The colossal bushfires that tore through Australia in 2019 and 2020 charred some 37,500 square miles, obliterating iconic ecosystems and pushing already-threatened species to the brink. The blazes were so big they spawned their own towering thunderclouds.
Thousands of miles away, clear across the Pacific Ocean, the bushfires were affecting something more subtle yet very consequential: New modeling shows that the smoke helped cool the waters off South America, greatly increasing the chances of the rare three-year La Niña that ensued. That’s the band of chilled water in the Pacific that lasted from late 2020 to early 2023. La Niña influences weather around the world, so the bushfires ended up having a widespread effect, long after the last embers stopped glowing in Australia.
“It skewed the probabilities toward a La Niña event for multiple years,” says John Fasullo, a climate scientist at the US National Center for Atmospheric Research and lead author of a new paper describing the modeling, which was published today in Science Advances. “This is actually—in terms of the historical records—one of the largest perturbations to the Southern Hemisphere that we’ve seen.”
Check out the map above. As fires raged in Australia in December 2019, smoke (high concentrations are shown in red) blew not only across the Pacific Ocean but also wrapped around the Southern Hemisphere. Up in the atmosphere, those innumerable aerosol particles—charred plant material, even bits of structures that had burned—became little nuclei for water to condense upon. This is how clouds form normally, when water aggregates around dust particles. But in the Southern Hemisphere, there aren’t a whole lot of these nuclei blowing around. (In the Northern Hemisphere, there are more land masses to provide particulate matter.)
“Having this huge source of the Australian wildfires actually had a big effect and created all these cloud condensation nuclei. And, in turn, that made the clouds brighter,” says Fasullo.
When the clouds brightened off the Pacific coast of South America, more of the sun’s energy got bounced back into space. That cooled the Pacific Ocean. On the map above, the blue blob in the red box indicates where there was less solar heating on the water’s surface in January 2020, at the peak of Australia’s brutal wildfire season, when smoke was brightening clouds near Peru and Chile. “They reflected a lot of energy to space,” says Fasullo. “And so they ended up cooling off the Southern Hemisphere, and did so in a region that’s actually very important for La Niña.”
La Niña forms when east-to-west winds strengthen, pushing surface water away from South America’s west coast. To fill the void, colder waters upwell from the depths, creating a smear of coolness that stretches across the Pacific. The maps below show how this looked in October 2020 and 2021.
Typically, a La Niña lasts a year or two and can have a wide range of global effects—the southern US tends to get drier, for instance, and the Pacific Northwest gets wetter. But this one began in 2020 and ended just a few months ago.
This La Niña was also weird in that it didn’t follow a strong El Niño, when warm water instead forms in the same region. (There can also be “neutral” conditions between those cooling and warming trends.) “These things have happened before, but it’s very unusual to have either one of those things, not to mention both of them,” says Fasullo. “That alone leads us to think that something unusual was happening there.”
That something unusual may have been the additional cooling provided by a months-long stream of bushfire smoke across the Pacific, which initiated a kind of cooling feedback loop. “A cooling patch cannot just sit there. It has to propagate,” says Shang-Ping Xie, a climate scientist at the Scripps Institution of Oceanography, who wasn’t involved in the research. “The cooling is going to increase the local atmospheric pressure, and atmospheric pressure is going to change the winds, and winds are going to further modify sea surface temperature patterns.” As the winds pushed water to the west and away from the equator, that led to more upwelling that brought up more cold water.
This may not have been the first time that bushfire smoke significantly affected La Niña. Fasullo and his colleagues are now investigating Australia’s notoriously awful 1974–75 fire season. In 1975 and 1976, scientists had forecast a warm El Niño, but that turned into what researchers instead dubbed an “aborted El Niño event,” when a cool La Niña formed instead. “As it turns out, we do have some case studies that we’re looking at from the ’70s,” says Fasullo. “We think it may have been due to Australian bushfires.”
That could mean that wildfires play a more active role in La Niña and El Niño than previously believed. “This is especially important given the background warming of the climate is going to increase the frequency and severity of wildfires,” says Xie. The more the world warms and dries, the bigger and hotter wildfires get, potentially creating more smoke that can drift across the Pacific. The route of smoke traveling from Australia is perfectly positioned to mess with the natural variability of ocean temperatures off the coast of South America.
And there’s another X factor: Wildfires are only one source of aerosols in the atmosphere. Others arise from the burning of fossil fuels. Like smoke, these actually help cool the planet by reflecting sunlight and acting as cloud nuclei. (Particulate pollution from cargo vessels, for example, is famous for creating “ship tracks” of cooling clouds.) But as humanity switches to green energy, we’ll produce fewer of these aerosols, and wildfire smoke aerosols may become even more impactful.
“We are pretty sure that anthropogenic aerosols are going to reduce, so that means those natural aerosols could be more important to the climate system,” says Hailong Wang, an earth scientist at the Pacific Northwest National Laboratory, who wasn’t involved in the new research.
Incorporating wildfire smoke into La Niña and El Niño forecasts could make them more accurate. That’s critical, because it would allow policymakers to prepare for what’s coming. For example, if La Niña ends up causing extreme precipitation, cities need to get their infrastructure ready. And if it brings drought, water managers need to handle potential supply issues.
Luckily, with more data and increasingly sophisticated modeling, predictions will get better. Back in June 2020, Fasullo says, the National Oceanic and Atmospheric Administration had actually expected neutral conditions in the Pacific. “This was a month before one of the most prolonged La Niña events on record—a kind of historic missed forecast,” says Fasullo. Today, he says, “we still don’t ourselves understand the full potential here. But certainly the take-home from this paper alone is that wildfires in certain circumstances provide some seasonal predictability that we’re not taking advantage of.”