September’s ozone hole was the biggest in 20 years. The warning signs appeared months ago

The area covered by the Antarctic ozone hole peaked at more than 27 million square kilometres in September, the second-largest ever observed for this time of year.

We first predicted a severe ozone hole in May this year, when we observed changes in atmospheric circulation. A very strong polar vortex and cold temperatures over the pole are driving the early and large growth of the ozone hole.

But rest assured, the Montreal Protocol – a landmark global treaty designed to protect the ozone layer by phasing out ozone-depleting substances – is still working.

The levels of ozone-depleting substances in the stratosphere have dropped by about a third since their peak around the year 2000.

Despite its larger area, the 2026 ozone hole hasn’t reached the depths of other large ozone holes from the past, meaning there is less “missing” ozone overall.

This year’s ozone hole area started growing earlier than average in August, accompanied by low temperatures in the stratosphere.

The hole then rapidly grew from the start of September. At 27.4 million square kilometres, this record area has only been broken twice: in 2000 the ozone hole briefly reached more than 28 million square kilometres and in 2015, it peaked during October at 27.9 million square kilometres.

How the Antarctic ozone hole works

The ozone hole forms in early spring inside the stratospheric polar vortex, a strong wind pattern that forms every winter and encircles Antarctica and the Southern Ocean.

The very low temperatures inside the vortex lead to the formation of polar stratospheric clouds. Chemical reactions on particles in this rare type of cloud then activate ozone-depleting gases.

Once formed, the ozone hole’s longevity depends on how well isolated the air is within the polar vortex. A cold and robust vortex leads to a long-lasting ozone hole. Hence, large-scale atmospheric dynamics play a key role.

Since the signing of the Montreal Protocol in 1987, concentrations of ozone-depleting gases have reduced.

However, sometimes the dynamics of the polar vortex can be “primed” leading into spring. Over several months prior, patterns of airflow can change and lead to a stronger and colder vortex.

If this happens, the ozone hole may grow larger and deeper, and last longer, even if chemical healing is underway.

Early predictions and the outlook for 2026

Improving the predictability of severe ozone hole events remains an ongoing scientific challenge.

By May 2026, changes in atmospheric composition observed from satellites were the first sign that atmospheric dynamics were priming the polar vortex. We predicted a “major” ozone hole for September 2026.

So far, September observations are matching early predictions. But what will happen next?

Unless the dynamical conditions above the pole shift, our prediction is that we are set for a prolonged season, with a major hole continuing through October and November. Recent work from our team will allow us to quantify just how much atmospheric dynamics exacerbated this year’s ozone hole.

We can expect 2026 to have an overall ranking within the top ten most severe ozone holes of all time.

A severe ozone hole in 2026 isn’t entirely unexpected. Since 2020, we have experienced a run of other large and long-lasting Antarctic ozone holes during the years from 2020 to 2023.

Smoke from the Australian wildfires in 2019-20 and aerosols from the 2021 La Soufrière eruption in the Caribbean have been linked to worsened ozone depletion in those years.

The eruption of the Hunga Tonga–Hunga Haʻapai submarine volcano in 2022 led to an early start to the 2023 ozone hole season. All things considered, the 2020s have been a unique decade for ozone recovery.

What this means for weather

The ozone hole affects weather and climate across the Southern Hemisphere.

Following a severe ozone hole, a climate pattern called the Southern Annular Mode (SAM) shifts towards its positive phase, which brings calmer winds and warmer, wetter weather to Aotearoa New Zealand, particularly on the west coast. For Australia, this means increased rainfall along the southeast coast.

The opposite phase can lead to more extreme wind events, such as the series of destructive storms that lashed parts of New Zealand during October last year.

In the long term, the Antarctic ozone hole is projected to reach full recovery before 2090.

But in recent years, the ozone hole has routinely persisted late into spring. Changes in global atmospheric dynamics could be making longer-lived ozone holes more likely, and the projected timeline of ozone recovery could be missing critical aspects of this behaviour.

For Aotearoa New Zealand, this means the ozone hole will continue to affect winds and temperatures on our shores for several more decades.

Hannah E. Kessenich receives funding from the Ministry of Business, Innovation and Employment.

Annika Seppälä receives funding from the Ministry of Business, Innovation and Employment.

Dan Smale receives funding from the Ministry of Business, Innovation and Employment.