Global Warming Fuels U.S. Forest Fires
By Sara Goudarzi, LiveScience Staff Writer
Posted: 06 July 2006 12:14 pm ET
A recent increase in wildfire activity has been
correlated with rising seasonal temperatures and the earlier
arrival of spring, a new study concludes. Looking at a
database of 1,166 forest wildfires from 1970 to 2003 in the
western United States, researchers compared the number
and potency of wildfires to spring and summer temperatures
and the timing of snowmelts. Wildfire season and potency
increased "suddenly and dramatically" in the late 1980's, the
scientists say. "The increase in large wildfires appears to be
another part of a chain of reactions to climate warming,"
said study co-author Dan Cayan, director of Scripps
Institution of Oceanography's Climate Research Division.
"The recent ramp-up is likely, in part, caused by
natural fluctuations, but evidence is mounting that
anthropogenic effects have been contributing to warmer
winters and springs in recent decades."
Fired up
Cayan and colleagues noted that in the mid
GRAPHIC: In recent years, more acres have
1980's there was a jump of four times the average number
been burned by wildfire in the United States.
of wildfires in the West compared with the early 1980's
Scientists now say climate change is partly to
and 1970's. The total area burned was six-and-a-half
blame for the trend especially in the Western
times greater in the mid 1980's than the earlier years
United States, where the shift is even more
examined. [Graphic: National Acres Burned] The
dramatic than seen in this national data.
wildfire season has also extended by 78 days in the more
Credit: LiveScience.
recent period of 1987 to 2003 compared to 1970 through
1986. The researchers also found that 56 percent of the wildfires and 72 percent of the total burnt area
occurred during the years when the snow melted early. When the snowmelt season occurred later than
average, only 11 percent of wildfires occurred.
"At higher elevations what really drives the fire season is the temperature. When you have a warm spring
and early summer, you get earlier snowmelt," said study coauthor Anthony Westerling of Scripps
Institution of Oceanography at the University of California, San Diego. "With the snowmelt coming out a
month earlier, areas then get drier earlier overall and there is a longer season in which a fire can be startedthere's more opportunity for ignition."
Changing the forest
The increased frequency of large wildfires could reduce the density of trees and change forest
landscapes. The country's western forests, which traditionally act as storage "sinks" for sequestering 20 to
40 percent of all U.S. carbon output, are now transforming into a source of atmospheric carbon dioxide as
they burn up, the authors write. "I see this as one of the first big indicators of climate change impacts in the
continental United States," said study coauthor Thomas Swetnam, director of the Laboratory of Tree-Ring
Research at the University of Arizona in Tucson. "Lots of people think climate change and the ecological
responses are 50 to 100 years away. But it's not 50 to 100 years away-it's happening now in forest
ecosystems through fire."
The study is detailed in this week's online version of the journal Science.