Is Global Warming Causing More, Larger Wildfires?

Fire is an important process for recycling dead biomass in the arid west, where natural decomposition rates are extremely slow (historical repeat photography has shown untreated wooden fenceposts still intact after 100 years). However, this benefit is balanced by the annual damages in the western United States from wildfires that have exceeded $1.0 billion in 6 of the past 15 years.
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. A similar increase in wildfire activity has been reported in Canada from 1920 to 1999. Could this increase be due to Global warming?
A. L. Westerling took to studying 1166 wildfires ranging over 400ha. He found that the length of the active wildfire season (when fires are actually burning) 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. Based on comparisons with climatic indices that use daily weather records to estimate land surface dryness, Westerling et al. attribute this increase in wildfire activity to an increase in spring and summer temperatures by ∼0.9°C and a 1- to 4-week earlier melting of mountain snowpacks. Snow-dominated forests at elevations of ∼2100 m show the greatest increase in wildfire activity. In the 34 years studied, years with early snowmelt had five times as many wildfires as years with late snowmelt. High-elevation forests between 1680 and 2690 m that previously were protected from wildfire by late snowpacks are becoming increasingly vulnerable. Thus, four critical factors—earlier snowmelt, higher summer temperatures, longer fire season, and expanded vulnerable area of high-elevation forests—are combining to produce the observed increase in wildfire activity.
n 2002, the U.S. Departments of Agriculture and the Interior embarked on a controversial new land management policy called “Healthy Forests,” whose stated objective is to clean out dead and dying trees in the west to reduce the risk of wildfires. This initiative blames increasing wildfire activity in the western United States solely on increasing stand density and the buildup of dead fuel as a result of fire exclusion policies; it does not acknowledge any role of changing climate in recent wildfire trends. In contrast, the analysis of Westerling et al. suggests that observed increased wildfire activity is at least partially the result of a longer wildfire season acting over a larger forested area. This increased wildfire trend correlates with observed higher temperatures and reduced moisture availability.As part of the upcoming 4th Assessment of the Intergovernmental Panel on Climate Change , seven general circulation models have run future climate simulations for several different carbon emissions scenarios. These simulations unanimously project June to August temperature increases of 2° to 5°C by 2040 to 2069 for western North America. The simulations also project precipitation decreases of up to 15% for that time period. Even assuming the most optimistic result of no change in precipitation, a June to August temperature increase of 3°C would be roughly three times the spring-summer temperature increase that Westerling et al. have linked to the current trends. Wildfire burn areas in Canada are expected to increase by 74 to 118% in the next century, and similar increases seem likely for the western United States.

Wildfires add an estimated 3.5 × 1015 g to atmospheric carbon emissions each year, or roughly 40% of fossil fuel carbon emissions. If climate change is increasing wildfire, as Westerling et al. suggest, these new sources of carbon emissions will accelerate the buildup of greenhouse gases and could provide a feed-forward acceleration of global warming.