Stratospheric ozone depletion, first observed in the 1980s, has been caused by the increased production and use of substances such as chlorofluorocarbons (CFCs), halons and other chlorine-containing and bromine-containing compounds, collectively termed ozone-depleting substances (ODSs). Following controls on the production of major, long-lived ODSs by the Montreal Protocol, the ozone layer is now showing initial signs of recovery and is anticipated to return to pre-depletion levels in the mid-to-late twenty-first century, likely 2050–2060. These return dates assume widespread compliance with the Montreal Protocol and, thereby, continued reductions in ODS emissions. However, recent observations reveal increasing emissions of some controlled (for example, CFC-11, as in eastern China) and uncontrolled substances (for example, very short-lived substances (VSLSs)). Indeed, the emissions of a number of uncontrolled VSLSs are adding significant amounts of ozone-depleting chlorine to the atmosphere. In this Review, we discuss recent emissions of both long-lived ODSs and halogenated VSLSs, and how these might lead to a delay in ozone recovery. Continued improvements in observational tools and modelling approaches are needed to assess these emerging challenges to a timely recovery of the ozone layer.
Ozone recovery is expected mid-century, owing to adherence to the Montreal Protocol, but a number of recent trends could challenge its timely recovery.
The apparent illicit production of CFC-11 is one such challenge to ozone recovery, but the added damage to the ozone layer in this case depends on how rapidly the CFC-11 emissions are mitigated.
A number of industrial processes that are allowed by the Montreal Protocol contribute considerable amounts of chlorinated gas emissions to the atmosphere.
Increases in ozone-depleting chlorine from a number of human-produced, short-lived gases have led to some increased ozone depletion, although their future impacts on ozone depend on future uses.
Natural processes also affect the balance of ozone in the stratosphere in a number of ways and could change in the future as climate responds to increases in atmospheric greenhouse gas concentrations.
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The authors thank S. Dhomse (University of Leeds) for the data provided in Fig. 6. M.P.C. and R.H. acknowledge support through the Natural Environment Research Council (NERC) Sources and Impacts of Short-Lived Anthropogenic Chlorine (SISLAC) grant NE/R001782/1. R.H. is supported by a NERC Independent Research Fellowship (NE/N014375/1).
The authors declare no competing interests.
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United Nations Environment Programme Ozone Country Data: https://ozone.unep.org/countries/data-table
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Layer of atmosphere (approximately 15–50 km).
Layer of atmosphere (surface to approximately 15 km).
- Ozone-depleting substances
(ODSs). Man-made chlorine- and bromine-containing gases that cause ozone depletion once they reach the stratosphere and are controlled by the Montreal Protocol.
Ozone-depleting substance included in the Montreal Protocol for limits on consumption and production.
Reservoirs of produced ozone-depleting substances stored in equipment or materials and not yet released to the atmosphere.
Application that does not lead to the eventual emission of ozone-depleting substances.
A halogenated chemical whose production and/or international trade is not controlled by the Montreal Protocol.
- Very short-lived substances
(VSLSs). Substances with an atmospheric lifetime of less than half a year.
- Ozone-depletion potentials
Relative amounts of ozone loss caused by the emission of 1 kg of a substance compared with the emission of 1 kg of CFC-11.
Measure of the removal rate (e-folding time) of emitted species by atmospheric processes.
Reaction of a feedstock with a fluorine-containing compound to produce a substance.
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Chipperfield, M.P., Hossaini, R., Montzka, S.A. et al. Renewed and emerging concerns over the production and emission of ozone-depleting substances. Nat Rev Earth Environ 1, 251–263 (2020). https://doi.org/10.1038/s43017-020-0048-8
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