EXPERT REACTION: How the Black Summer bushfires depleted the ozone layer

The health of the stratospheric ozone layer is critical for life on this planet – without it, damaging high energy ultra-violet radiation would make the surface of the Earth unhabitable.

Ozone destruction due to CFCs has begun to be reversed through prolonged international negotiations (starting with the Montreal Protocol, first signed in 1987 just two years after its discovery).  Every nation has ratified the protocol and its revision, making it the most successful international agreement in history.

Had the Montreal Protocol not been negotiated and CFC production continued unabated, the Antarctic and (rarely occurring) Arctic Ozone holes would likely have expanded over highly populated areas causing untold damage to people, animals and crops.

The Solomon et al. paper shows that smoke from extreme bushfires entering the stratosphere increases the potency of the chlorine in the atmosphere, risking the progress made through the Montreal Protocol to date.  This highlights the complex nature of interactions in the Earth System, and the potential for dangerous and unanticipated outcomes from human induced global warming.

Large bushfires are set to become more common as dry seasons become drier and hotter and summers lengthen under global warming. The largest of these events can inject huge quantities of smoke into the stratosphere; the 2019/2020 Australian bushfires are a prominent example. A new study in Nature shows that this aerosol plays a role in atmospheric chemistry.

The Antarctic ozone hole forms because in the stratosphere on the surfaces of cloud particles chlorine is “activated” to form compounds that then deplete ozone. This new study finds that smoke aerosol can activate chlorine too, just like polar stratospheric clouds but at higher, more ubiquitous temperatures and also away from the poles. The authors (led by atmospheric chemist Susan Solomon who in the 1980s also famously explained the ozone hole) highlight a hitherto ignored mechanism of ozone depletion, and one that might become more important as more such bushfire events occur. More research is needed into the chemical properties of such complex bushfire aerosol though.

The paper describes the coming-together of two dimensions of human interference in the climate system that had previously been considered separate.

Following the widespread use of chlorofluorocarbons (CFCs) in the 1970s and 1980s, there are still substantial amounts of chlorine in the stratosphere (approximately 10-50 km above Earth’s surface), where the protective ozone layer resides. Stratospheric chlorine causes the Antarctic ozone hole to form each spring, although early signs of Antarctic ozone recovery have been visible since approximately the mid-2010s. In the absence of any major changes, we expect that stratospheric chlorine concentrations will gradually decrease this century and that the ozone hole will get smaller year by year.

This study looks at the massive wildfires that occurred in Australia during the summer of 2019-2020. These fires injected vast amounts of wildfire smoke into the stratosphere – a rare event. The authors find that wildfire aerosols in the stratosphere can “activate” chlorine into a more destructive form and enhance ozone loss. Of concern is that while the ozone hole usually forms over Antarctica because of the cold temperatures there, wildfire aerosols appear to be capable of promoting ozone losses at the relatively warmer temperatures present at mid-latitudes which are heavily populated.

Overall, the authors show 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. Ozone depletion of 3-5% in a single year is not an overwhelming loss, however as the authors note, it is significant given that ozone should be increasing by 1% per decade because of bans on CFCs. Given that wildfires are likely to become more frequent and severe in a warming world, the implications of this study for future ozone recovery are concerning. We cannot afford to be negligent when it comes to protecting the ozone layer.

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.

Nearly fifty years ago it was observed that stratospheric ozone concentrations over the Antarctic were reduced (the so-called 'ozone hole') in early Spring and the blame was sheeted home to chlorofluorocarbons (CFCs). Research showed that it was chemical reactions with chlorine atoms, derived from the CFCs. that destroyed some of the ozone. Under the intergovernmental Montreal Protocol the uses of these and  related substances have ceased and the 'ozone hole' is shrinking year by year.

Over the last two years it has become evident that chemical reactions with constituents of bushfire (wildfire) smoke can also destroy ozone molecules. The authors of this latest paper modelled the absorption of hydrogen chloride (HCl, a combustion product) onto the partly-burnt organic matter in smoke particles. This provides a pathway for the transport of a range of chlorinated substances to the stratosphere where they can release their destructive chlorine atoms. The results of the modelling agree well with the experimental results.

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.

The study by Solomon and coauthors studies how the black summer bushfires could have influenced the ozone hole observed the year after in 2020. While previous studies have already made the link between bushfire smoke and stratospheric ozone depletion, this work provides a detailed description of the chemical reactions behind the depletion, and what makes bushfire smoke so special.

Of particular interest for Australia is the extension of the ozone hole further equatorward, which means that the ozone layer can become thinner much closer to where millions of Australians live. An important implication is that as we expect more bushfires in the future, the ozone hole might recover more slowly than expected.

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.