Study Ties Earlier Fire Season to Climate Warming

California’s wildfire season is starting significantly earlier than in past decades, with new research in Science Advances attributing much of this shift to anthropogenic warming. The study finds that in 11 of the state’s 13 ecoregions, human-driven climate change has advanced the start of the fire season by 6 to 46 days between 1992 and 2020, creating longer windows for fire activity and increasing the risk to lives, property, and ecosystems.

“Understanding how anthropogenic warming influences fire onset is critical for assessing the increased probability of fire occurrence,” said Gavin D. Madakumbura, lead author and atmospheric scientist at UCLA. “Our findings have major implications for wildfire seasonal predictions and disaster management strategies in the years ahead.”

A Statewide Shift Toward Earlier Fire Onset

The analysis examined fire occurrence records from 1992–2020, covering 13 ecoregions across California. Researchers defined fire season onset as the earliest five to 15 percent of annual fire start dates, depending on regional fire patterns.

Key findings include:

• Earlier start dates in most regions: All but one ecoregion saw fire onset shift earlier, with the Cascades, Eastern Cascades Slopes and Foothills, and Central California Foothills and Coastal Mountains experiencing the largest changes—about 2 days earlier per year.
• Regional differences: Northern California’s forested and higher-elevation areas experienced the most pronounced shifts due to warming-driven snowmelt acceleration and earlier fuel drying. Southern California’s arid desert ecoregions saw smaller, statistically insignificant changes, and in some cases slight delays, likely due to fuel limitations.

Fuel Moisture and Climate Are Primary Drivers

The study concludes that the timing of fire onset is strongly linked to fuel moisture, which in turn depends on winter precipitation, snowpack levels, and evaporative demand. Remote-sensing data and live fuel moisture measurements both confirmed this relationship.

• Winter soil moisture correlation: In many regions, cool-season soil moisture had correlations above 0.8 with fire onset timing.
• Spring and summer vapor-pressure deficit (VPD): Higher VPD, indicating drier air, was strongly linked to earlier fire starts.
• Snowpack influence: In the Sierra Nevada and other high-elevation regions, deeper snowpacks delayed fire onset by several weeks.

Disentangling Natural Variability from Anthropogenic Warming

Using a causal effect network and dynamical adjustment techniques, researchers separated natural climate variability (e.g., multi-year droughts) from long-term anthropogenic warming effects.

• Natural variability impact: Severe droughts, particularly 2012–2015, advanced fire onset by up to 28 days in some regions.
• Anthropogenic warming impact: In northern ecoregions, warming advanced fire onset by up to 46 days. In contrast, some southern desert areas saw slight delays because warming without sufficient precipitation can limit fuel growth.

Implications for Risk Management

The findings suggest that wildfire management agencies, utilities, insurers, and local governments will need to adjust preparedness timelines. Earlier fire onset could:

• Require shifting seasonal staffing, training, and equipment readiness earlier in the year.
• Affect insurance risk models and infrastructure hardening schedules.
• Increase the importance of year-round vegetation management and community outreach.

“These findings underscore the very real impact climate change has on people’s lives and livelihoods,” said Alex Hall, a co-author of the report and director of both the  Sustainable LA Grand Challenge  and Institute of the Environment and Sustainability, or IoES.

Given that the warming trend is expected to continue, the study warns that northern California may see further departures from historical fire season timing—beyond what natural variability alone can explain.