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The aerosol outburst from bushfires that burned over more than 70,000 square miles of Australia in 2019 and 2020 was so persistent and widespread that it lit up a vast expanse of cloud over the subtropical Pacific.
Beneath those clouds, the ocean surface and atmosphere cooled, shifting a major tropical shower belt northward and pushing the Equatorial Pacific toward an unexpected and prolonged cool phase of the La Niña-El Niño cycle, according to research released today. has been published in Science Advances.
Wildfire aerosols are basically fire dust — microscopic bits of charred mineral or organic matter that can carry superheated wildfire clouds into the stratosphere and spread across hemispheres with different climatic effects depending on where they are produced and where they end up.
In the new modeling study, the scientists quantified how aerosols from the Australian bushfires caused clouds over the tropical Pacific to reflect more sunlight back into space. The cooling effect was equivalent to turning off a 3-watt light bulb on every square foot of the ocean area. And that cooling, their data showed, shifted the cloud and rain belt, called the Intertropical Convergence Zone, to the north.
Combined, the effects may have helped create the rare three-year La Niña, from late 2019 to 2022. The effects of the La Niña rippled around the world, intensifying drought and famine in East Africa and preparing the Atlantic on hurricanes. as 2020 became the most active tropical storm season on record with 31 tropical and subtropical systems, including 11 storms making landfall in the US, including four in Louisiana alone.
“The findings highlight widespread multi-year climate impacts caused by an unprecedented wildfire season,” said lead author John Fausulloan atmospheric scientist with the National Center for Atmospheric Research in Boulder, Colorado.
“From November to January, huge amounts of smoke were continuously pumped into the atmosphere,” he said. “The more particles you have in the atmosphere, the more cloud droplets you get, the brighter those cloud droplets are and the longer they live in the atmosphere. These stratus cloud decks are so important to the Earth’s energy budget that it makes a big difference if you can disrupt them a little bit.”
Such large-scale interactions between wildfire aerosols and climate “may become more common under climate change, as wildfires are expected to intensify and become more frequent,” the authors warned.
Wildfire emissions are part of wider changes
The new paper joins other recent research into how global warming is affecting ocean currents in the Southern Ocean, said NCAR climate scientist Stephen Yeager, who was not involved in the new study.
“The link to climate change is kind of hidden in there,” he said. “The wildfires triggered this chain of events that led to cooling in the eastern Pacific. It’s a chain reaction that we can expect more of in the future… as we expect wildfire emissions to rise. We expect Australia to become drier and these bushfires to get worse.”
Yeager said the new research is valuable because it shows how changes in ecosystems and disturbances on land can affect the ocean and atmosphere.
