Greater Temperature and Precipitation Extremes Intensify Western U.S. Droughts, Wildfire Severity, and Sierra Nevada Tree Mortality

Abstract Extensive drought in the western United States (WUS) during the twenty-first century and associated wildfire and tree mortality incidence has highlighted the potential for greater area of severity within widespread droughts. To place recent WUS droughts into a historical context, the authors analyzed gridded daily climate (temperature, precipitation, and climatic water deficit) data to identify and characterize the spatiotemporal evolution of the largest WUS droughts of the last 100 years, with an emphasis on severe cores within drought extents. Cores of droughts during the last 15 years (2000–02 and 2012–14) covered a greater area than in earlier droughts, driven by greater temperature and precipitation extremes. Comparing fire extent and severity before, during, and after drought events using the monitoring trends in burn severity dataset (1984–2014), the authors found fire size and high-severity burn extent were greater during droughts than before or after. Similarly, recent Sierra Nevada forest mortality was greatest in cores immediately after the drought. Climate simulations anticipate greater extremes in temperature and precipitation in a warming world; droughts and related impacts of the last 15 years may presage the effects of these extremes.