What was done
The authors "compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it to hydro-climatic and land-surface data."

What was learned
The findings of Westerling et al. are succinctly summarized by Running (2006) in an accompanying Perspective, wherein he writes that "since 1986, longer warmer summers have resulted in a fourfold increase of major wildfires and a sixfold increase in the area of forest burned, compared to the period from 1970 to 1986," noting also that "the length of the active wildfire season in the western United States has increased by 78 days, and that the average burn duration of large fires has increased from 7.5 to 37.1 days." In addition, he notes that "four critical factors - earlier snowmelt [by one to four weeks], higher summer temperatures [by about 0.9°C], longer fire season, and expanded vulnerable area of high-elevation forests - are combining to produce the observed increase in wildfire activity."

What it means
It is difficult to argue with the objective findings of Westerling et al. with regard to the recent fire history of the western U.S. It is also difficult to argue with their conclusions about the immediate causes of the increase in fire activity. Last of all, we find no fault with their conclusion that "whether the changes observed in western hydro-climate and wildfire are the result of greenhouse gas-induced global warming, or only an unusual natural fluctuation, is presently unclear." However, as our Medieval Warm Period Project goes forward, we believe this lack of clarity will ultimately give way to a decisive resolution of the issue.

Reference
Running, S.W. 2006. Is global warming causing more, larger wildfires? Sciencexpress 6 July 2006 10.1126/science.1130370.