Controls and consequences of forest wildfires under a changing climate
The type and arrangement of the vegetation that fuels wildfires, and the occurrence, extent and severity of those wildfires, are influenced by climate, topography, and human actions to varying degrees across diverse forest ecosystems of the western United States. Climate change is likely to alter wildfire regimes, but the magnitude and timing of potential climate-driven changes in regional fire regimes are not well understood. We consider how the occurrence, size, spatial location and severity of large fires might respond to changing climate in forests of the western United States. We rely on both a survey of the recently published literature, and statistical analysis of historic wildfire, climate and land surface characteristics in the Sierra Nevada and Rocky Mountains. We develop a suite of statistical models that relate monthly climate data to the occurrence, size and severity of fires >200 ha in Sierra Nevada and Rocky Mountain forests; these models are cross-validated and then used with downscaled (∼12 km × 12 km) climate projections from a range of global climate models to predict changing fire occurrence and area burned through 2099.
Wildfire activity in western forests has increased substantially in recent decades, with large (>1000 acre) fires in the decade through 2012 on the order of five times as frequent and burned area approximately ten times as great as the 1970s and early 1980s. These changes are closely linked to increased temperatures and a greater frequency and intensity of drought. Projected additional future warming implies that wildfire activity may continue to increase in western forests. However, the interaction of changes in climate, fire and other disturbances, vegetation and land management may eventually transform some forest ecosystems and fire regimes, with changes in the spatial extent of forest and fire regime types. In particular, forests characterized by infrequent, high severity stand replacing fire may be highly sensitive to warming. Increased wildfire combined with warming may transform these ecosystems such that fuel availability, rather than flammability, becomes the dominant constraint on fire activity. Climate will continue to warm for some time regardless of future greenhouse gas emissions, requiring adaptation to warmer temperatures. Changes in forest location, extent and type may result in substantial changes in ecosystem services.