Large wildfires create weather conditions conducive to more fires

A new study from UC Riverside shows that soot from large California wildfires traps sunlight, making days hotter and drier than they should be.

Many studies examine the effect of climate change on wildfires. However, this study sought to understand the opposite: whether large fires also change the climate.

“I wanted to know how weather patterns are affected by aerosols emitted from wildfires as they burn,” said James Gomez, lead author of the study and a UCR doctoral student.

To find his answers, Gomez analyzed peak fire days and emissions from each fire season over the past 20 years. Of those fire days, he looked at a subset that occurred when temperatures were lower and humidity was higher. “I have observed unusually cool or wet days during fire season, with or without fires. This largely eliminates the effects of fires,” Gomez said.

Published in the journal Atmospheric chemistry and physics, the study found that large fires did indeed have an effect. They made it hotter and drier than usual on days when the fires were burning. The additional heat and aridity can then create favorable conditions for more fires.

“It seems like these fires create their own fire climate,” Gomez said.

The most intense fires have been in Northern California, where the vegetation fueling the fires is denser than elsewhere in the state. On average, temperatures were about 1 degree Celsius higher per day during the fires.

There are probably two reasons for this. First: the soot traps heat, and second: the extra heat reduces the humidity in the atmosphere, making it more difficult for clouds to form.

“Fires emit smoke containing black carbon, or soot. Because it's so dark, soot absorbs sunlight more easily than shiny or reflective objects,” Gomez said.

There are two types of aerosols: reflective and absorbent. Sulfate aerosols, which are byproducts of fossil fuel combustion, are reflective and can cool the environment. These particles reflect the sun's energy back into space, keeping it out of the atmosphere.

Recent UCR research highlights an unfortunate byproduct of improving air quality by reducing sulfate aerosols. Since these particles have a cooling effect, their removal worsens climate change and leads to an increase in wildfires, particularly in forests in the Northern Hemisphere.

Sulfate aerosols can also help make clouds brighter, more reflective, and more effective at cooling the planet.

The researchers note that the only way to prevent more wildfires when cleaning up air pollution reflected from sulfates is to simultaneously reduce emissions of greenhouse gases like carbon dioxide and methane.

Absorbent aerosols have the opposite effect. They trap light and heat in the atmosphere, which can cause the temperature to rise. Black carbon, the most common aerosol emission from wildfires, is an absorbent aerosol. Not only do they directly increase temperatures, but also indirectly by discouraging cloud formation and precipitation.

“What I found is that the black carbon emitted by these wildfires in California does not increase the number of clouds,” Gomez said. “It’s hydrophobic.” Fewer clouds mean less precipitation, which is a problem for drought-prone states.

While some studies have shown an association between fires and brighter, more numerous clouds, this one has not.

Notably, the study found that days with fewer fire emissions had a more muted effect on the weather. “If the aerosols come out in smaller amounts and more slowly, the heating effect is not as pronounced,” Gomez said.

Gomez hopes CO mitigation₂ CO2 emissions, combined with better land management practices, can help reduce 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,” Gomez said. “With more forest management and more prescribed burning, we could have fewer giant fires. That's within our control.”