Warmer temperatures are promoting wildfires in the American West
In years that are unusually warm, a larger total area tends to burn.[[Field, C.B., L.D. Mortsch,, M. Brklacich, D.L. Forbes, P. Kovacs, J.A. Patz, S.W. Running and M.J. Scott: North America. Climate Change 2007: Impacts, Adaptation and Vulnerability. (PDF) Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 617-652.]] [[Westerling, A. L., H. G. Hidalgo, D. R. Cayan, and T. W. Swetnam. “Warming and earlier spring increase western US forest wildfire activity.” (PDF) Science 313, no. 5789 (2006): 940–943.]] In addition, in mountain ecosystems, wildfire is more destructive under conditions of drought and high temperature.[[Littell, Jeremy S., Donald McKenzie, David L. Peterson, Anthony L. Westerling. “Climate and wildfire area burned in western U.S. ecoprovinces, 1916–2003.” (Abstract) Ecological Applications: Vol. 19, No. 4 (2009), pp. 1003-1021.]]
Increasing temperatures in the American West are promoting wildfires in a number of key ways. Warmer springs cause decreased snowfall and earlier snowmelt, which in turn is associated with longer fire seasons and more wildfire. And hotter summers are drying out forests more, increasing risk further.
Here is more detail on two of the recent studies linking rising temperatures and wildfire in the West. The first led by Anthony Westerling at the University of California-Merced, focused on fires in National Forests and National Parks. These areas represent most of the forested land in the West. The scientists narrowed their focus still further by looking only at large fires (at least 1000 acres in size), but records tend to be better for large fires.
As a result, while this narrowing procedure eliminated over 99% of the total reported fires, the data still include 73% of the total area burned. The authors were able to rule out other explanations for this increase in fire such as changes in forest management and natural climate variability. The years chosen for the study covered a period with relatively high quality and consistent fire data, coordinated nationally by multiple federal agencies.
The second study, led by Jeremy Littell, of the University of Washington, covered a longer time period over a similar region and found the same basic trends and linkages.
An additional factor behind the trend toward an increase in wildfire size, in some regions appears to be decades of fire suppression, which has led to a build up of brush and other fuel that would historically have been kept in check by frequent fires.[[Donovan, G. H., and T. C. Brown. “Be careful what you wish for: The legacy of Smokey Bear, 2007.” (Abstract) Frontiers in Ecology and the Environment. 5(2): 73-79.]]