Smoke from historic wildfires in Australia a few years ago widened the Antarctic ozone hole by millions of square kilometres, a recent study has found.
Chemists from the Massachusetts Institute of Technology (MIT) looked at the effect of wildfire smoke from the devastating megafire, known as "Black Summer," that burned in eastern Australia from December 2019 to January 2020.
The fires burned tens of millions of acres and released more than one million tons of smoke into the atmosphere, making it the country's most devastating on record, the researchers say.
As part of their study, published last week in Nature, the scientists found the fires ate away at the ozone hole over Antarctica, widening it by 2.5 million square kilometres or 10 per cent by late 2020 compared to the previous year.
"The Australian fires of 2020 were really a wake-up call for the science community," Susan Solomon, Lee and Geraldine Martin Professor of Environmental Studies at MIT and a leading climate scientist, said in a story from MIT News.
"The effect of wildfires was not previously accounted for in (projections of) ozone recovery. And I think that effect may depend on whether fires become more frequent and intense as the planet warms."
The ozone layer protects the Earth from ultraviolet radiation from the sun.
While not technically a hole, the ozone hole refers to a region where ozone has been "exceptionally depleted" over the Antarctic, occurring annually between August and October, NASA says.
Solomon and her colleagues first identified in 2022 a chemical link between wildfires and ozone depletion.
They found that chlorine-containing compounds, first released by factories as chlorofluorocarbons or CFCs, could react with fire smoke, creating a chemical reaction that produces chlorine monoxide, described by the researchers as "the ultimate ozone-depleting molecule."
"But that didn't explain all the changes that were observed in the stratosphere," Solomon said. "There was a whole bunch of chlorine-related chemistry that was totally out of whack."
Using three independent sets of satellite data, the scientists discovered that in the months after the Australian wildfires, concentrations of hydrochloric acid dropped significantly at mid-latitudes, while chlorine monoxide levels spiked.
Hydrochloric acid is found in the stratosphere as CFCs break down naturally over time, the researchers say.
Chlorine can't destroy ozone if it is in the form of hydrochloric acid. But if hydrochloric acid breaks apart, the researchers say chlorine can react with oxygen to form chlorine monoxide.
While hydrochloric acid can break apart as it interacts with cloud particles in frigid temperatures, such as those found over polar regions, the researchers were surprised to find this reaction at warmer mid-latitudes.
"The fact that hydrochloric acid at mid-latitudes dropped by this unprecedented amount was to me kind of a danger signal," Solomon said.
The scientists say this chemical reaction likely contributed to a three to five per cent depletion of total ozone at mid-latitudes in the Southern Hemisphere over Australia, New Zealand and parts of Africa and South America.
The long-term effects of the wildfires on ozone remain unclear, but the United Nations reported in January that the ozone hole is healing slowly, although it would not be fully fixed until 2066.
While hydrochloric acid is believed to be the most likely way that wildfires can deplete ozone, Solomon says there may be other chlorine compounds present in the stratosphere.
"There's now sort of a race against time," Solomon said. "Hopefully, chlorine-containing compounds will have been destroyed, before the frequency of fires increases with climate change. This is all the more reason to be vigilant about global warming and these chlorine-containing compounds."
With files from The Associated Press
