Observed impacts of anthropogenic climate change on wildfire in California

California is so big and so variable, there is no one-size-fits-all explanation for how climate might affect wildfires across the board. We have tried to provide one-stop shopping to show people how climate has or, in some cases, hasn't affected fire activity.

Park Williams, lead author and bioclimatologist at Columbia University's Lamont-Doherty Earth Observatory

• States that recent fire seasons have fueled intense speculation regarding the effect of anthropogenic climate change on wildfire in western North America, and especially in California
• States that during 1972–2018, California experienced a five‐fold increase in annual burned area, mainly due to more than an eight‐fold increase in summer forest‐fire extent
• States that increased summer forest‐fire area very likely occurred due to increased atmospheric aridity caused by warming
• States that since the early 1970s, warm‐season days warmed by approximately 1.4°C as part of a centennial warming trend, significantly increasing the atmospheric vapor pressure deficit (VPD)
• These trends were consistent with anthropogenic trends simulated by climate models
• Finds that the response of summer forest‐fire area to VPD is exponential, meaning that warming has grown increasingly impactful
• Robust interannual relationships between VPD and summer forest burned area strongly suggest that nearly all of the increase in summer forest‐fire area during 1972–2018 was driven by increased VPD
• Climate‐change effects on summer wildfire were less evident in non‐forest
• Finds that, in fall, wind events and delayed onset of winter precipitation are the dominant promoters of wildfire
• Finds that, while these variables did not change much over the past century, background warming and consequent fuel drying is increasingly enhancing the potential for large fall wildfires
• Among the many processes important to California's diverse fire regimes, warming‐driven fuel drying is the clearest link between anthropogenic climate change and increased California wildfire activity to date

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