Climate change is increasing the likelihood of extreme autumn wildfire conditions across California

These autumn wildfires have coincided with extreme fire weather conditions during periods of strong offshore winds coincident with unusually dry vegetation enabled by anomalously warm conditions and late onset of autumn precipitation. In this study, we quantify observed changes in the occurrence and magnitude of meteorological factors that enable extreme autumn wildfires in California, and use climate model simulations to ascertain whether these changes are attributable to human-caused climate change.

We show that state-wide increases in autumn temperature (similar to 1 degrees C) and decreases in autumn precipitation (similar to 30%) over the past four decades have contributed to increases in aggregate fire weather indices (+20%). As a result, the observed frequency of autumn days with extreme (95th percentile) fire weather-which we show are preferentially associated with extreme autumn wildfires-has more than doubled in California since the early 1980s.