It’s hard to fathom that there’s an upside to air pollution. But it’s becoming clear that, paradoxically, cleaning up tailpipes and smokestacks comes with a price for the planet.

As pollution controls cut emissions of aerosols such as sulfur dioxide, scientists are uncovering the myriad ways these tiny, sunlight-reflecting particles have been taking some of the sting out of global warming.

Cuts in aerosol pollution have been linked to increased hurricanes in the North Atlantic, a spate of devastating underwater heatwaves in the North Pacific, and nearly half of the global rise in heat driving global warming over the first two decades of the 2000s.

Now, add to the list the increasing flammability of the world’s largest forest biome. “Cleaning up the air, which is something we all want to do, will accelerate global warming and also impact wildfires,” warned Robert Allen, a climate scientist at the University of California Riverside.

Allen is the lead author of new research that finds falling aerosol pollution could have a bigger effect than rising greenhouse gas emissions on the future risk of wildfires in the boreal forests.

These forests ring the upper reaches of the Northern Hemisphere like a green halo, covering vast parts of Alaska, Canada, Scandinavia and Russia. They account for nearly a third of the planet’s forestland and are the single largest store of land-based carbon.

While wildfires are a natural part of these ecosystems, there are concerns that rising temperatures are making them bigger. Over the last 60 years, the amount of boreal forest burned in North America has nearly doubled. In 2021, fires burning in boreal forests were responsible for a quarter of all carbon dioxide emissions from wildfires that year, prompting one scientist to warn that boreal forests could be a “time bomb of carbon.”

Allen, along with collaborators in the United Kingdom and Norway, wanted to find out how changes in aerosol pollution might factor into the future of these forests. They turned to a leading “coupled” climate model. The Community Earth System Model is a sophisticated computer program designed to represent the complex interplay between the atmosphere, the oceans and land. That includes forecasts of carbon emissions from fires—a gauge of the scale of wildfires.

The researchers then ran the model with two different future scenarios—one with few air pollution regulations and relatively high aerosol levels, and another with strict pollution limits. In both cases greenhouse gas emissions continued to rise. The results showed that between 2015 and 2060, a crackdown on aerosol pollution could lead to a more than 150% increase in carbon emissions from fires in northern boreal forests, the scientists reported in Science Advances.

The model results are driven chiefly by shallow soil becoming more parched due to the interplay between sunshine and moisture evaporating from the leaves of plants. As the number of reflective aerosols in the atmosphere decline, more sunlight reaches the Earth’s surface, rather than bouncing back into space. That increase in solar energy can drive higher levels of moisture loss from forests, particularly boreal forests. That translates into drier soil, which in turn sets the stage for wildfires. “When you dry out the soil, that also dries out plants, which makes them more combustible,” said Allen.

The solution obviously isn’t to stop cleaning up polluting factories. There are proponents of a controversial strategy that amounts to simulated air pollution – injecting sulfur dioxide particles high into the atmosphere to cool the planet. While some have touted this as a relatively quick and easy way to slow global warming, it has encountered strong resistance amid concerns about the unintended consequences of further meddling with planet-wide forces.

But Allen points to the results as further evidence for the need to tackle the greenhouse gas emissions that also increase wildfire risk in boreal forests. The computer models showed future carbon releases by wildfires would increase by nearly 120% due to rising climate pollution.

Cuts in release of methane, a potent greenhouse gas, could cancel out the wildfire-related effects of aerosols, Allen said. “We’re on the right trajectory with regards to aerosols and methane,” he said. “I still have hope, but there is a narrow window of time to make this happen and avoid the worst projected climate impacts.”

Allen, et. al. “Are Northern Hemisphere boreal forest fires more sensitive to future aerosol mitigation than to greenhouse gas-driven warming?” Science Advances. March 29, 2024.