he said, but the new research indicates a neutral or slight cooling.
Even if that is correct, however, it would be far too small to reduce overall global warming, said Overpeck, who did not participate in the study. Scaled up to larger fires, the amount of soot and ash deposited would increase snow and ice melt.
The study looked at the Donnelly Flats fire in central Alaska, which burned 16,549 acres in 1999. Researchers measured incoming and outgoing radiation, carbon dioxide being absorbed or emitted by plants, wind speed and other conditions. They took similar measurements on nearby land that burned in 1987 and on land that burned around 1920.
They found a lot of carbon dioxide was indeed released in the fire, there were increases in ozone levels and ash fell on icy areas, causing more light to be absorbed.
But the following spring, the land was brighter than before the fire because fewer trees shaded the ground. Snow was more exposed and reflected more light back into space.
The dark spruce trees of the forest were replaced by lighter-colored deciduous trees such as aspen and birch. When these trees lost their leaves in winter, more snow was exposed. The younger trees also take in carbon dioxide faster than the older conifers, the researchers said.
It took some 80 years before the dark conifers dominated the forest again, they reported.
"The reflectivity effect in the long run is larger than the carbon effect," Michelle Mack of the University of Florida, a co-author of the paper, said in a statement.
The report indicates that forest management strategies may need to be reconsidered.
Many ecologists have urged efforts to reduce global warming by fighting fires and increasing forested areas in an effort to take up, or sequester, carbon dioxide.
"What we're showing is that if you are going to manage an ecosystem to have an effect on the climate via carbon sequestration, you need to consider all the other climate forcing factors you may be changing at the same time," said Ted Schuur of the University of Florida, another co-author of the paper.