Climate Change and Future Wildfire in the Western United States: An Ecological Approach to Nonstationarity

Abstract

We developed ecologically based climate-fire projections for the western United States. Using a finer ecological classification and fire-relevant climate predictors, we created statistical models linking climate and wildfire area burned for ecosections, which are geographic delineations based on biophysical variables. The results indicate a gradient from purely fuel-limited (antecedent positive water balance anomalies or negative energy balance anomalies) to purely flammability-limited (negative water balance anomalies or positive energy balance anomalies) fire regimes across ecosections. Although there are other influences (such as human ignitions and management) on fire occurrence and area burned, seasonal climate significantly explains interannual fire area burned. Differences in the role of climate across ecosections are not random, and the relative dominance of climate predictors allows objective classification of ecosection climate-fire relationships. Expected future trends in area burned range from massive increases, primarily in flammability limited systems near the middle of the water balance deficit distribution, to substantial decreases, in fuel-limited nonforested systems. We predict increasing area burned in most flammability-limited systems but predict decreasing area burned in primarily fuel-limited systems with a flammability-limited (“hybrid”) component. Compared to 2030–2059 (2040s), projected area burned for 2070–2099 (2080s) increases much more in the flammability and flammability-dominated hybrid systems than those with equal control and continues to decrease in fuel-limited hybrid systems. Exceedance probabilities for historical 95th percentile fire years are larger in exclusively flammability-limited ecosections than in those with fuel controls. Filtering the projected results using a fire-rotation constraint minimizes overprojection due to static vegetation assumptions, making projections more conservative.