The recovery of stratospheric ozone from past depletion is underway owing to the 1987 Montreal Protocol and its subsequent amendments, which have been effective in phasing out the production and consumption of the major ozone-depleting substances (ODSs). However, there is uncertainty about the future rate of recovery. This uncertainty relates partly to unexpected emissions of controlled anthropogenic ODSs such as CCl3F and slower-than-expected declines in atmospheric CCl4. A further uncertainty surrounds emissions of uncontrolled short-lived anthropogenic ODSs (such as CH2Cl2 and CHCl3), which observations show have been increasing in the atmosphere through 2017, as well as potential emission increases in natural ODSs (such as CH3Cl and CH3Br) induced by climate change, changes in atmospheric concentrations of greenhouse gases N2O and CH4, and stratospheric geoengineering. These challenges could delay the return of stratospheric ozone levels to historical values, (for example, the abundance in 1980), by up to decades, depending on the future evolution of the emissions and other influencing factors. To mitigate the threats to future ozone recovery, it is crucial to ensure that the Montreal Protocol and its amendments continue to be implemented effectively in order to have firm control on future levels of ODSs. This action needs to be supported by an expansion of the geographic coverage of atmospheric observations of ODSs, by enhancing the ability of source attribution modelling, and by improving understanding of the interactions between climate change and ozone recovery.
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X.F. and R.G.P. were supported by NASA grant numbers NAG5-12669, NNX07AE89G, NNX11AF17G and NNX16AC98G to MIT. S.P. was supported by the National Strategic Project-Fine particle of the NRF funded by the MSIT, ME and MOHW (grant no. NRF-2017M3D8A1092225). We thank the station personnel at AGAGE stations for continuously measuring atmospheric N2O, CH2Cl2, CHCl3 and other referenced species, and R. H. Wang at the Georgia Institute of Technology for producing global monthly mean data of these species from the measurements from individual AGAGE stations. We thank Z. Dai from Harvard University for useful discussions on stratospheric geoengineering.
The authors declare no competing interests.
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