The aerosol fallout from wildfires that burned across more than 70,000 square miles of Australia in 2019 and 2020 was so persistent and widespread that it brightened a vast area of clouds above the subtropical Pacific Ocean.

Beneath those clouds, the ocean surface and the atmosphere cooled, shifting a key tropical rainfall belt northward and nudging the Equatorial Pacific toward an unexpected and long-lasting cool phase of the La Niña-El Niño cycle, according to research published today in Science Advances.

Aerosols from wildfires are basically fire dust—microscopic bits of charred mineral or organic matter that can ride super-heated wildfire clouds up to the stratosphere and spread across hemispheres with varied climatic effects, depending on where they’re produced and where they end up.

In the new modeling study, the scientists quantified how aerosols from the Australian wildfires made clouds over the tropical Pacific reflect more sunlight back toward space. The cooling effect was equivalent to switching off a 3-watt light bulb over every square meter of the ocean region. And that cooling, their data showed, shifted the cloud and rain belt called the Intertropical Convergence Zone northward.

Combined, the effects may have helped trigger the rare three-year La Niña, from late 2019 through 2022. The impacts of the La Niña rippled around the world, intensifying drought and famine in Eastern Africa, and priming the Atlantic Ocean region for hurricanes, as 2020 became the most active tropical storm season on record with 31 tropical and subtropical systems, including 11 storms that made landfall in the US, including four alone in Louisiana.

“The findings highlight widespread multi-year climate impacts caused by an unprecedented wildfire season,” said lead author John Fasullo, an atmospheric scientist with the National Center for Atmospheric Research in Boulder, Colorado.

“From November through January, massive amounts of smoke were pumping continually into the atmosphere,” he said. “The more particulates 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 for Earth’s energy budget that if you’re able to perturb them a little bit, it makes a big difference.”

Such large-scale interactions between wildfire aerosols and the climate “may become more prevalent under climate change as wildfires are projected to intensify and become more frequent,” the authors warned.

Wildfire emissions are part of broader changes

The new paper fits together with other recent research on how global warming affects 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 set off this chain of events that lead to cooling in the eastern Pacific. It’s a chain reaction that we can expect more of in the future… because we expect wildfire emissions to go up. We expect Australia to get drier and for these wildfires to get worse.”

Yeager said the new research is valuable because it highlights how ecosystem changes and disruptions on land can affect the ocean and atmosphere.

https://arstechnica.com/?p=1938497