Antarctic ozone hole spreads to 27.4 million square kilometres on a cold, strong polar vortex
Antarctica's ozone hole reached 27.4 million square kilometres in September, a size topped only in 2000 and 2015. Its forecasters blame a cold, strong polar vortex, so the size says little about the Montreal Protocol until the weather's share is measured.
The Scientist · Science desk
What happened
- This year's hole began growing earlier than average in August, under low stratospheric temperatures, then expanded rapidly from the start of September.
- In May the researchers predicted a major September hole after satellites showed changes in atmospheric composition that pointed to a primed polar vortex.
- September observations match that forecast, and the team expects a major hole to persist through October and November unless conditions over the pole shift.
Why it matters
- What it limits constraint This year's area cannot be read as a verdict on the Montreal Protocol, because a cold vortex enlarges the hole; depth and the amount of missing ozone track the chemistry more closely.
- What's now possible capability A satellite composition signal flagged this season about four months before its September peak; if it holds across more years, severe holes could be forecast before spring begins.
- Who's exposed exposure New Zealand and southeast Australia get the weather effect: after a severe hole the Southern Annular Mode tends to turn positive, bringing warmer, wetter weather to New Zealand's west coast and more rain to Australia's southeast coast.
Next story · Science · Sep 18
WMO logs a second consecutive Antarctic ozone hole well below the 1990-2010 averageConfirmed2 publishers
The ranking depends on which date is compared. The phys.org headline calls September's hole the biggest in 20 years [2], and the account itself calls it the second-largest ever observed for the time of year [1]. On the account's own figures, though, the 2015 hole peaked in October at 27.9 million square kilometres [7]. That is 0.5 million larger than this year's, and 11 years ago [20][21]. The other larger hole is 2000's, which briefly passed 28 million [7].
Area responds to weather as well as chemistry. The hole forms in early spring inside the stratospheric polar vortex, a belt of strong winter winds around Antarctica and the Southern Ocean [8]. Low temperatures in the vortex produce polar stratospheric clouds, and reactions on the cloud particles activate ozone-depleting gases [8]. The colder and better isolated the vortex, the longer the hole lasts [9]. The authors wrote that airflow changes in the months before spring can "prime" the vortex [10]. If that happens, they wrote, "the ozone hole may grow larger and deeper, and last longer, even as the ozone layer recovers" [10]. They put this year's early, rapid growth down to a very strong vortex and cold temperatures over the pole [3].
The chemistry points the other way. Concentrations of ozone-depleting gases have fallen since the Montreal Protocol was signed in 1987 [16], and stratospheric levels are about a third below their peak around 2000 [4]. Despite its area, the 2026 hole has not reached the depths of other large holes, so less ozone is missing overall, according to the authors [5]. The account does not give a depth figure or a total for missing ozone, so the gap between this year and 2000 or 2015 cannot be sized from it.
The thing this doesn't tell you is the control: how large the hole would have been under this year's vortex with 2000-level gases, or what share of its size is weather. Until that number exists, the vortex explanation rests on sound physics and on timing. I'd call it well supported, with its share unmeasured. The team says recent work will let it quantify how much atmospheric dynamics made this year's hole worse [17].
The forecast is the part I find most interesting. By May, satellites had picked up changes in atmospheric composition that the team read as the first sign of a primed vortex, and it predicted a "major" hole for September [11]. It is one forecast and one hit. A single season cannot establish forecasting skill. The team expects 2026 to rank among the 10 most severe ozone hole seasons of all time [13].
Large holes have been common this decade. From 2020 to 2023 there was a run of large, long-lasting holes [18]. The authors link worse depletion in those years to smoke from Australia's 2019-20 wildfires and aerosols from the 2021 La Soufriere eruption [18]. The 2022 Hunga Tonga-Hunga Ha'apai eruption brought an early start to the 2023 season [18]. In recent years the hole has routinely persisted late into spring [15]. Full recovery of the Antarctic hole is still projected before 2090 [14].
What to watch
- Publication of the team's estimate of how much atmospheric dynamics enlarged the 2026 hole, the figure that would separate weather from chemistry.
- Depth and missing-ozone figures for 2026 set against 2000 and 2015, so the shallower-hole claim can be sized.
- October's extent, since the major hole is expected to last through the month in which 2015 set its larger peak.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence72
- Adoption
- Not scored
- Hype gap+15
- Incentives30
- Confidence68
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
The area of the Antarctic ozone hole peaked at 27.4 million square kilometres in September 2026, the second-largest area ever observed for this time of year.
- [2]
The phys.org headline states that September's ozone hole was the biggest in 20 years.
- [3]
A very strong polar vortex and cold temperatures over the pole are driving the 2026 ozone hole's early and rapid growth.
- [4]
Levels of ozone-depleting substances in the stratosphere have dropped by about a third since their peak around 2000.
- [5]
Despite its larger area, the 2026 ozone hole has not reached the depths of other large ozone holes in the past, meaning there is less missing ozone overall.
- [6]
The 2026 ozone hole started growing earlier than average in August, accompanied by low stratospheric temperatures, then grew rapidly from the start of September.
- [7]
The 27.4 million square kilometre area has been exceeded only twice: in 2000 the hole briefly reached more than 28 million square kilometres, and in 2015 it peaked in October at 27.9 million square kilometres.
- [8]
The ozone hole forms in early spring inside the stratospheric polar vortex, a strong wind pattern encircling Antarctica and the Southern Ocean; very low temperatures inside it form polar stratospheric clouds, and chemical reactions on their particles activate ozone-depleting gases.
- [9]
The ozone hole's longevity depends on how well isolated the air is within the polar vortex; a cold, robust vortex leads to a long-lasting ozone hole.
- [10]
Atmospheric dynamics can 'prime' the polar vortex before spring, leading to a stronger, colder vortex. The authors wrote: "If this happens, the ozone hole may grow larger and deeper, and last longer, even as the ozone layer recovers."
- [11]
By May 2026, changes in atmospheric composition observed from satellites were the first sign that dynamics were priming the polar vortex, and the team predicted a 'major' ozone hole for September 2026.
- [12]
September observations are matching the team's early predictions; unless conditions above the pole shift, the team expects a prolonged season with a major hole continuing through October and November.
- [13]
The team expects 2026 to rank among the 10 most severe ozone hole seasons of all time.
- [14]
In the long term, the Antarctic ozone hole is projected to reach full recovery before 2090.
- [15]
In recent years the ozone hole has routinely persisted late into spring.
- [16]
Since the Montreal Protocol was signed in 1987, concentrations of ozone-depleting gases have declined.
- [17]
Recent work from the team will allow it to quantify how much atmospheric dynamics exacerbated this year's ozone hole.
- [18]
From 2020 to 2023 there was a run of large, long-lasting Antarctic ozone holes; smoke from the 2019-20 Australian wildfires and aerosols from the 2021 La Soufriere eruption have been linked to worsened depletion, and the 2022 Hunga Tonga-Hunga Ha'apai eruption led to an early start to the 2023 season.
- [19]
Following a severe ozone hole, the Southern Annular Mode shifts toward its positive phase, bringing calmer winds and warmer, wetter weather to New Zealand, particularly the west coast, and increased rainfall along Australia's southeast coast.
- [20]
The 2015 October peak was about 0.5 million square kilometres larger than the 2026 September peak.
- [21]
The 2015 hole, larger than 2026's, occurred 11 years earlier, inside a 20-year window, so 'biggest in 20 years' holds only for September comparisons.
- [22]
The May forecast came about four months before the September peak it predicted.
Sources
2 independent publishers whose own reporting we read for this story.
- livescience.comThe ozone layer was supposed to be healing. So why is this year's Antarctic ozone hole so big?
1 article · October 6, 2026
- phys.orgSeptember's ozone hole was the biggest in 20 years
1 article · September 25, 2026
Topics and entities
Topics
- Southern Hemisphere climate
- Antarctic ozone hole
- Stratospheric polar vortex
Entities
- Southern Annular Mode
- Hannah Kessenich
- Copernicus Atmosphere Monitoring Service
- University of Otago
- Montreal Protocol