Patrick T Brown , Holt Hanley, Ankur Mahesh, Colorado Reed, Scott Strenfel, Steven J Davis, Adam Kochanski, Craig B. Clements

California has experienced increased instances of extreme wildfire behavior in recent years, but the extent to which this is due to anthropogenic warming has been difficult to determine. Here we quantify empirical relationships between temperature and the risk of extreme daily wildfire growth (>10,000 acres) in California and use these relationships to estimate how extreme growth risk is changing under anthropogenic warming. We subject fires from 2003 to 2020 to differing background climatological temperatures and aridity metrics and find that the fraction of the risk of extreme daily growth attributable to anthropogenic warming to date averages 19% but varies substantially depending on whether background warming pushed fires over critical aridity thresholds. When the historical fires from 2003 to 2020 are subjected to projected end-of-century temperatures, the expected frequency of extreme daily growth events increases by 59% under an emissions scenario in line with the Paris Agreement, compared to an increase of 172% under a very high emissions scenario.