Four decades of international action to ban chlorofluorocarbons (CFCs) have put the Antarctic ozone hole on a demonstrable path to recovery. Yet the win is partial, suggest two recent studies. One of the studies, led by the Massachusetts Institute of Technology, US, shows for the first time with high statistical confidence that the recovery owes overwhelmingly to declines in CFCs, a group of human-made long-lived ozone-depleting substances once widely used as refrigerants, propellants and solvents. The other study, by a group of researchers in Canada and the UK, identifies an obstacle to a faster rebound: increasing emissions of chlorinated very-short-lived substances (CL-VSLS) that are topping up stratospheric chlorine. CL-VSLS are chlorine containing gases used in solvents and industrial processes that have short atmospheric lifetimes of around six months. The two studies together suggest that the ozone layer is healing because of human action, yet human activity is also nudging the recovery off course.
Ozone is a naturally occurring gas within the Earth’s stratosphere that acts as a sort of sunscreen, protecting the planet from the sun’s harmful ultraviolet radiation. In 1985, scientists discovered a “hole” in the ozone layer over Antarctica that opened up between September and December. This seasonal ozone depletion was suddenly allowing UV rays to filter down to the surface, leading to skin cancer and other adverse health effects.
Satellites and models have hinted for years that the ozone hole was shrinking. The MIT-led paper, published in Nature on March 5, 2025, elevates those hints into proof. The team separated the different drivers of change (natural variability, rising greenhouse gases and the decline in long-lived ozone-depleting substances) and found that the fall in CFCs and related chemicals is the dominant cause of the recent improvement. This means that the Montreal Protocol, an international treaty signed in 1987 to protect the Earth’s ozone layer, and its ammendments are delivering the results scientists promised.
But the other study, also published in Nature on June 20, 2025, makes it clear that the recovery is not as fast as expected. The team looked at satellite measurements of stratospheric chlorine—the reservoir gases that carry chlorine into the ozone layer—and found that they are not declining as quickly as ground-level measurements of banned CFCs would predict. “The difference is because of CL-VSLS that are not regulated by the Montreal Protocol,” Kimberlee Dube, the corresponding author of the study, told Down to Earth. Atmospheric abundances of CL-VSLS have been increasing since the early 2000s and so they are contributing to the total stratospheric chlorine reservoir that is measured by satellites.
Many VSLS have anthropogenic roots although some also arise naturally from the ocean and marine organisms. Crucially, a fraction of their chlorine can hitch a ride into the lower stratosphere where, once converted into reactive forms, it participates in ozone-destroying chemical cycles. The majority of VSLS enter the stratosphere through upwards transport (convection) in the tropics. They can also be injected directly into the stratosphere via the Asian monsoon.
“The effect of CL-VSLS is small compared to that of the regulated chlorine-containing gases, which is why ozone is still recovering, but it will take longer for the ozone to return to pre-1980 levels than it would have without increasing CL-VSLS emissions, says Dube, a postdoctoral re-searcher at the University of Saskatchewan, Canada. A 2023 study conducted by Julián Villamayor, a researcher in Spain, and others found that controlling emissions of anthropogenic VSLS would prevent up to 25 per cent of the projected tropical lower stratospheric ozone depletion by the end of the 21st century.
Dube warns of another threat to the ozone layer that can become the biggest problem moving forward: the greenhouse gases. In the lower stratosphere these gases are altering the ozone distribution by changing the atmospheric temperature and circulation. And in the middle stratosphere, nitrogen dioxide, produced from the greenhouse gas nitrous oxide, is destroying the ozone layer. “Nitrous oxide and other greenhouse gas emissions are rising, so they could become an important future source of ozone depletion,” she says.
Her final plea is for observation: decades of progress in ozone science rest on long-term, high-quality measurements from both the ground and space. “Many of the satellite instruments that scientists rely on are at the end of their lifetimes and new missions meant to replace them are at risk of being cancelled,” she says. “We hope that governments across the world can come together, just like they did to protect the ozone layer, and continue funding instruments that measure atmospheric composition,” she adds. This is crucial because lately the stratosphere has been changing in new and unexpected ways due to wildfires and volcanic eruptions. It is impossible to figure out how these events impact the atmosphere without long-term observations.
This article was originally published in the September 16-30, 2025 print edition of Down To Earth