Climate and Wildfire in Mountains of the Western United States

Since the mid-1980s, there has been a dramatic increase in the area burned in wildfires in mountain forests of the western United States, with mean annual area burned nearly three and a half times higher compared to the preceding one and a half decades.(1) Concomitant increases in variability in annual area burned and in fire suppression costs pose a serious challenge for land management in the mountainous West. The variance in annual area burned since 1987 is nineteen times its previous level. Since managers must be prepared for the worst possible scenarios in every fire season, increased uncertainty about the scale of the western fire season each year imposes high costs on public agencies. Annual real suppression costs in western forests have more than doubled for the Forest Service since 1987, while the variance in annual suppression costs is over four times higher. Although federal agencies' fire suppression budgets have increased recently, they are still close to what would be spent in an "average" year that seldom occurs, while costs tend to fluctuate between low and high extremes. Modeling area burned and suppression costs as a function of climate variability alone, Westerling (2004, unpublished work) found that the probability of the Forest Service's suppression expenses exceeding the current annual suppression budget has exceeded 50% since 1987, a substantial increase from the one-in-three chance over the preceding 40 years. Recent progress in our understanding of the links between climate and wildfire, and in our ability to forecast some aspects of both climate and wildfire season severity a season or more in advance, offers some hope that these costs might be ameliorated through the integration of climate information into fire and fuels management. In addition to the effects of climate variability on wildfire, long-term biomass accumulations in some western ecosystems have fueled an increasing incidence of large, stand-replacing wildfires where such fires were previously rare. These severe large fires can result in erosion and changes in vegetation type, with consequences for water quality, stream flow, future biological productivity of the affected areas, and habitat loss for endangered species. Apart from their deleterious ecological consequences, severe fires can also dramatically affect amenity values for public lands and for homeowners living in the wildland-urban interface. In the National Fire Plan, land management agencies have committed to reducing fuels on millions of hectares of public lands. The primary means are mechanical removal, prescribed fire and wildland fire use. The Forest Service estimates they will need to spend hundreds of millions of dollars per year to meet their fuel reduction targets, while efforts in recent years have not kept up with the current rate of biomass increase. Use of climate information for targeting resources and scheduling prescribed burns could increase the efficiency of these efforts. In this study we review the fire history since 1970 for western mountain forests, and demonstrate apparent links between regional climate variability and decadal-scale changes in annual area burned. This analysis explores how wildfire size and frequency have varied over the past thirty-five years by elevation and latitude, and how climate indices such as precipitation, temperature, drought indices and the timing of spring runoff vary in importance for fire season severity by elevation in forests around the western United States.