Snowpack decline kindles more severe fire in the western United States

Climate change is reducing winter snowpack and advancing spring snowmelt across the western United States (US), interacting with El Niño-Southern Oscillation (ENSO) teleconnections that drive spatially predictable interannual fluctuations that contribute to high- or low-snow winters. Early snowmelt extends the fire season, enhancing opportunities for ignition and increasing fuel dryness, both of which contribute to greater burned areas. However, relationships between snowpack on burn severity, a measure of forest loss and expected biogeochemical and hydrological impacts of fire, have not been examined. Here, using remotely sensed snow and fire data spanning 1985-2021, we examined how snowpack quantity and timing of spring snowmelt influence annual area burned and burn severity at the watershed scale. Early snowmelt was associated with earlier occurrences of fire ≥400 ha and greater annual area burned, whereas low snowpack water content was associated with more severe burn outcomes including greater mean composite burn index (CBI) and larger proportions of high severity fire (CBI ≥ 2.25). Thus, low-snow winters with early snowmelt may prime forested watersheds to dry, burn, and experience high severity fire. These outcomes are consistent with enhanced fuel dry-down: early snowmelt extends the dry-down window while low snowpack quantity portends greater fuel aridity during the dry period. Our findings also highlight how the ENSO interacts with directional warming: El Niño phases amplify trends of snowpack loss and increasing area burned severely in northwestern watersheds but dampen these trends in southwestern watersheds, while La Niña phases exert the opposite effect. Projected warming, potentially accompanied by greater ENSO variability and extremes, points toward a future of reduced snowpack, earlier snowmelt, and increased area burned at high severity in forests where snowpack historically buffered fire risk, with attendant losses in forest carbon storage and disrupted hydrological function of forested watersheds.