November 17, 2006 — By Randolph E. Schmid, Associated Press
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.