The Antarctic ozone hole, ultraviolet radiation and bushfires

In the 1980s, British Antarctic scientists (Farman et al. 1985) discovered the hole in the ozone layer over Antarctica, and we are now familiar with images of springtime ozone depletion extending beyond the continental margins (Fig. 1a). The largest Antarctic ozone hole occurred in 2006 (Fig. 1b), but in recent years recovery has started to become apparent, with the total column ozone predicted to return to 1980 levels by 2066 (WMO 2022). However, there is still reason to be concerned about the timing and extent of ultraviolet (UV) radiation exposure in Antarctica, as well as how ozone recovery may be jeopardized by climate change-mediated events such as wildfires. The ozone layer protects the Earth's surface from damaging UV-B radiation. Recent reports demonstrate that over the period of maximum ozone depletion (1990–2020) the maximum spring UV index at Palmer Station (64°S) has increased by 2.5 times compared to the pre-ozone hole era as measured in the early 1970s. Despite the solar angle being much lower in Antarctica, the maximum UV index at Palmer Station in spring can now sometimes exceed that experienced in summer in subtropical regions (San Diego, CA, 32°N; Bernard et al. 2022; Environmental Effects Assessment Panel in press). Antarctic ozone depletion generally peaks between September and October, when most Antarctic terrestrial vegetation and soil biota will be frozen, dormant and hopefully protected under snow cover. Similarly, much marine life will be protected by sea-ice cover, although some seals and birds might be breeding on the ice at this time. Usually by the time summer arrives the ozone layer has recovered (see white lines in Fig. 1c). However, for the past 3 years ozone depletion has been extensive (Fig. 1b) and long-lasting, extending into early summer (e.g. 2022; see black lines in Fig. 1c). Since 2019, November–December total ozone column depth for latitudes 60–90°S have been the lowest since records began in the 1980s (NASA 2023). From a biologist's perspective, ozone depletion in early December is far more concerning, given that this is closer to the solstice, meaning that all solar radiation is higher, including the UV index. A peak in UV index coincident with snowmelt and the emergence of vegetation as well as during the peak breeding season at the start of the summer is of particular concern, as more biota are likely to be exposed to this higher incident UV-B radiation. For some organisms, such exposure may also occur at a more vulnerable time in their life cycles. The effects of climate change through earlier snowmelt and heatwaves (Robinson et al. 2020, Environmental Effects Assessment Panel in press) is likely to be enhancing the spring and summer UV exposure of Antarctic organisms, as noted in the latest United Nations Environment Program Environmental Effects Assessment Panel report (Barnes et al. 2023; Environmental Effects Assessment Panel in press). The Montreal Protocol is an extremely successful environmental treaty. As a result of this treaty, the release of ozone-depleting compounds (e.g. chlorofluorocarbons; CFCs) has been controlled, and the ozone layer is predicted to recover by 2066 (WMO 2022). Actions taken due to the Montreal Protocol and its Kigali 2016 amendment have also contributed to reducing greenhouse gas emissions and slowing global temperature increases through the substitution of those CFCs that contributed to greenhouse gas emissions with compounds that do not contribute to climate forcing. It is estimated that this has prevented at least 0.5–1.0°C of warming in the mid-latitudes and prevented more than 1.0°C of warming in the Arctic (WMO 2022; Environmental Effects Assessment Panel in press). But we cannot afford to be complacent. The extended ozone depletion observed in recent years may have been exacerbated in part by climate change impacts. Two of the recent large ozone holes were influenced by distinct events: the Australian Black Summer bushfires of 2019–2020 and the eruption of the La Soufriere volcano in 2021 (Yook et al. 2022). Research shows that the extensive Australian wildfires injected aerosols into the stratosphere that are capable of depleting ozone (Damany et al. 2022; Yook et al. 2022) and probably contributed to the larger ozone hole area in 2020 (Fig. 1b; Solomon et al. 2023). Unlike the original causes of ozone doi:10.1017/S0954102023000081