Researchers at University of California have examined wildfire activity in the Western United States from 2001 to 2024 and compared heat wave data to see if there was a notable impact on the fires.
Published in Science Advances, the findings were emphatic- despite heat waves only accounting for 12 percent to 15 percent of warmseason days, the team found that 42 percent of all the area burned by fires had occurred simultaneously or immediately after a heat wave.
Defining heat waves as three or more consecutive days with temperatures in the top tenth of hottest days, the study also observed that the amount of area burned each day was more than 50 percent larger during heat waves than during the cooler days immediately before heat waves began.
In some areas, the increase of area burned was up to 300 percent. High temperatures mean that the rate at which the air evaporates moisture from land and vegetation increases. This creates more dry material to burn through dead vegetation.
This increase in kindling is at increased risk of ignition through an increased chances of cloud-to-ground lightning, as the study also observed. The hot unstable atmosphere present during heatwaves is more likely to see lighting storms, with a higher risk of ‘dry lightning.’
This occurs when a storm system’s rain evaporates before it can reach the ground, meaning there is insufficient rainfall to douse flames. Nighttime humidity is also limited by heat waves, with drier air enabling fires to burn for longer periods and often through the night. Forests are particularly susceptible to fires.
Heat waves are becoming more common in the US as global temperatures rise and the research suggested that forest fires has also increased, with the amount forest area burned 2.5 times larger.
Without the increase in heat wave days, the cumulative area of burned forest would have been 37 percent smaller. However, grassland and shrublands did not see as strong a correlation between area burned and heat waves.
Wildfires in these environments was more influenced by the amount of available vegetation than by heat alone. Summers in the Western US are trending hotter and drier, meaning the consequential impact on wildfires is of extreme importance in management strategies.
Wildfire forecasts must include heat waves along with wind, humidity and fuel dryness, the paper urges, as well as awareness of longstanding fire deficits. By focusing on quickly extinguishing wildfires rather than allowing them to burn away forest debris, there is a huge stock of potential material to burn in future wildfires
