September’s ozone hole was the biggest in 20 years

The area covered by the Antarctic ozone hole peaked at more than 27 million square kilometers (10.4 million square miles) in September, the second-largest area 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 ozone hole’s early and rapid growth.

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 2000.

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

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

The hole then grew rapidly from the start of September. Its area of 27.4 million square kilometers (10.6 million square miles) has been exceeded only twice: In 2000, the ozone hole briefly reached more than 28 million square kilometers (10.8 million square miles), and in 2015, it peaked in October at 27.9 million square kilometers (10.8 million square miles).

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, robust vortex leads to a long-lasting ozone hole. Large-scale atmospheric dynamics therefore play a key role.

Since the Montreal Protocol was signed in 1987, concentrations of ozone-depleting gases have declined.

However, atmospheric dynamics can sometimes “prime” the polar vortex before spring. Over the preceding months, airflow patterns can change and lead to a stronger, colder vortex.

If this happens, the ozone hole may grow larger and deeper, and last longer, even as the ozone layer recovers.

Early predictions and the outlook for 2026

Improving the ability to predict 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 our early predictions. But what will happen next?

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

We expect 2026 to rank among the 10 most severe ozone hole seasons of all time.

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

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 toward its positive phase, bringing 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 in 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-lasting ozone holes more likely, and the projected timeline for ozone recovery could be missing critical aspects of this behavior.

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

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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This article is republished from The Conversation under a Creative Commons license. Read the original article.