Every spring in the Southern Hemisphere, a vast wound opens in the sky above Antarctica. For about three months, the protective ozone layer over the South Pole thins dramatically — and this year, it opened wider than it has in any September for a quarter of a century.
According to data from the European Union's Copernicus Atmosphere Monitoring Service (CAMS), the 2026 Antarctic ozone hole peaked on September 20 at 27.4 million square kilometres (10.6 million square miles) — the second-largest September extent ever recorded, surpassed only by a briefly larger hole in September 2000. Scientists who had been watching the warning signs since May called it correctly months in advance.
Headlines like that can sound alarming, as if decades of environmental protection just unravelled. But the real story is more nuanced — and, in the end, reassuring. Here's what happened, why it happened, and what it tells us about the long-term healing of the ozone layer.
September 2026: the numbers
The ozone hole is defined as the region south of 60°S where total-column ozone falls below 220 Dobson Units — a measure of how much ozone sits in the full atmospheric column above a location. By that definition, the 2026 hole crossed the 15 million square kilometre threshold (already larger than Antarctica itself) by the end of August — slightly earlier than average, at a pace similar to 2025.
Then came the surge. In the first half of September, the hole's area jumped to an estimated 25 million square kilometres by September 12 — roughly 5 million square kilometres above the long-term average for that point in the season. On September 20 it peaked at 27.4 million square kilometres.
How does that rank historically? The all-time record, also measured by Copernicus, was set on September 9, 2006, when the hole spanned 29.6 million square kilometres. In September 2000 it briefly exceeded 28 million square kilometres, and in October 2015 it peaked at 27.9 million. The 2026 hole sits just behind those three — the fourth-largest hole of any month ever recorded, and the biggest September since 2000.
There is an important subtlety hidden in those rankings. Size is only one way to measure an ozone hole, and on the other measures 2026 looks far less dramatic. CAMS found that the lowest ozone reading inside the hole and the total amount of ozone lost — the "mass deficit" — stayed close to historical averages. As the New Zealand scientists who tracked this year's hole put it, 2026 "hasn't reached the depths of other large ozone holes from the past, meaning there is less 'missing' ozone overall."
How the ozone hole actually forms
To understand why this year went big, it helps to know the machinery behind the hole — one of the most remarkable pieces of atmospheric chemistry ever uncovered.
The ozone layer sits in the stratosphere, roughly 15 to 35 kilometres above the surface. It absorbs most of the Sun's harmful ultraviolet (UV) radiation, shielding humans, plants, and animals. In the mid-1980s, scientists discovered that this shield thins dramatically over Antarctica every spring. The culprits turned out to be chlorofluorocarbons (CFCs) — industrial chemicals once used in refrigerators, air conditioners, and aerosol sprays. CFCs drift up into the stratosphere, where they linger for decades, slowly releasing chlorine and bromine atoms that chew through ozone molecules.
But chemistry alone doesn't make the hole. It needs a special container: the stratospheric polar vortex, a vast band of fast-spinning westerly winds that encircles Antarctica and the Southern Ocean each winter. The vortex acts like a wall, trapping frigid air inside and keeping warmer, ozone-rich air from mixing in.
Inside that freezing isolation, something rare happens. When stratospheric temperatures drop low enough, ghostly ice crystals form — polar stratospheric clouds, among the highest clouds in the world. Chemical reactions on the surfaces of these cloud particles convert the dormant chlorine and bromine compounds into their active, ozone-destroying forms. But they still need sunlight to start the destruction, which is why the hole only blossoms when the Sun returns to the pole in spring — roughly August through November.
In short: CFCs supply the weapon, the vortex supplies the freezer, the clouds supply the trigger, and spring sunlight pulls it. Any ingredient that is stronger in a given year makes a bigger hole.
Why this year's hole grew so large
The 2026 hole was not caused by new emissions or a failure of pollution controls. The levels of ozone-depleting substances in the stratosphere are still falling steadily. What happened was weather — just very high, very cold weather.
This year the polar vortex was unusually strong and exceptionally cold. Data from the US Climate Prediction Center showed temperatures 20 to 30 kilometres above the Antarctic region sat among the lowest 10 percent of observations in the historical record for most of September. When air gets that cold, polar stratospheric clouds form more abundantly, the chemistry runs hotter, and the vortex holds together longer, giving destruction more time.
CAMS directly linked the rapid early-September growth to a sudden drop in minimum stratospheric temperature about 20 kilometres above the South Pole. The vortex was essentially "primed" in the months leading up to spring: patterns of airflow shifted in ways that locked in a stronger, colder circulation. A robust, cold vortex means a large, long-lasting hole — even while chemical healing quietly continues underneath.
This also fits a recent pattern. Since 2020, Antarctica has seen a run of large and long-lasting ozone holes. Researchers have linked the worst of those years to unusual natural events: smoke from the catastrophic 2019–2020 Australian bushfires injected particles high into the stratosphere, and aerosols from the 2021 La Soufrière volcanic eruption in the Caribbean added more ozone-depleting fuel. Nature can still throw punches; the chemistry just determines how much they hurt.
Interestingly, greenhouse gases play a double role here. Warming at Earth's surface — the subject of our guide to the causes and consequences of the greenhouse effect — actually helps cool the stratosphere, which can modestly favour ozone depletion even as CFC levels decline. Climate and ozone chemistry are tangled together in ways scientists are still unpicking.
The scientists who saw it coming in May
One of the most striking parts of the 2026 story is that it was predicted. Back in May 2026, a team of New Zealand atmospheric scientists — University of Otago physicists Hannah Kessenich and Annika Seppälä, together with Dan Smale of Earth Sciences New Zealand — noticed changes in atmospheric circulation from satellite data that suggested the vortex was being primed for a severe season.
They forecast a "major" ozone hole for September 2026. Writing in The Conversation in late September, after the peak had arrived, they confirmed the prediction had been borne out. Unless the dynamics above the pole shifted, they warned, the season would be prolonged, with a major hole continuing through October and November — likely landing 2026 among the top ten most severe ozone holes of all time.
That kind of forecast skill matters. Predicting severe ozone events months ahead is still an emerging science, but it has practical value: it tells researchers where to point instruments, warns Antarctic programmes about elevated UV exposure, and helps climate models account for how the hole nudges Southern Hemisphere weather patterns — including winds and rainfall that affect New Zealand and Australia.
Just as researchers track record-breaking climate swings like the super El Niño of 2026, reading early circulation signals gives a head start on what the spring will bring.
Why this is not a failure of the Montreal Protocol
Here is the headline that got less attention: the long-term recovery trend is intact.
The 1987 Montreal Protocol — the treaty that phased out CFCs and other ozone-depleting substances worldwide — is widely regarded as the most successful environmental agreement ever signed. Concentrations of ozone-depleting gases in the stratosphere have fallen by roughly a third since their peak around the year 2000. The UN's scientific assessment still expects the Antarctic ozone layer to fully recover around the middle of this century.
Year-to-year swings like 2026 are natural variability layered on top of that slow healing. The atmosphere's plumbing changes from year to year: a strong vortex one spring, a disrupted one the next. Because CFCs decline only gradually — they can persist for decades — any given year can still produce a large hole when the weather cooperates with the remaining chlorine. What the protocol changed is the baseline: the same cold vortex that produced a 29.6-million-square-kilometre hole in 2006 is not expected to produce holes of that scale decades from now, when far fewer ozone-destroying chemicals remain.
The 2026 numbers themselves support this reading. A hole can be wide without being deep, and this year's was wide but shallow-ish: minimum ozone values and total ozone lost stayed near historical averages rather than hitting the extremes of the late 1990s and early 2000s. That is exactly what a healing-but-variable system looks like.
What to watch for the rest of the season
The ozone hole story of 2026 is not over. Holes typically persist through October and November, filling in only when the vortex finally breaks down and ozone-rich air from lower latitudes floods back over the pole. The Otago team's prediction of a prolonged season means 2026 could finish among the most severe holes on record by cumulative measures.
A few things to watch in the coming weeks:
First, the timing of the vortex breakdown. An early breakup would let the hole heal quickly; a late one keeps UV exposure elevated over the Southern Ocean and parts of the Southern Hemisphere landmasses into late spring.
Second, total ozone loss. Area makes the headlines, but the mass deficit — how much ozone is actually destroyed — is the better gauge of severity. So far that measure looks unremarkable, which is quietly good news.
Third, weather downstream. A large ozone hole cools the stratosphere over Antarctica, which can shift the jet stream and influence spring weather across the Southern Hemisphere. Researchers will be watching whether this year's hole leaves a fingerprint on rainfall and wind patterns in Australia, New Zealand, and southern South America.
The bottom line
The 2026 Antarctic ozone hole was the biggest September opening in 25 years — a genuine record, driven by an unusually strong and frigid polar vortex, and correctly forecast months in advance by scientists reading early circulation signals. But its size is a story about weather, not about a broken treaty.
The chemicals that caused the hole are still declining. The hole of 2026 was wide but not especially deep. And the slow, multi-decade recovery projected by the world's atmospheric scientists remains on schedule. Big September holes can still happen on the road to healing — the Montreal Protocol promised a destination, not a smooth ride.