The stratosphere, just above where Flying airplanes does not normally vary much in temperature due to events on the Earth’s surface – with the exception of volcanic eruptions.
But in the first few months of 2020, sudden and unexpected warming of the global stratosphere was detected — up to 3 degrees Celsius (5.4 degrees Fahrenheit) around Australia and about 0.7 degrees Celsius (1.26 degrees Fahrenheit) worldwide. The researchers say it was the highest temperature in the stratosphere since the 1991 eruption of Mount Pinatubo in the Philippines, sending aerosols of sulfate and smoke high into the sky.
Lilly Damany-Pearce, a researcher at the English University of Exeter who led the study, said both the warming of the stratosphere and a significant ozone hole that spread across most of the Antarctic continent in 2020 were likely caused by the violent fire caused by storm clouds, or “pyrocumulonimbus” events, that injected huge 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 it to rise smoke-laden air in a process similar to that in which hot air balloons rise.
Once the particles are 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 done under the Montreal Protocol … could be undone by the impact of global warming on intense fires,” said study co-author Jim Haywood, an atmospheric scientist at the University of Exeter, in an email.
The ozone layer helps absorb the sun’s incoming ultraviolet radiation and protects life on Earth from its harmful effects, such as skin cancer and cataracts. The ozone hole that formed over Antarctica after the 2020 fires was the longest-lasting and one of 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 plume of smoke drifting over the South Pacific — “is unprecedented as far as the sighting record goes.”
Morgenstern, who was not involved in the study, explained that smoke aerosols don’t stay in the upper atmosphere as long as the harmful man-made chemicals, which can linger in the atmosphere for up to 80 years.
“The big problem here is that under global warming, the frequency and intensity of wildfires is expected to increase, leading to more in the future” fire-induced warming of the stratosphere and depletion of the ozone layer, Haywood said.
“I don’t believe it’s a coincidence that we’ve had these massive fires in Australia. They are part of the trend,” Morgenstern said, pointing to this devastating summer fires in Europe – also fueled by waves of extreme heat and widespread drought, similar to those that led to the Black Summer fires in Australia.
Previous research has shown that Australia’s 2020 fire season was so extreme that it changed large-scale wind patterns more than 10 miles overhead. Another study from last year observed changes in temperature and ozone based on satellite data.
The big contribution of the latter paper, according to Martin Jucker, a climate expert at the Australian University of New South Wales who was not involved in the study, is that the researchers used satellite observations from that period in a state-of-the-art climate model “to to prove that the wildfires were actually the reason for what we observed.”
“Heating the stratosphere doesn’t really have a direct impact on us on the surface [of the Earth]but stopping the ozone from repairing or destroying the ozone for a year has a real impact on the surface,” he said. “Before the 2019 wildfires, we didn’t even think we thought [fires] can have such an impact. That a forest fire can have just as impact as a volcano.”