Climate Change Has Pushed California’s Fire Season Earlier — By As Much as a Month and a Half

By Dave Levitan | August 7, 2025 | 11:08am

Photo via BLM California/Wikimedia Commons

Climate wildfires

Climate Change Has Pushed California’s Fire Season Earlier — By As Much as a Month and a Half

California serves as a sort of ground zero for American wildfires, generally experiencing more fires and more burned acres than any other state. It is an easy matter of observation to see that California’s fire season has gotten worse — more fires, more acres, more large and damaging individual incidents — but a new study has put some hard data on climate change’s specific contribution to those changes. Spoiler, it’s a lot.

Researchers at UCLA and elsewhere in the state used an “extensive record of fire occurrence data” between 1992 and 2020 to examine onset of fire season and the drivers behind its change; their results were published Wednesday in Science Advances. Writing an accompanying piece in The Conversation, they found significant variation across the state but a consistent and occasionally strong signal:

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“The typical onset of summer fire season, which is in May or June in many regions, has shifted earlier by at least one month in most of the state since the 1990s, and by about 2½ months in some regions, including the northern mountains. Of that, we found that human-caused climate change was responsible for advancing the season between six and 46 days earlier across most of the state from 1992 to 2020.”

Forty-six days! The analysis divided the state into 13 “ecoregions,” and the large differences in local climate and terrain produced big differences — for the most part, the biggest changes and the largest warming contribution occurred in mountainous areas to the north, with less adjustment in the more arid southern part of California. The 46-day warming-induced jump occurred in the northeastern Cascades, with other large variations seen in the Northern Basin and Range (31 days) and the Sierra Nevada area (24 days). Overall, 11 of the 13 ecoregions had at least some climate change-influenced advance to the start of wildfire season.

There are other things that are contributing to worsening fire seasons — changes to the amount of available fuel in the form of grasses and trees, for example, as well as human shifts into wildland areas and all the power lines and other ignition sources that follow them. Fuel aridity was found to be the most important predictor of the season’s timing — and that aridity is most heavily influenced by climatic changes. The mountain regions offer a clear stepwise process: warmer temperatures mean earlier snowmelt, which means vegetation dries out faster. Dry fuel, earlier in the year, equals earlier fire season start.

And because no story like this can avoid the ways the current administration is making all of this more dangerous and dire, it is important to remember that the potentially massive cuts to the US Forest Service are already making their mark. The National Interagency Fire Center says that 3.4 million acres have burned nationwide so far this year, and 36 large fires are currently burning; red flag warnings from the National Weather Service on Thursday covered parts of Wyoming, Colorado, Idaho, Nevada, and essentially all of Utah.

Of course, this trend is not going to reverse anytime soon. “The anthropogenic warming component in predicting fire onset is indicative of what is to come, as warming trends continue,” the authors wrote. “Our findings have major implications for wildfire seasonal predictions and disaster prevention and management strategies in coming seasons and years.”