The bushfires also unleashed plumes of smoke so voluminous they could be seen from space.
Surprisingly, that smoke may have helped spark an important climate phenomenon half a world away: a rare 3-year stint of cool La Niña conditions in the tropical Pacific Ocean, according to a new modeling study published today in Science Advances.
The result underscores the idea that fires and other events “can have really strong impacts on the climate that we don’t fully understand,” says Samantha Stevenson, a climate scientist at the University of California, Santa Barbara, who was not part of the study team. It also suggests forecasts of La Niña and other seasonal events could be improved if scientists could account for wildfires.
La Niña refers to the periodic cooling of surface waters in the tropical Pacific caused by chilly waters welling up from the depths off the coast of South America. The shift impacts weather patterns around the world. This latest “triple-dip” La Niña brought floods to Pakistan and eastern Australia, and worsened droughts in the Horn of Africa, where tens of millions of people are contending with severe hunger and water shortages.
In most cases, La Niña only sticks around for a year or two, trading off with its opposite, El Niño, which warms Pacific surface waters. But the world is only now emerging from La Niña conditions that began in 2020. And unlike other triple-dip La Niñas, the latest event didn’t follow a strong El Niño. Many forecasts failed to predict its arrival. “It’s almost unprecedented,” says John Fasullo, a climate scientist at the National Center for Atmospheric Research (NCAR) who led the study.
Using NCAR’s climate model, Fasullo and his colleagues simulated global atmospheric conditions from 2019 through 2021. In one set of simulations, the model accounted for smoke in a general way, by factoring in an estimate of background wildfire emissions. In a second set, the model incorporated specific satellite data for wildfire smoke—much of it coming from Australia.
Wildfires reshape forests and change the behavior of animals
Past studies suggested the gaseous plumes from volcanic eruptions could produce La Niña–like conditions by reflecting sunlight and cooling the surface. But volcanic aerosols tend to linger longer and float higher in the atmosphere than wildfire smoke, prolonging their impacts. So the team assumed the effect of smoke on La Niña would pale in comparison. “But we were wrong,” Fasullo says.
The simulations using the satellite wildfire data pushed the world closer to La Niña than the models that relied on estimated emissions—and revealed a likely chain of events that implicated the Australian bushfires. The smoke particles, which can coax water vapor to condense into tiny droplets, created brighter and longer lasting clouds across the Southern Hemisphere. These clouds reflected more sunlight back into space, cooling and drying out the air.
Next, this cooler air shifted the location of the Intertropical Convergence Zone (ITCZ), a band of rain usually perched along the equator where the easterly winds of the Northern and Southern hemispheres collide. The cool air nudged the ITCZ north, which intensified the easterly winds above the tropics. These winds pushed warm surface waters to the west, which helped cold waters rise from the deep and solidified La Niña’s reign.
Although the wildfires seemed to clearly impact La Niña in the simulations, no model is perfect, says Robert Jnglin Wills, a climate scientist at ETH Zürich who didn’t work on the study. “Can we trust this is a good representation of how the real world responds?”
Testing multiple models would help verify the findings, especially ones used for real-time forecasting, says Michelle L’Heureux, a climate scientist at the National Oceanic and Atmospheric Administration’s Climate Prediction Center. She says it’s also difficult to disentangle the effect of the wildfires from the influence of a separate unexplained trend that has been nudging the tropical Pacific toward La Nina conditions since the 1970s. “It’s possible we’re just seeing two factors here,” L’Heureux says, “and we still need to understand how much [of each] is driving what.”
Fasullo’s team is testing the model with other potential wildfire-influenced La Niñas. He says it’s probably not a coincidence that one of the three other recorded triple-dip La Niñas since 1950 coincided with another set of massive Australian bush fires in 1974–75.
Fasullo says the study also underscores the need for seasonal forecasts and climate models to properly account for wildfires. A 2022 United Nations report found that the likelihood of catastrophic fires could increase up to 57% by the end of the century because of climate change and shifts in land use. Untangling the global impact of these regional events may become even more important in the future, Fasullo says. “Wildfires, in certain circumstances at least, might offer a very important source of predictability.”
