Climate change drives earlier wildfire season onset in California

Abstract Wildfires in California have become more frequent in recent decades, with increasingly devastating impacts. The fire season is also lengthening, with an earlier onset. This trend has been hypothesized to be driven by climate change, but it has yet to be quantitatively attributed to climate drivers. Using a comprehensive fire occurrence dataset, we analyze fire season onset and climate controls on its variability and change during 1992-2018 in California’s forested ecoregions. Onset shows an advancing trend, by 14 days in Northern California and 17 days in Southern California. In Southern California, this trend is dominated by decreasing winter precipitation, possibly natural in origin. By contrast, in Northern California, the largest contributor to the advancing onset is a springtime temperature increase, driven to a large degree by climate change. No matter what the dominant contributor to the trend is, the influence of climate-change-driven warming on onset has already emerged from the influence of natural temperature variability in both regions. Given inevitable future warming trends, this influence will continue to accelerate fire season onset and exacerbate wildfire risks in California in coming years.