Multiple Fire Index Examination of Future Climate Change Affecting Wildfire Seasonality and Extremes in the Contiguous United States

Climate change is impacting wildfires in the contiguous United States; thus, projections of fire danger under climate change have the potential to inform responses to changing wildfire risks. We calculate fire indices for 13 dynamically downscaled regional climate models, then count days exceeding relevant fire danger thresholds, and compare future changes for mid-and late-twenty-first century relative to a historical reference period. We then compare the responses of the fire indices to highlight areas of agreement and disagreement on the sign and magnitude of future change in fire danger days. Many regions in the domain experience increases in the number of days exceeding fire danger thresholds by the midcentury. The regions which exhibit agreement across the simulation ensemble on the sign of change, and the magnitude of that change, vary greatly between indices. The timing and frequency of fire danger days (defined as days exceeding fire danger thresholds) throughout the year change, both in the shoulder season and during existing peaks in fire danger. By the end of the century, most of the domain experiences statistically significant increases in the number of fire danger days. Complex interactions between input variables, and the sensitivities to inputs, affect the response of fire indices under climate change. The projected increase in fire weather risk could place greater demands upon fire management resources, pose elevated hazards for populations exposed to fire, and potentially disrupt landscapes and infrastructure more frequently. SIGNIFICANCE STATEMENT: The purpose of this study is to examine future changes in multiple fire indices, calculated for an ensemble of regional climate models, under a high emission scenario for the contiguous United States. We compare the fire indices to one another and examine their responses to climate change to better understand how climate change may impact weather conducive to wildfires. We find that the fire indices do not respond to climate change uniformly and that in the midcentury, some regions disagree on the sign of change, while other regions agree on the sign of change and project increases in fire conducive weather. More frequent fire conducive weather poses challenges for fire management and human safety.