The ozone hole over Antarctica was the biggest in a decade this spring, and the reason is the wind, not a failing treaty

For a few weeks this spring, the thinning of the ozone layer over Antarctica reached a size not seen since the mid-2010s.

The annual measurements by the US National Oceanic and Atmospheric Administration (NOAA) and NASA show the 2026 ozone hole averaged about 25 million square kilometres during its peak season, from 7 September to 13 October. That makes it the seventh-largest in 48 years of satellite monitoring, and more than twice the area of the United States.

The hole peaked on 18 September at about 27.1 million square kilometres. The lowest ozone level measured directly above the South Pole was 105 Dobson units on 17 October. The record low, 92, was set in October 2006.

If the ozone layer is healing, why was the hole so big?

Weather in the stratosphere. The scientists said an unusually powerful polar vortex, the ring of strong winds that circles Antarctica each winter and spring, lasted longer than usual this year.

Amy Butler, a research scientist at NOAA’s Chemical Sciences Laboratory, said the atmospheric waves that usually disturb the vortex “were especially weak in August and September”. That kept the strongest winds several degrees of latitude further from the pole, widening the cold area where ozone is destroyed.

Earlier in the season, Europe’s Copernicus Atmosphere Monitoring Service also tracked the hole growing faster than usual. It reached 15 million square kilometres by the end of August, and about 25 million by 12 September, some 5 million above the average for that date, after a sudden drop in stratospheric temperatures over the South Pole.

So is the Montreal Protocol still working?

Yes, the scientists say. Paul Newman, the long-time leader of NASA’s ozone research team, said the hole could have been 1 to 2 million square miles larger if ozone-depleting substances were still at their peak, “which tells us the ozone layer is healing”.

NOAA’s measurements show the amount of chlorine from those chemicals in the Antarctic stratosphere has fallen by about a third from its 1980s peak. The depletion this year was not as severe as in the record years 10 and 20 years ago. The 1987 treaty that phased out ozone-destroying chemicals, now signed by every country, continues to drive the long-term recovery, NOAA said.

The year-to-year swings are large. Last year’s hole was among the smallest in years.

What does it mean for Australians?

The ozone hole forms over Antarctica, not over Australia, and the scientists have not linked this year’s size to any change in UV levels here. But Australia’s sun protection advice does not depend on it. UV in most of the country is already high enough in spring and summer to burn unprotected skin, and the Bureau of Meteorology publishes a daily UV forecast. The size of the hole each year is one of the clearest measures of whether the world’s most successful environmental treaty is still working.