The stratosphere, just above where airplanes fly, doesn’t normally vary much in temperature because of events on the Earth’s surface — with the exception of volcanic eruptions.
But a sudden and unexpected warming of the global stratosphere was detected in the first few months of 2020 — reaching up to 3 degrees Celsius around Australia and around 0.7 degrees Celsius globally. The researchers say it was the highest temperature recorded in the stratosphere since Mount Pinatubo erupted in the Philippines in 1991, sending aerosols from sulfate and smoke high into the air.
Lilly Damany-Pearce, a researcher at England’s University of Exeter who led the study, said that both the stratospheric warming and a sizable ozone hole that spread over most of the Antarctic continent in 2020 were likely to have been caused by the violent fire-induced thunderclouds, or “pyrocumulonimbus” events, which injected enormous plumes of smoke into the lower stratosphere.
She said smoke particles are about 50 times more efficient at absorbing sunlight than volcanic sulfate particles — because of the black soot in smoke aerosols. Sunlight heats the air containing the smoke particles, causing this smoke-laden air to rise in a process similar to that which causes hot-air balloons to rise.
Once in the stratosphere, the researchers said, continued heating can cause changes in ozone through changes in atmospheric circulation, and chemical reactions on the surface of the smoke particles can deplete the ozone layer.
“It is plausible that the good work carried out under the Montreal Protocol … could be undone by the impact of global warming on intense fires,” study co-author Jim Haywood, an atmospheric scientist at the University of Exeter, said in an email.
The ozone layer helps absorb incoming ultraviolet radiation from the sun, shielding life on Earth from its harmful effects, such as skin cancer and cataract formation. The ozone hole that formed over Antarctica following the fires in 2020 was the longest-lasting and among the largest and deepest in decades, according to the World Meteorological Organization.
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Olaf Morgenstern, a scientist at New Zealand’s National Institute of Water and Atmospheric Research, said the impact of the Australian fires on the stratosphere — including a smoke plume that drifted over the South Pacific — is “unprecedented as far as the observational record goes.”
Morgenstern, who was not involved in the study, explained that smoke aerosols don’t remain in the upper atmosphere for as long as the harmful human-manufactured chemicals, which can linger in the atmosphere for as long as 80 years.
“The big issue here is that, under global warming, the frequency and intensity of wildfires is expected to increase, which would lead to more” fire-induced stratospheric warming and ozone depletion in the future, Haywood said.
“I don’t believe it’s a coincidence that we had these massive fires in Australia. They are part of the trend,” Morgenstern said, pointing to this past summer of devastating blazes in Europe — also fueled by similar waves of extreme heat and widespread drought conditions that led to Australia’s Black Summer fires.
Previous research has shown the 2020 Australian fire season was so extreme that it altered large-scale wind patterns more than 10 miles overhead. Another study last year observed temperature and ozone changes from satellite data.
The big contribution of the latest paper, according to Martin Jucker, a climate expert at Australia’s University of New South Wales who was not involved in the study, is that the researchers put satellite observations from the period into a state-of-the-art climate model “to prove that the bush fires were actually the reason for what we observed.”
“Heating the stratosphere doesn’t really have a direct impact for us at the surface [of the Earth], but keeping the ozone from recovering or destroying the ozone for a year has a real impact at the surface,” he said. “Before the 2019 bush fires, I don’t think we even thought [fires] could have such an impact. That a bush fire could be as impactful as a volcano.”