Copious amounts of smoke aerosols from the 2019–20 bushfires may have contributed to the onset of the rare, triple-dip La Niña.
In a study published in the journal Science Advances, a team of researchers ran climate models with and without bushfire emissions to determine the role the months-long bushfire event played.
The idea for the study was sparked by research into emission reductions from the COVID pandemic.
Lead author John Fasullo from the National Centre for Atmospheric Research in the United States said the team intuitively thought the bushfires may have had a larger climate response than the drop in emissions that resulted from the pandemic.
And they were right.
"In fact, we underestimated the effects, if anything, of the wildfires," Dr Fasullo said.
"But what we found in the initial two years itself was actually pretty interesting – we have this La-Niña-like response."
It is the first time a bushfire event has been widespread enough to have an impact on climate models.
The findings could provide important insight into the link between large bushfires and climate events of the past when the necessary satellite data wasn't available, Dr Fasullo said.
Clouds brightened, surface cooled
During what's become known as Black Summer, bushfires of unprecedented and uncontrollable scale razed the landscape and burned with unusual intensity and size.
The months-long event frequently pumped vast smoke aerosols into the atmosphere that could be seen from space – on par with major volcanic eruptions.
At one point in December, more than 100 fires were simultaneously burning in New South Wales alone.
The ongoing bushfire emissions from August 2019 to July 2020 caused clouds to become brighter, thicker and longer lasting, Dr Fasullo said, and, in turn, created a cooling effect across the southern hemisphere.
"So when you introduce smoke into the system, all of a sudden you have all these little particulates for the clouds to condense on," he said.
"And two things happen — one is that those cloud droplets become brighter, but also when they collide with each other they don't rain out of the atmosphere as quickly.
"So the cloud droplets stay in the atmosphere, they don't get as big, they don't rain out and because of that more sunlight gets reflected to space."
Chain reaction may have triggered La Niña
The influence of the bushfire emissions lasted well beyond the relatively short amount of time it took for the particles to clear the troposphere, Dr Fasullo said.
By cooling the surface through cloud, it set off a chain reaction of events that created a "lasting memory" in the ocean and atmosphere, he said.
This was likely to have created favourable conditions for the La Niña to form in 2020, he said.
He said this was by influencing a large tropical cloud band northwards, which allowed trade winds to come in underneath it."
"So that ties together the fires all the way to the emergence of the La Niña," Dr Fasullo said.
"The trade winds over the equator lead to this steep ocean upwelling, and that is a La Niña event.
"I would say that the bushfires roughly doubled the chances of a prolonged La Niña event.
"They certainly added in a material way to both the probability of La Niña and also the intensity of La Niña for at least two years."
He said the process was similar to the effects caused by large volcanic eruptions, which were large enough to push ash particles into the stratosphere.
But Dr Fasullo said the bushfire event's climate influence came largely from the smoke emissions in the troposphere.
The triple-dip La Nina began in 2020 and lasted until 2022, dominating the weather pattern across Australia and helping deliver record-breaking flooding events to the country's east.
Understanding the 'strange' three-year event
The research may provide an explanation for some of the "unusual" features of the multi-year La Niña event, Dr Fasullo says.
"There have been a number of things about this La Niña event that were quite curious," he said.
"Typically, your prolonged La Niña events will follow an El Niño event. But that was not the case.
"This time, this La Niña event just kind of jumped out of nowhere."
In 2020, when the La Niña first formed, Dr Fasullo said it was a very poorly forecast event.
"So as late as June of 2020, the National Oceanic and Atmospheric Administration (NOAA), which is the chief forecast body in the US, was predicting neutral conditions and no La Niña," he said.
"As it turns out, they were right on the cusp of this record three-year event that had extraordinarily strong cooling anomalies in the Pacific Ocean.
"So a number of strange things about this event made us wonder if something else was going on."
Other research has found that the Black Summer bushfires likely damaged the ozone layer, causing a deep and long-lived hole and the highest temperatures in the stratosphere in 30 years.
