Australia’s bushfire season of 2019 to 2020 was one of the worst on record for the country, scorching millions of acres, killing at least 33 people and releasing more than a million tons of smoke into the atmosphere. Research later revealed that the smoke had eaten away at the ozone layer—the Earth’s protective shield against the sun’s ultraviolet rays. But exactly how that happened remained unclear.
Now, in a new study published last week in Nature, researchers found that particles from wildfire smoke can set off chemical reactions in the atmosphere that erode its ozone.
The smoke from Australia’s wildfires temporarily depleted 3 to 5 percent of the ozone above parts of the Southern Hemisphere, per the study. It also worsened the ozone hole over Antarctica, widening it by 2.5 million square kilometers, or 10 percent of its area compared to the previous year.
“From a scientific point of view, it’s very exciting to see this brand new effect,” lead author Susan Solomon, an atmospheric scientist at MIT, tells the Guardian’s Donna Lu. “From a planetary point of view … it would be just tragic to have mankind screw up solving the ozone hole by deciding that we’re going to [allow] a lot more of these fires if we don’t mitigate climate change.”
Humans created the ozone hole by using now-banned substances called chlorofluorocarbons (CFCs). When these break down, they release chlorine, which eats away at the ozone. Some of this destructive chlorine, however, can get safely stored in the atmosphere as hydrochloric acid (HCl). In this form, it won’t eat away at Earth’s protective blanket. But, the researchers found, wildfire smoke broke down HCl in the lower stratosphere, generating a chemical called chlorine monoxide—“the ultimate ozone-depleting molecule,” per an MIT statement.
In the study, the researchers examined three sets of satellite data from after the fires. They saw that at mid-latitudes—above Australia, New Zealand and regions of Africa and South America—concentrations of HCl had dropped, while the harmful chlorine monoxide had risen.
This surprised the scientists—previously, researchers had only seen HCl break apart in chemical reactions at cold temperatures in the atmosphere.
“I thought, gosh, this looks just like Antarctica,” Solomon tells Science News’ Carolyn Gramling. “How can this be happening over Australia?”
She wondered if a new chemical reaction involving wildfire smoke could explain what the data was showing. After sifting through other published research to look for clues, the team discovered that smoke particles that have hung around in the atmosphere contain organic acids that drastically raise HCl’s ability to be dissolved, even in warm temperatures.
“It does seem to be a big missing piece of the puzzle,” Jim Haywood, an atmospheric scientist at the University of Exeter in England who was not involved in the research, tells Nature’s Dyani Lewis, adding that this breakdown of HCl following wildfires has not been studied before.
Thanks to the 1987 Montreal Protocol, which regulates the production and consumption of destructive CFCs, the ozone layer is on the mend. The U.N. estimates recovery is expected by around 2066 over the Antarctic and by 2045 over the Arctic. But with these new findings, researchers are concerned that an increase in wildfires could threaten this progress.
“There’s now sort of a race against time. Hopefully, chlorine-containing compounds will have been destroyed before the frequency of fires increases with climate change,” Solomon says in the statement. “This is all the more reason to be vigilant about global warming.”