The Antarctic ozone hole in 2025 reached its fifth smallest extent since measurements began in 1992. On 1 December it closed completely, three weeks earlier than the ten-year average. Behind this recovery stands one of the most remarkable achievements in global environmental policy: the 1987 Montreal Protocol, the only UN agreement ever ratified by all 198 member states.
1985: The discovery that changed everything
British scientists from the Antarctic Survey discovered in 1985 a dramatic hole in the stratospheric ozone layer above the South Pole. The cause was clear: chlorofluorocarbons, or CFCs, used in refrigerators, aerosol cans and solvents, were systematically depleting ozone in the stratosphere. That protective layer, which shields life on Earth from harmful UV radiation, had already opened a hole of several million square kilometres above Antarctica. Two years after the discovery, in 1987, 46 countries signed the Montreal Protocol, committing to a global phase-out of CFCs and related substances. It was the beginning of a historic experiment: can humanity reverse atmospheric damage it had caused itself?
2006: The lowest point
Ozone-depleting substances linger in the atmosphere for decades. The ozone hole therefore reached its record only in September 2006, almost twenty years after the Protocol was signed: 27.5 million square kilometres, an area larger than North America. The CFC phase-out was well underway internationally, but the substances already present in the stratosphere could not be recalled. Waiting was the only option.
The Protocol found its rhythm during this period. In 2009, all 198 UN member states ratified the agreement, a universal ratification no other UN environmental treaty has achieved to this day. Since 1987, more than 99 percent of controlled ozone-depleting substances have been taken out of use worldwide.
2025: The measure of recovery
On 9 September 2025, the ozone hole reached its maximum extent for the year: 22.86 million square kilometres, nearly five million less than in 2023. The concentration of ozone-depleting substances in the Antarctic stratosphere has fallen by one third since its peak around the year 2000. Had chlorine concentrations remained at their level of 25 years ago, the hole would have been 2.6 million square kilometres larger in 2025, NASA scientists calculated. The Protocol has left a measurable trace in the atmosphere.
Susan Solomon of the Massachusetts Institute of Technology, one of the first scientists to describe the CFC mechanism, wrote in a 2025 study published in Science: in a favourable year, there could be no Antarctic ozone hole at all as early as 2035. The World Meteorological Organization and NASA take a more conservative view. Their consensus projection puts the full return of ozone levels above Antarctica to their 1980 baseline at 2066.
In comparison: why Montreal succeeded where Kyoto failed
The Montreal Protocol can only be properly understood when set against the failure of other environmental agreements. The 1997 Kyoto Protocol was meant to limit CO₂ emissions but achieved no universal ratification: the United States never entered into ratification, Canada withdrew in 2012. Researchers identify three conditions that aligned favourably for Montreal: a clear causal link between a defined group of substances and the measured damage, industrially available substitutes for the affected industries, and enforceable trade barriers against non-signatories. For CO₂, none of those three conditions holds in the same clean form.
The Protocol also carries a climate benefit that receives little attention. Climate research analyses estimate that without the CFC reductions, the world would be roughly 2.5 degrees Celsius warmer by the end of the century. CFCs are potent greenhouse gases in their own right. The agreement thus simultaneously rescued the ozone layer and incidentally counteracted global warming.
From 1987 to 2066: the full arc
Forty years separate the signing of the Montreal Protocol from the 2025 measurement. Another forty years will pass before the ozone hole above Antarctica, according to WMO projections, fully returns to its pre-industrial baseline. That is slow, but it follows a plan. The ozone curve shows what is possible when an atmospheric damage problem is identified early enough, clearly attributed to a group of substances, and addressed through binding international agreements with concrete phase-out obligations.
Someone born in 2026 might still witness the complete closure of the ozone hole. That is one of the rare statements about environmental policy for which there is empirical grounding today.