Recovery of the ozone layer could face minor setbacks

Earlier this year, the United Nations announced that the stratospheric ozone layer is on track to recover within 4 decades thanks to actions taken under the Montreal Protocol. Adopted in 1987, the international agreement guided the phaseout of ozone-depleting compounds such as chlorofluorocarbons (CFCs), which were once widely used as refrigerants, solvents, and aerosol propellants.

Although substantial progress has been made, the ozone layer’s recovery has also had its share of setbacks over the years. For example, illegal production of CFC-11 and leaks from aging equipment have slowed the decline of ozone-depleting compounds from the atmosphere.

Most recently, a new study has found that atmospheric concentrations of five CFCs have been increasing since 2010, despite bans on their use and production (Nat. Geosci. 2023, DOI: 10.1038/s41561-023-01147-w). If the trend continues, it’s possible that the timeline for stratospheric ozone recovery could be extended.

More work is needed to definitively pinpoint where these five CFCs are coming from. However, the researchers think they could be released during the production of other chemicals, such as hydrofluorocarbons (HFCs), which are more ozone-friendly refrigerants. “Tighter controls on leakage and proper destruction of byproducts would help to reduce emissions,” says lead author Luke Western, a researcher at the National Oceanic and Atmospheric Administration’s Global Monitoring Laboratory, in an emailed response.

The Montreal Protocol does not ban the use of CFCs as chemical feedstocks.

Aside from CFCs, other compounds, such as bromoform, can contribute to ozone depletion, says Susann Tegtmeier, an atmospheric scientist at the University of Saskatchewan. Bromoform is not regulated by the Montreal Protocol, in part because it’s thought to be derived primarily from natural sources.

However, Tegtmeier and her colleagues found that human activity may be responsible for a larger portion of bromoform emissions than previously assumed (Geophys. Res. Lett. 2023, DOI: 10.1029/2023GL102894). As a result, anthropogenic sources, such as power and desalination plants, could boost the amount of bromoform that can enter the stratosphere and subsequently react with ozone.

Compared to CFCs, bromoform’s impact on the ozone layer is likely to be small, Tegtmeier says. Yet, she points out that bromoform is important to monitor as it may prove to have a larger impact in the future, especially as concentrations of CFCs in the atmosphere trend downward.

As for the five increasing CFCs, Ross Salawitch, an atmospheric chemist at the University of Maryland who wasn’t involved in either study, says that understanding the sources of these emissions are of the utmost importance to the research community. He is confident that if concentrations of the five CFCs rise to levels of concern, decision-makers for the Montreal Protocol will “do the right thing.”