This year's Antarctic ozone hole was the second-largest on record for its time of year. Scientists say the ozone layer is still healing. Here is how both can be true

Every southern spring, a thinning in the ozone layer opens over Antarctica. This year's was unusually large. The hole began growing early, passed 15 million square kilometers by the end of August, and reached about 25 million square kilometers on September 12, roughly 5 million square kilometers above the average for that date, according to the Copernicus Atmosphere Monitoring Service, known as CAMS, as reported by Down To Earth.

It kept growing. The hole peaked on September 20 at about 27.4 million square kilometers, or 10.6 million square miles, the second-largest for that time of year, Live Science reported. The largest comparable hole was in October 2015, at about 27.9 million square kilometers. The all-time record remains 29.6 million square kilometers, set on September 9, 2006. By early October the hole had shrunk to about 23 million square kilometers, making a new 2015-sized record unlikely this year.

Why was the 2026 ozone hole so big?

The short answer is weather in the stratosphere. This year's large hole stems mainly from a strong polar vortex, the ring of winds around Antarctica, that trapped unusually cold air over the continent, Live Science reported. Down To Earth noted a sudden temperature drop about 20 kilometers above the South Pole. Cold air allows polar stratospheric clouds to form, and those clouds help activate the halogen chemistry that destroys ozone when sunlight returns in the southern spring.

Crucially, scientists have not found more of the chemicals doing the damage. Researchers have not detected an increase in reactive chlorine, and the colder vortex drove more depletion than usual, MIT atmospheric chemist Susan Solomon told Live Science. Hannah Kessenich, a physicist at the University of Otago in New Zealand, attributed the size mostly to natural variability in atmospheric dynamics. "This year's ozone hole shouldn't be a cause for alarm," she said.

Other measures support that reading. CAMS said indicators such as the minimum ozone column and the ozone mass deficit, which estimates how much ozone is missing, stayed close to average. "The Antarctic ozone hole's development can vary considerably from year to year," said Laurence Rouil, director of CAMS, adding that those other indicators "remain close to historical averages." Kessenich told ABC News that the hole's depth, rather than its area, is the better measure of recovery, according to Smithsonian Magazine.

Is the ozone layer healing?

Yes, on the measures scientists say matter most. The ozone hole is "slowly recovering as expected," Solomon told Live Science, and the longer-term trends show steady recovery even when individual years jump around.

Last year illustrated the other side of that variability. The World Meteorological Organization's bulletin on the 2025 season found it was among the least severe ozone holes of the past 20 years, the fourth or fifth smallest since systematic observations began in 1992, according to Meteored. Its maximum ozone mass deficit, 36.7 million metric tons on September 29, 2025, was more than 25% below the 1990 to 2010 average of 50.1 million metric tons. Mid-September warming in the stratosphere slowed ozone destruction that year, and the vortex broke down early.

The long-run path comes from the Montreal Protocol, signed in 1987 and in force since 1989, which phased out the chemicals responsible. A 2023 assessment by the WMO and the UN Environment Programme found that nearly 99% of banned ozone-depleting substances had been phased out. If current policies stay in place, it projected that the ozone layer would return to 1980 levels around 2040 for most of the world, around 2045 over the Arctic and around 2066 over the Antarctic, the WMO said. Because major chlorofluorocarbons remain in the atmosphere for 50 to 100 years, researchers expect recovery to take about six decades, Live Science reported.

The treaty also has a climate dimension. Its 2016 Kigali Amendment targets hydrofluorocarbons, which do not damage ozone but are potent greenhouse gases. It aims to cut their use by more than 80% over the coming decades, which could avoid up to 0.5°C of warming by 2100 if fully implemented, Meteored noted. "The impact the Montreal Protocol has had on climate change mitigation cannot be overstressed," Meg Seki, executive secretary of UNEP's Ozone Secretariat, said when the 2023 assessment was released.

What to watch for the rest of the season

The Antarctic hole typically reaches its maximum between mid-September and early October, CAMS noted, and it usually stays open for about three months, Smithsonian Magazine reported. Kessenich told Live Science that if the vortex stays strong, the hole could remain large into November, and she raised the possibility of a new pattern of holes lasting later into spring for reasons other than chlorofluorocarbons. That is a question for future seasons. A late-lasting hole matters because the ozone layer shields life on Earth from harmful ultraviolet radiation, as the EU's Copernicus program noted on the UN's International Day for the Preservation of the Ozone Layer, September 16.

The scientists quoted here say this year's large hole does not change their assessment that recovery is under way. The next readings to watch are the Copernicus end-of-season data and the WMO's bulletin on the 2026 season.