Larger wildfires create favorable conditions for more fire to spread

Smoke traps sunlight and makes days warmer and drier.

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The increase in large wildfires in the southwestern United States, particularly in northern California (nCA), has had a significant impact in the last decade. These fires have been linked to substantial emissions of absorbing aerosols and heat into the troposphere.

A recent study conducted by UC Riverside has revealed that soot from these wildfires in California is causing sunlight to be trapped, leading to warmer and drier days than usual. While many studies focus on the impact of climate change on wildfires, this study delves into the reverse perspective, exploring how large fires may also be influencing the climate.

“I wanted to learn how the weather is affected by aerosols emitted by wildfires as they’re burning,” said lead study author and UCR doctoral candidate James Gomez.

Gomez’s quest led him to scrutinize emissions and peak fire days over the past two decades. He focused on a subset of fire days coinciding with lower temperatures and higher humidity to isolate the effects of aerosols from fire-generated weather patterns.

“I looked at abnormally cool or wet days during fire season, both with and without fires. This mostly takes out the fire weather effects,” Gomez said.

The study’s findings revealed the significant impact of large fires on the environment. These fires not only intensified the heat and dryness but also created favorable conditions for more fire to spread. According to Gomez, it seems that these fires are generating their own fire weather.

The most severe fires were observed in Northern California, where the dense vegetation acted as fuel for the fires.

On average, temperatures rose by approximately 1 degree Celsius per day during the fires. This is likely due to two factors: soot trapping heat and the additional heat reducing atmospheric humidity, making it challenging for clouds to form.

“Fires emit smoke with black carbon or soot. Since it is very dark, the soot absorbs sunlight more readily than bright or reflective things,” Gomez said.

Research indicates that aerosols come in two types: reflective and absorptive. Reflective sulfate aerosols, a result of fossil fuel burning, play a crucial role in cooling the environment by reflecting the sun’s energy back into space.

However, reducing these aerosols to improve air quality may unintentionally exacerbate climate change, leading to more severe consequences, such as increased wildfires in northern hemisphere forests.

Furthermore, sulfate aerosols also contribute to making clouds brighter and more reflective, enhancing their effectiveness in cooling the planet.

The researchers note that mitigating the impact of reflective sulfate air pollution requires a simultaneous reduction in greenhouse gas emissions such as carbon dioxide and methane to prevent additional wildfires. Absorptive aerosols, such as black carbon emitted from wildfires, have the opposite effect as they trap light and heat, leading to increased temperatures. Additionally, they indirectly discourage cloud formation and precipitation, further exacerbating the impact on the climate.

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

Some studies have shown an association between fires and brighter, more numerous clouds, but this one did not. The study found that days with fewer fire emissions had a less significant effect on the weather. According to Gomez, if the aerosols are released in smaller amounts and more slowly, the heating effect is not as pronounced.

Gomez hopes that mitigating CO2 emissions and improving 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 burns, we could have fewer giant fires. That is in our control.”

Journal reference:

  1. James L. Gomez, Robert J. Allen, and King-Fai Li. California wildfire smoke contributes to a positive atmospheric temperature anomaly over the western United States. Atmospheric Chemistry and Physics, 2024; DOI: 10.5194/acp-24-6937-2024

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