Not long after the Black Summer bushfires generated enough smoke to be seen from space, scientists noticed something concerning in the stratosphere.
The ozone layer had become much thinner — thanks in part to the record-breaking plume of smoke which emanated from Australia — but scientists weren't sure exactly how it happened.
New research reveals how bushfire smoke can spark a chemical reaction that eats through ozone, and also sheds light on the scale of the damage.
The findings have raised concerns that repairing the existing hole in the ozone layer may take longer as climate change worsens bushfires.
But how exactly does bushfire smoke damage the ozone layer — and what does that mean for the hole near Australia?
How much of the ozone layer was destroyed by bushfires?
Between 3 to 5 per cent of the ozone layer was destroyed in the months following the 2019-2020 bushfires, the research published in scientific journal Nature on Thursday said.
The damage was so great, in some places researchers recorded "the lowest ozone that we've ever seen", the study's lead author, Susan Solomon, an atmospheric chemist at Massachusetts Institute of Technology (MIT), told the ABC.
The losses temporarily cancelled out the gains made by the Montreal Protocol, a 1987 treaty that agreed to phase out substances that damage ozone.
We'll talk more about that treaty further down.
The other concerning effect of the bushfires was that the expanded ozone hole lasted for a record-long time, the paper said.
What is the ozone layer?
The ozone layer sits about 15-35 kilometres above the Earth's surface and shields us from the brunt of the sun's harmful UV rays.
"Life on earth was only able to come out of the ocean and live on land because the ozone layer evolved, so we really can't afford to damage it," Dr Solomon said.
The thinnest part of the ozone layer — known as the "ozone hole" — hovers over Antarctica, over the South Pole.
Every year that hole expands and shrinks but in the months after the Black Summer bushfires, it expanded by about 2.5 million square kilometres, the new paper says.
How did Australia’s bushfire smoke damage the ozone layer?
The damage is all connected to everyday items that were popular more than 50 years ago.
Late last century, the ozone layer started thinning and the famous "ozone hole" appeared over Antarctica.
In the 1970s and 1980s, scientists concluded that chemicals called chlorofluorocarbons (CFCs) — which were used in aerosol sprays, solvents, and refrigerators — were responsible for the damage.
CFCs can rise into the atmosphere and break down, releasing a molecule that destroys ozone.
They can also last a long time, lingering in the atmosphere for up to 150 years.
That brings us to the 2019-2020 Black Summer bushfires.
The sheer scale of the catastrophe meant bushfire smoke travelled into the stratosphere, sparking a chemical reaction that essentially "activated" some of those ozone-destroying compounds still lingering there from decades ago.
The good news is, it appears bushfire smoke on its own does not shred the ozone layer.
So once those CFC remnants eventually fade away, the ozone-gobbling effect of bushfire smoke should stop.
The other good news is that not every bushfire has the potential to do this — just the huge ones, explains Martin Jucker, a lecturer at the Climate Change Research Centre at the University of New South Wales.
"The one that's currently happening in New South Wales will probably not have an effect on the ozone layer next year," Dr Jucker said.
Is it just Australian bushfire smoke that does this, or could any huge fire eat up ozone?
Good question! Scientists are wondering the same thing.
The paper said more research should be done into whether any huge bushfire could have this ozone-destroying effect, or whether it could be from burning eucalyptus in particular.
"If it's not special [to Australia] and every fire everywhere could do this … then I think the problem becomes much worse for everyone," Dr Solomon said.
Why was the ozone hole repairing before the bushfires?
After scientists confirmed CFCs were destroying the Ozone layer, the world agreed to phase out those compounds.
This agreement was called the Montreal Protocol, and it was hailed as a huge success in creating policy based on science.
Eventually, the ozone layer started recovering by about 1 per cent per decade.
It was those gains that were temporarily wiped out by the black Summer fires, said Ian Rae, an honorary professor at the University of Melbourne's School of Chemistry.
"It cancelled out the good work of the Montreal Protocol for that year," Dr Rae said.
This smoke study has some scientists worried that ozone recovery could be slowed or delayed by extreme fire events.
Those concerns are amplified by the knowledge that climate change is projected to make bushfires more frequent and more severe.
"We can be proud of what humanity achieved in saving the ozone layer, but this could slow it down and it's all the more reason to be concerned about climate change," Dr Solomon said.
