The effects of the Black Summer bushfires on the ozone layer

Wildfire smoke promotes chemical reactions that contribute to the destruction of stratospheric ozone, a paper in Nature has suggested. These findings increase concerns that more frequent and intense wildfires could delay ozone recovery as the climate warms.

The Australian wildfires of 2019–2020 sent plumes of smoke high into the atmosphere, where it was transported around the world. The smoke was associated with changes in the chemical composition of the upper atmosphere, including a decline in stratospheric levels of ozone. However, the mechanism of how wildfire smoke might contribute to ozone depletion has remained uncertain.

Susan Solomon and colleagues proposed that the mixture of chemicals in wildfire smoke enhances the activation of chlorine radicals — molecules that can destroy ozone. The authors tested their hypothesis by comparing atmospheric observations to model simulations, which reproduce the observed ozone depletion during the Australian wildfires. Their findings indicated that wildfire aerosol chemistry has the potential to contribute to ozone depletion.

Experts have reacted to this research, giving their thoughts on the study by Solomon. The authors note that other reactions beyond those studied here may also be important and recommend further investigation of the effects of different aerosols in the stratosphere.

Roger Dargaville from the Faculty of Engineering at Monash University has conducted research on stratospheric ozone depletion. He said high-energy ultraviolet radiation would render the surface of the Earth uninhabitable without a healthy stratospheric ozone layer. The Montreal Protocol, signed in 1987, has contributed to lower levels of dangerous chlorofluorocarbons (CFCs) entering the environment; however, smoke from extreme bushfires entering the stratosphere has increased the potency of chlorine in the atmosphere, risking the progress made through the Montreal Protocol.

Olaf Morgenstern, Principal Scientist – Atmosphere and Climate at NIWA, said large bushfires are set to become more common as a consequence of global warming. This can inject large quantities of smoke aerosol into the stratosphere, contributing to the Antarctic ozone hole by activating chlorine at a high temperature.

Laura Revell, Associate Professor in the School of Physical and Chemical Sciences at the University of Canterbury, said that there are still substantial amounts of chlorine in the stratosphere following the widespread use of CFCs in the 1970s and 1980s. These cause the Antarctic ozone hole to form each spring, though early signs of its recovery have been visible since the mid-2010s. Without any major changes, chlorine concentrations will gradually decrease this century and the ozone hole will get smaller year by year.

The Australian bushfires were a rare event that can enhanced ozone loss, with Solomon and colleagues finding that wildfire aerosol chemistry led to a 3–5% depletion of southern mid-latitude stratospheric ozone during 2020 and a larger-than-expected Antarctic ozone hole the same year. According to Revell, this is not a huge loss, but it is significant considering the ozone layer should only be increasing by 1% per decade due to CFC bans. With wildfires likely to become more frequent, this is a concern for the future of ozone recovery.

Professor Ian Rae from the University of Melbourne’s School of Chemistry said, “Ozone is continuously formed and destroyed in the stratosphere. As a result of these competing processes, there is a steady, albeit small, concentration of ozone up there and because it absorbs ultraviolet light, we gain a measure of protection from this skin-damaging radiation.”

He spoke further of the Montreal Protocol and CFCs, supporting the views of the other scientists in the matter.

“The nature of the chemical reactions is not completely nailed down but the overall picture is probably correct. Unlike the way nations dealt with the CFCs, I don’t think there will be another ‘Montreal Protocol’ for bushfires, which the authors warn are likely to become more frequent under climate change that we can already see happening,” Rae said.

Martin Jucker from the University of New South Wales and Associate Investigator at the ARC Centre of Excellence for Climate Extremes said Australia is of particular interest because its bushfires may lead to an extension of the ozone hole equatorward, which would see the ozone layer move closer to where Australians live. If more bushfires are expected in the future, it may lead to a slower ozone layer recovery.

“In addition, the study confirms once again that when it comes to climate, all things are connected, and events of which we think we know the effects can in fact have many more far-reaching consequences. The authors also rightly point out that there are still many basic mechanisms we don’t understand, even in a subject like the ozone hole which we thought was a solved problem,” Jucker said.

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