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Published Online July 6, 2006
Science DOI: 10.1126/science.1128834

Research Articles

Submitted on April 17, 2006
Accepted on June 28, 2006

Warming and Earlier Spring Increases Western U.S. Forest Wildfire Activity

Anthony Leroy Westerling 1*, Hugo G. Hidalgo 2, Daniel R. Cayan 3, Thomas W. Swetnam 4

1 Scripps Institution of Oceanography, La Jolla, CA 92093, USA; University of California, Merced, CA 95344, USA.
2 Scripps Institution of Oceanography, La Jolla, CA 92093, USA.
3 Scripps Institution of Oceanography, La Jolla, CA 92093, USA; US Geological Survey, La Jolla, CA 92093, USA.
4 Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721, USA.

* To whom correspondence should be addressed.
Anthony Leroy Westerling , E-mail: awesterl{at}ucsd.edu

Western United States forest wildfire activity is widely thought to have increased in recent decades, but surprisingly, the extent of recent changes has never been systematically documented. Nor has it been established to what degree climate may be driving regional changes in wildfire. Much of the public and scientific discussion of changes in western United States wildfire has focused rather on the effects of 19th and 20th century land-use history. We compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it to hydro-climatic and land-surface data. Here, we show that large wildfire activity increased suddenly and dramatically in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks, and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.


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