Warming climate plays a role in wild fires
Warming Climate Plays Large Role in Western U.S. Wildfires,
Scripps-led Study Shows
Research published in Science
shows rising temperatures expected in the years ahead will
exacerbate the number of large and costly fires
A new study led by scientists at Scripps Institution of
Oceanography at UCSD implicates rising seasonal temperatures and
the earlier arrival of spring conditions in connection with a
dramatic increase of large wildfires in the western United
States.
In the most systematic analysis to date of recent changes in forest
fire activity, Anthony Westerling, Hugo Hidalgo and Dan Cayan of
Scripps Oceanography, along with Tom Swetnam of the University of
Arizona, compiled a database of recent large western wildfires
since 1970 and compared it with climate and land-surface data from
the region. The results show that large wildfire activity increased
suddenly and dramatically in the 1980s with longer wildfire
seasons and an increased number and more potent wildfires.
The new findings, published in the July 6 issue of Science Express
(the advance online version of the printed journal Science), point
to climate change, not fire suppression policies and forest
accumulation, as the primary driver of recent increases in large
forest fires.
The increase in large wildfires appears to be another part of a
chain of reactions to climate warming, said Cayan, a coauthor of
the paper and director of Scripps 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.
Western U.S. wildfires have garnered broad public and political
attention in recent years due to the severity and expanse of the
areas they have consumed, including hundreds of homes burned and
devastating damage to natural resources. Fire-fighting expenditures
for wildfires now regularly exceed one billion dollars per
year.
The scientists compiled a comprehensive time series of 1,166 forest
wildfires of at least 1,000 acres that had occurred between 1970
and 2003 from wildfire data covering western U.S. Forest Service
and National Park Service lands. To investigate what role climate
might play, the researchers compared the time series, the timing of
snowmelt and spring and summer temperatures for the same 34
years.
For the timing of peak snowmelt in the mountains for each year,
they used the streamflow gauge records from 240 stations throughout
western North America. The team also used other climatic data such
as moisture deficit, an indicator of dryness.
The results point to a marked increase in large wildfires in
western U.S. forests beginning around 1987, when the region shifted
from predominantly infrequent large wildfires of short duration
(average of one week) to more frequent and longer-burning wildfires
(five weeks). The authors found a jump of four times the average
number of wildfires beginning in the mid-1980s compared with the
1970s and early 1980s. The comparison showed that the total area
burned was six and a half times greater. Also in the mid-1980s, the
length of the yearly wildfire season (March through August)
extended by 78 days, a 64 percent rise when comparing 1970-1986
with 1987-2003.
The researchers determined that year-to-year changes in wildfire
frequency appear to be strongly linked to annual spring and
summer temperatures with many more wildfires burning in hotter
years than in cooler years.
They established a strong association between early arrivals of the
spring snowmelt in the mountainous regions and the incidence of
large forest fires. An earlier snowmelt, they said, can lead to an
earlier and longer dry season, which provides greater opportunities
for large fires. Overall, 56 percent of the wildfires and 72
percent of the total area burned occurred in early snowmelt years.
By contrast, years when snowmelt happened much later than average
had only 11 percent of the wildfires and 4 percent of the total
area burned.
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 Westerling. 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 startedtheres more
opportunity for ignition.
The greatest wildfire increases occurred in the Northern Rockies,
where forest ecosystems in middle elevations were found to be
highly susceptible to temperature increases. Other significant
wildfire increases were found in the Sierra Nevada, the southern
Cascades and the Coast Ranges of northern California and southern
Oregon.
I see this as one of the first big indicators of climate change
impacts in the continental United States, said research team
member Thomas Swetnam, director of the Laboratory of Tree-Ring
Research at The University of Arizona in Tucson. Were showing
warming and earlier springs tying in with large forest fire
frequencies. Lots of people think climate change and the ecological
responses are 50 to 100 years away. But its not 50 to 100 years
awayits happening now in forest ecosystems through fire.
The authors state that climate model projections, driven by
potential increases in atmospheric greenhouse gas concentrations,
indicate that warmer springs and summers will likely continue and
intensify in the coming decades, accentuating conditions favorable
to large wildfires.
Westerling says that the papers results indicate that measures to
limit future climate change could help to curtail catastrophic
increases in future summer wildfires. If climate warms markedly
over todays levels, intensified fuels management and fire
suppression are not likely to be effective in much of the western
U.S., he said.
The overall importance of climate in wildfire activity underscores
the urgency of ecological restoration and fuels management to
reduce wildfire hazards to human communities and to mitigate
ecological impacts of climate change, especially in forests that
have undergone substantial alterations due to past land uses, the
authors note in the paper.
The authors conclude that the increased frequency of large and
devastating wildfires may significantly change forest composition
and reduce tree densities, transforming the western U.S. forests
role as a storage sink for sequestering some 20 to 40 percent of
all U.S. carbon to a source for increasing carbon dioxide in the
atmosphere.
The research was supported by the National Oceanic and Atmospheric
Administrations Office of Global Programs, the National Fire Plan
via the United States Forest Services Southern Research Station
and the California Energy Commission.