Australia’s Black Summer bushfires linked to largest stratospheric warming in three decades

Australia’s devastating Black Summer bushfires released smoke particles into the Earth’s upper atmosphere, contributing to the highest stratospheric temperatures in three decades.

Scientists believe the impact may have prolonged the life of the hole in the ozone layer.

The 2019-20 bushfires burned more than 24 million hectares, directly caused 33 deaths and killed or displaced an estimated three billion animals.

The fires released millions of tonnes of smoke aerosols, and researchers from the UK have found a significant amount reached the lower stratosphere, about 15 kilometres above the ground.

The study, published in Nature Scientific Reports, links the smoke aerosols to abnormally high stratospheric temperatures, which were the highest recorded since the early 1990s.

The authors report an initial plume of smoke reached an altitude of 16 kilometres on December 31, 2019 followed by a similar plume on January 12, 2020. The smoke aerosols eventually reached altitudes of 25 to 35 kilometres detected throughout 2020.

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Read more: Fire and rain and the smoke that lingers

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Using data from the CALIPSO and Suomi National Polar-orbiting Partnership satellites to monitor aerosol levels in the stratosphere in the southern hemisphere between December 2019 and 2020, the researchers concluded the smoke aerosols emitted from the Australian bushfires led to increased stratospheric temperatures of about 0.7 degrees Celsius.

The study backs up findings from Massachusetts Institute of Technology (MIT) earlier this year that bushfire smoke may have set back efforts to reduce ozone-depleting emissions under the Montreal Protocol.

The MIT report in February found the fires injected over one million tons of smoke particles into the atmosphere, reaching up to 35 km above Earth’s surface — a mass and reach comparable to that of an erupting volcano.

Jennifer Chu, a science reporter at the MIT News Office says atmospheric chemists at MIT found the smoke from those fires set off chemical reactions in the stratosphere that contributed to the destruction of ozone, which shields the Earth from incoming ultraviolet radiation.

“The study was the first to establish a chemical link between wildfire smoke and ozone depletion.

“In March 2020, shortly after the fires subsided, the team observed a sharp drop in nitrogen dioxide in the stratosphere, which is the first step in a chemical cascade known to end in ozone depletion. The researchers found this drop in nitrogen dioxide directly correlates with the amount of smoke that the fires released into the stratosphere. They estimate that this smoke-induced chemistry depleted the column of ozone by 1%.

“To put this in context, they note that the phaseout of ozone-depleting gases under a worldwide agreement to stop their production has led to about a 1 percent ozone recovery over the past ten years — meaning that the wildfires cancelled those hard-won gains for a short period. If future wildfires grow stronger and more frequent, as they are predicted to do with climate change, ozone’s projected recovery could be delayed by years. 

“The Australian fires look like the biggest event so far, but as the world continues to warm, there is every reason to think these fires will become more frequent and more intense,” says lead author Susan Solomon, Professor of Environmental Studies at MIT. “It’s another wakeup call, just as the Antarctic ozone hole was, in the sense of showing how bad things could actually be.”