Annual acres
burned by wildfire in the United States. Credit: LiveScience
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 1980's there was a jump of four
times the average number of wildfires in the West compared with the early 1980's
and 1970's. The total area burned was six-and-a-half times greater in the
mid 1980's than the earlier years examined. [Graphic:
National Acres Burned]
The wildfire season has also extended by 78 days in the more 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
started—there'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.
