Heat waves are playing a much larger role in fueling wildfires across the western United States than previously understood, according to a new study that examined wildfire activity from 2001 to 2024.
Researchers found that although heat waves accounted for only 12% to 15% of warm-season days, they were associated with 42% of the total area burned by wildfires during the study period. The findings highlight the growing connection between climate change, extreme heat and wildfire activity.
The study, conducted by a team of fire and climate scientists, analyzed two decades of wildfire data across the western U.S. and quantified the impact of heat waves on fire behavior for the first time. Researchers defined a heat wave as a period of at least three consecutive days with temperatures ranking among the hottest 10% of days.
Their analysis showed that daily burned area during heat waves was more than 50% larger than on the cooler days immediately preceding them. In some regions, wildfire activity increased by as much as 300% during heat-wave conditions.
Scientists say heat waves increase wildfire risk through several interconnected mechanisms. High temperatures intensify the atmosphere’s demand for moisture, rapidly drying vegetation and making it easier to ignite. Heat waves also reduce nighttime humidity, allowing fires to remain active for longer periods and continue spreading overnight.
In addition, the hot and unstable atmospheric conditions associated with heat waves increase the likelihood of lightning strikes, including dry lightning events. Dry lightning occurs when rainfall evaporates before reaching the ground, leaving lightning as a potential ignition source without the benefit of rain to suppress emerging fires.
Researchers found that wildfire risk often remains elevated even after a heat wave ends because vegetation and dead plant material continue to stay unusually dry for several days.
The study also revealed a strong long-term link between increasing heat waves and expanding wildfire activity in forests. Since 2001, the number of heat-wave days across western U.S. forests has nearly doubled, while the total forest area burned has increased by approximately 2.5 times.
According to the researchers, without the increase in heat-wave days observed since 2001, the cumulative area of burned forest would have been about 37% smaller.
However, the relationship varies by ecosystem. Forests showed a clear connection between more frequent heat waves and increased fire activity, while grasslands and shrublands did not exhibit the same long-term trend. In these landscapes, the amount of available vegetation plays a larger role in determining fire extent than temperature alone.
The study also notes that climate change is making western U.S. summers both hotter and drier. Relative humidity during heat waves has declined in recent decades, particularly in the forested regions of California, Oregon and Washington. These increasingly dry heat waves create ideal conditions for large and destructive wildfires.
Researchers warn that decades of aggressive fire suppression have also contributed to growing wildfire risks by allowing combustible vegetation and forest debris to accumulate. Combined with hotter and drier conditions driven by climate change, these fuel buildups are increasing the potential for larger and more intense fires across the region.
The findings underscore how rising global temperatures are amplifying wildfire hazards and suggest that future increases in extreme heat could further escalate wildfire activity unless greenhouse gas emissions are reduced and land management strategies are adapted to changing climate conditions.