UC Berkeley

Human-caused climate change has increased wildfire above natural levels across extensive areas of the world and caused vegetation biome shifts, detected and attributed from field observations. These changes threaten ecosystem integrity, yet detailed spatial patterns of risks remain unknown. We identify high risk areas in the globally unique ecosystems of Yosemite and three other United States national parks in California through analyses at 10 m spatial resolution. Our results reveal that climate change increased national park average temperatures up to 2.1 °C above pre-industrial levels from 1895 to 2023, exceeding the global average increase of 1.2 ± 0.1 °C. Forests in the region naturally require periodic fire but suppression generated severe fire risks on 6%–13% of park area by 2024. Under the highest emissions scenario (+4.6 °C–5 °C locally), climate change could shift potential biomes hundreds of meters upslope, on up to half of park area, increase fire frequencies 150%–350%, and shift wildfire into new areas, placing 8%–23% of park area at severe fire risk. Reduced emissions (+1.6 °C–1.9 °C locally) could avert biome shifts and limit fire frequency increases to <40%. Use of naturally ignited fires and prescribed burning reduces understory fuel, decreasing fire risks. To identify sites for these fire management actions, managers currently use 1849 pre-European American settlement as a reference condition, missing extensive areas at risk under climate change. Here, we provide results for a new approach of using climate change conditions as a reference to target fire management for future ecological resilience. Ultimately, cutting the emissions from human activities that cause climate change remains the fundamental solution to reduce fire risks.