Anthropogenic warming drives earlier wildfire season onset in California
/Over the last few decades, California’s wildfire season has been shifting, with increasingly severe impacts on communities. This study tracks that shift by examining when fire season truly begins across 13 ecoregions in California between 1992 and 2020, using an extensive record of fire occurrence data. In most of the state, fires are now starting earlier in the year with the strongest advances being observed in northern regions like the Cascades, the Northern Basin and Range, and the Sierra Nevada. In contrast, one of southern California’s desert ecoregions, the Sonoran Basin and Range, experienced a slight delay in fire season onset. Earlier starts in the northern region are closely attributed to warmer and drier conditions, drying out fuels sooner and making landscapes more prone to fires weeks earlier than they used to be. From this analysis, it was shown that anthropogenic warming has advanced fire season onset by 6–46 days in 11 out of the 13 ecoregions, while continued warming is expected to continue to promote earlier fire onsets.
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Madakumbura, Gavin D., Max A. Moritz, Karen A. McKinnon, A. Park Williams, Stefan Rahimi, Benjamin Bass, Jesse Norris, Rong Fu, and Alex Hall. “Anthropogenic warming drives earlier wildfire season onset in California.” Science Advances 11, no. 32 (2025): eadt2041. https://doi.org/10.1126/sciadv.adt2041
