Smoke plumes from Australian bushfires had the biggest impact on the Earth's climate last year, even more so than the pandemic-related lockdowns, according to a new study.
The Australian fires cooled the Southern Hemisphere to the point that they ended up lowering the planet’s average surface temperatures, researchers wrote in the peer-reviewed study published Wednesday in Geophysical Research Letters.
Coronavirus lockdowns, on the other hand, had a slight warming effect on the planet, as reduced emissions led to clearer skies over many cities.
“The main climate forcing of 2020 wasn’t COVID-19 at all. It was the explosion of wildfires in Australia,” John Fasullo, a scientist at the National Center for Atmospheric Research and lead author of the study, said in a statement Wednesday.
As the bushfires ripped through Australia from late 2019 into 2020, plumes of smoke stretched into the stratosphere and ended up circling much of the Southern Hemisphere, Fasullo and his colleagues observed.
Sulfates and other smoke particles from the flames interacted with clouds to make their droplets smaller, which in turn reflected more incoming solar radiation back to space. With less solar radiation available to absorb, the Earth’s surface became significantly cooler.
The scientists also found that the pandemic-related lockdowns of 2020 will result in a gradual average warming worldwide of about 0.05 degrees Celsius by the end of next year. By comparison, the brush fires cooled the planet within six months by about 0.06 degrees Celsius.
The researchers said they used a computer modeling program to run more than 100 simulations that recreated global climates — both with and without emissions — under various atmospheric conditions from 2015 to 2024. They also evaluated the simulations in conjunction with real-world observations to better understand the impact emissions can have on the global climate.
“What this research shows is that the impact of regional wildfire on global climate can be substantial,” Fasullo said. “There are large-scale fingerprints from the fires in both the atmosphere and ocean. The climate response was on par with a major volcanic eruption.”
One reason that major fires can be so disruptive to the planet is that they inject huge amounts of sulfates and other particles into the atmosphere — pushing tropical thunderstorms northward from the equator and potentially influencing the El Niño and La Niña warming and cooling cycles in the tropical Pacific Ocean, according to the researchers.
While the climate response to the brush fires may have shared similarities with the impacts of volcanic eruptions, Fasullo stressed that volcanoes are much rarer events than wildfires.
“Those tend to happen every 30 years or so,” he said. “In contrast, major wildfires can occur every couple of years and therefore have more recurring impacts. We clearly need to learn more about how they affect global climate.”