Western wildfires are increasing

Across the American West, wildfire activity has spiked dramatically in the past two decades or so, compared with the two decades that preceded them. Climate Central graphics illustrate trends in the Northern Rockies and Washington State.

Two recent studies document this ongoing increase and also find that regional warming has been a key factor driving it. Warming leads to earlier snowmelt which leads to reduced streamflow in late summer and drier trees and a longer fire season.

The first study,[[Westerling, A. L., H. G. Hildago, D. R. Cayan, and T. W. Swetnam. “Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity.” (Abstract) Science Vol. 313. no. 5789, August 18, 2006. pp. 940 - 943.]] 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.  Also these are more destructive.

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 were cover a period with relatively high quality and consistency fire data, coordinated nationally by multiple federal agencies.

The second study,[[Littell, Jeremy S., Donald McKenzie, David L. Peterson, Anthony L. Westerling “Climate and wildfire area burned in western US ecoprovinces, 1916–2003.” (Abstract) Ecological Applications: Vol. 19, No. 4, 2009 pp. 1003-1021]] 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, specifically, 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). 2007. 73-79.]]