Soot from Wildfires Warms and Dries Days, According to New Study
A new study from UC Riverside reveals that soot from large wildfires in California traps sunlight, leading to warmer and drier days than expected. This groundbreaking research shifts the focus from the well-documented effects of climate change on wildfires to the reverse—how large fires themselves may be altering the climate.
"I wanted to learn how the weather is affected by aerosols emitted by wildfires as they're burning," said James Gomez, the study's lead author and a UCR doctoral candidate.
To uncover these effects, Gomez analyzed peak fire days and emissions from each fire season over the past 20 years. He focused on days that were unusually cool or wet during the fire season, both with and without fires, to isolate the impact of fire-related aerosols from typical fire weather.
Published in the journal Atmospheric Chemistry and Physics, the study found that large wildfires indeed made days hotter and drier. This additional heat and dryness could potentially create more favorable conditions for subsequent fires.
The most intense fires were found in Northern California, where the vegetation is denser. On average, temperatures during the fires were about 1 degree Celsius warmer per day.
There are likely two reasons for this phenomenon. First, soot traps heat. Second, the extra heat reduces atmospheric humidity, making it harder for clouds to form.
Aerosols come in two types: reflective and absorptive. Reflective aerosols, such as sulfate aerosols from fossil fuel burning, can cool the environment by reflecting sunlight back into space. However, the study highlights an unintended consequence of improving air quality by reducing sulfate aerosols: it can exacerbate climate change and increase wildfire occurrences.
Absorptive aerosols, like black carbon, trap light and heat in the atmosphere, raising temperatures and reducing cloud formation and precipitation. This study found that black carbon from California wildfires does not increase the number of clouds, as it is hydrophobic, leading to less precipitation.
Notably, the study observed that days with fewer fire emissions had a more muted weather effect. "If the aerosols are coming out in smaller amounts and more slowly, the heating effect is not as pronounced," Gomez said.
Gomez remains optimistic that reducing CO2 emissions and improving land management practices can help mitigate the number of large wildfires.
"There is a buildup of vegetation here in California. We need to allow more frequent small fires to reduce the amount of fuel available to burn," he said in a press release. "With more forest management and more prescribed burns, we could have fewer giant fires. That is in our control."
Wildfires in the United States are not only becoming more frequent but also more intense, establishing a new norm that requires urgent attention and comprehensive understanding. In recent years, these fires have burned millions of acres, displacing communities and transforming entire geographical regions.
In 2023 alone, there were 56,580 wildfires in the US, which burned a total of 2,693,910 acres. These wildfires destroyed a total of 4,318 structures in 2023, including 3,060 homes. The annual cost of wildfires in the United States ranges from $394 billion to $893 billion.
