Intensifying Fire Season Aridity Portends Ongoing Expansion of Severe Wildfire in Western <scp>US</scp> Forests

Area burned by wildfire has increased in western US forests and elsewhere over recent decades coincident with warmer and drier fire seasons. However, high-severity fire-fire that kills all or most trees-is arguably a more important metric of fire activity given its destabilizing influence on forest ecosystems and direct and indirect impacts to human communities. Here, we quantified area burned and area burned severely in western US forests from 1985 to 2022 and evaluated trends through time. We also assessed key relationships between area burned, extent and proportion burned severely, and fire season climate aridity. Lastly, using the strong relationships between fire season aridity and both area burned and area burned severely, we predicted future fire activity under ongoing warming. While annual area burned increased 10-fold over our study period, area burned severely increased 15-fold. Disproportionate increases in severe fire occurred across a wide range of forest types from 1985 to 2022. Importantly, we found that the proportion of area burned severely increased with fire extent at the scale of individual fires and total annual area burned. The relationships between fire season aridity and fire were strong, and our models predicted further increases in fire activity, leading to 2.9- and 4-fold increases in area burned and area burned severely, respectively, under mid-21st century climate. Without a substantial expansion of management activities that effectively reduce fire severity (e.g., thinning of understory and fire-intolerant trees combined with prescribed fire), wildfires will increasingly drive forest loss and degrade ecosystem services including carbon storage, biodiversity conservation, and water yield, with major impacts to human communities.