Wildfire response to changing daily temperature extremes in California’s Sierra Nevada

Abstract:

Burned area has increased over the last half-century across California, especially in the Sierra Nevada mountain range where recent fires have contributed to economic losses and decreases in forest carbon stocks. The interplay of controls on increasing burned areas remains unclear, with past work identifying forest management, human-related ignition sources, and climate change as important drivers. To better understand the impacts of new extremes in fire weather on wildfires in the Sierra Nevada, here we quantify the sensitivity of wildfire occurrence and burned area to daily climate variables during 2001-2018. We find that the likelihood of ignition of a large wildfire increases non-linearly with daily mean temperature during summer, with a 1°C increase yielding a 19 - 22% increase of large fire occurrence. Area burned has a similar, non-linear sensitivity, with 1°C of warming yielding a 20% increase. Our analysis provides quantitative information about how increasing summer daily temperature extremes, including more frequent heat waves, will influence the probability that wildfires will escape from human control. Combining this information with future climate projections, we estimate that by the 2040s, the number of large fires will increase by 30 ± 7% and that area burned will increase by 34 ± 7% relative to a 2011-2018 baseline, highlighting the threat posed to fire management by hotter and drier summer weather.