September’s Ozone Hole Was the Biggest It Has Been in 25 Years

The size of the hole in the stratosphere’s ozone layer fluctuates every year, but last month’s, measuring 10.6 million square miles, was especially large

Sara Hashemi | Daily Correspondent

October 9, 2026 10:55 a.m.

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In September, the Antarctic ozone hole peaked at 10.6 million square miles—the second largest ever observed for this time of year.

Using data from the Copernicus Atmosphere Monitoring Service, scientists found that the ozone hole started growing earlier than usual in August. By September 20, it reached a size close to its October 2015 peak, when it was 10.8 million square miles.

To understand what’s happening, we first need to understand the ozone layer. The ozone layer lies in the Earth’s stratosphere and protects our planet from harmful ultraviolet (UV) radiation that can harm humans, plants and animals. The infamous ozone hole is actually a patch of depleted ozone in the stratosphere over Antarctica that is mainly caused by chemicals called chlorofluorocarbons, or CFCs. While the 1987 Montreal Protocol phased out the use of CFCs globally, they can persist in the atmosphere for decades.

The ozone hole’s size fluctuates every year. For about three months each fall—the Southern Hemisphere’s spring—the hole opens inside the stratospheric polar vortex, a large band of fast-spinning westerly winds that encircles Antarctica and the Southern Ocean. The vortex traps freezing air inside and leads to the formation of polar stratospheric clouds, writes a New Zealand-based research team in The Conversation. “Chemical reactions on particles in this rare type of cloud then activate ozone-depleting gases,” they add.

This year brought a stronger and colder polar vortex. “We have unusually strong winds and cold temperatures over the pole that have created the conditions possible for the hole to grow to the size it has today,” research team member Hannah Kessenich, a physicist at the University of Otago in New Zealand, tells the Australian Broadcasting Corporation (ABC News). By late January, the hole will shrink again.

So, should we be worried? “This year’s ozone hole shouldn’t be a cause for alarm,” Kessenich told Live Science's Sascha Pare in an email. “However, there are still many unanswered questions. It’s something we need to keep a close eye on.”

Kessenich and other experts underscore the fact that the ozone hole size we’re currently seeing isn’t a sign that the Montreal Protocol isn’t working. “We look at the depth of the hole rather than the area to assess how we’re coming along in terms of the recovery from those chemicals,” Kessenich says to ABC News.

In fact, this year’s hole isn’t as deep as it has been in previous years, and year-to-year variability is to be expected. “We have to look at the longer term trends, and those metrics continue to tell us that the ozone hole is slowly recovering as expected,” adds Susan Solomon, an atmospheric chemist at MIT, in an email to Live Science.

There will, however, be impacts on weather across the Southern Hemisphere. A larger ozone hole shifts the Southern Annular Mode into a positive phase, which affects winds and precipitation in different areas. In Australia, for example, it causes increased rainfall on the country’s southeast coast, Kessenich and her colleagues write in The Conversation.

Researchers are still trying to understand how climate change impacts the strength of the polar vortex, and why bigger ozone holes are persisting longer than they used to. It could be that changes in global atmospheric dynamics are making longer-lived ozone holes more likely, which in turn could mean our timelines for ozone recovery need to be adapted.

“It's too soon to tell what has been driving dynamical variability in recent years, but more research is needed,” Kessenich says to Live Science.

Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.