Australia's catastrophic bushfires over 2019/2020 burned an estimated 46 million acres - and might have also helped tip the planet into a La Niña state. Photo / Nathan Edwards
Could Australia’s catastrophic bushfires have been partly to blame for the relentless run of warm, wet weather a rain-soaked North Island has endured under a rare “triple-dip” La Niña?
That’s an intriguing possibility raised in a major new study exploring how emissions from the unprecedented fires –
After three years of it - and a wrecked, record-wet summer for many in the North Island - Kiwis would now be all-too-familiar with La Niña meddling with our regional weather patterns.
It’s one end of the natural ocean-atmosphere seesaw we know as El Niño-Southern Oscillation (Enso), specifically measuring the movement of warm, equatorial water across the Pacific Ocean.
In New Zealand, La Niña tended to deliver more northeasterly winds that brought rainy conditions to the North Island’s northeast - and drier conditions to the south and southwest of the South Island.
While the system finally dissipated earlier in autumn, the wet weather that’s drenched the north over recent days has been linked to a “La Niña-like cough” - or a lag effect between ocean and atmosphere.
On top of its broad influence, scientists have also been observing increasingly extreme weather events: partly a result of the extra moisture that global heating has loaded into the atmosphere.
Still, it was remarkably rare to experience three of these back-to-back, something that last occurred in the early 1970s, and one of only three events of this type documented since 1950.
The recent La Niña streak was also unusual because it was the only one that didn’t follow a strong El Niño — a warming instead of cooling in the Tropical Pacific with similar but opposite climate impacts.
The new study - published in leading journal Science Advances and led by the US-based National Center for Atmospheric Research (NCAR) - happened to pin the phenomenon on a fascinating trigger.
“Many people quickly forgot about the Australian fires, especially as the Covid pandemic exploded, but the Earth system has a long memory, and the impacts of the fires lingered for years,” said the study’s lead author, Dr John Fasullo.
Scientists have previously established that events in the Earth system, including large volcanic eruptions in the Southern Hemisphere, can shift the odds towards a La Niña state emerging.
In the case of a volcano, emissions spewed high into the atmosphere could result in the formation of light-reflecting particles called aerosols, which could cool the climate and ultimately create favourable conditions for La Niña.
Given the massive scale of the Australian fires - which burned an estimated 46 million acres - Fasullo and his co-authors wondered what climate impacts the resulting emissions might have had.
To investigate the question, the researchers used an advanced NCAR-based computer model called the Community Earth System Model, version two, to run two batches of simulations.
All the simulations started in August of 2019, before the blazes in Australia became historically large, but only one set incorporated the emissions from the wildfires as observed by satellite.
The other used average wildfire emissions, the normal practice when running long-term climate model simulations.
The research team found that the emissions from the wildfires, which quickly encircled the Southern Hemisphere, kicked off a chain of climate interactions.
After the recent La Nina 3-peat, scientists detailed a possible link between this rare event & the effects of the Australian bushfires of 2019.@jfasullo discovered a “Rube Goldberg” machine of climate interactions that led from 🔥emissions to a La Niña.https://t.co/Zj4e9PTegF pic.twitter.com/nLOPZnrdGD
— National Center for Atmospheric Research (@NCAR_Science) May 10, 2023
Unlike a volcanic eruption, the bulk of wildfire emissions did not make it high enough in the atmosphere to cool the climate by directly reflecting sunlight.
Instead, the aerosols that formed from the emissions brightened the cloud decks across the Southern Hemisphere and especially off the coast of Peru, which cooled and dried the air in the region, ultimately shifting the zone where the northern and southern trade winds come together.
The net result was a cooling of the Tropical Pacific ocean, where La Niñas form, over multiple years.
“It’s a Rube Goldberg of climate interactions that we were only able to identify because our model now represents specific details in the evolution of smoke and cloud-aerosol interactions, a recent improvement to its capabilities,” Fasullo said.
In June 2020 - just a few months before the first of the three La Niñas formed - some seasonal forecasts were still predicting “neutral” conditions in the Tropical Pacific, meaning that neither La Niña nor El Niño was favoured.
Instead, a strong three-year La Niña materialised.
Fasullo said the new research helped explain this missed forecast and highlights the importance of using a coupled Earth system model, which includes the atmosphere and the ocean, as a forecast tool.
The research also underscored the importance of having realistic wildfire emissions, both in seasonal climate predictions and long-range climate projections.
Currently, biomass burning emissions in most climate model simulations were prescribed - meaning they were imposed on the model run and not determined by interactions happening within the model.
For example, a simulated hot and dry period in the model simulation would not lead to more wildfires, and therefore more emissions within the simulation.
“As the climate changes, the emissions from wildfires will also change,” Fasullo said.
“But we don’t have that feedback in the model. It is the goal of our current work to incorporate these effects as realistically as possible.”
Victoria University climate scientist Professor James Renwick said the suggestion the bushfires may have primed the three-peat was “really plausible”.
“We’ve known for a long time that volcanic eruptions can affect the [Enso] cycle, so it makes sense that you can have the same effect with a big bushfire that also sends a lot of stuff into the atmosphere and blocks out sunlight.”
Renwick said the jury was still out on what climate change meant for the Enso cycle: while IPCC models favoured a shift to more El Niño-like states, which we’re about to see later this year, some climate scientists have more recently suggested a skew toward La Niña.
Either way, it was becoming much more apparent that climate change was making weather patterns more extreme under Enso – meaning Kiwis in the north and east, particularly, could expect to see bigger downpours under La Niña, and more intense droughts under El Niño.
National Institute of Water and Atmospheric Research (Niwa) meteorologist Chris Brandolino also questioned whether – if the new study’s findings held – we might see an emerging feedback loop.
That was with La Niña conditions in eastern Australia providing more growth and “fuel” for disastrous bushfires under El Niño, which in turn could help flip the cycle back to La Niña - and all against a climate change background.
Climate scientist Dr Nathanel Melia also read the new study with interest.
“A triple-dip La Niña is super-rare, and the unprecedented magnitude of particles the Australian fires injected into the Southern Hemisphere, and their cooling effect, like microscopic mirrors, may explain La Niña’s permanence,” he said.
“While the authors are rightly cautious, this ingenuous piece of climate detective work could have solved the triple-dip La Niña conundrum.”
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