The Antarctic ozone hole grew to its largest size in a decade this year, ranking seventh by area in 48 years of monitoring, NASA and NOAA said on Nov. 4. Yet the same scientists say Earth's ozone shield is still healing, and that this year's hole would have been far bigger without the global phaseout of ozone-destroying chemicals.

The hole averaged about 9.6 million square miles (25 million square kilometers) during the peak season of Sept. 7 to Oct. 13, according to NASA's 2026 ozone report. That average area is more than twice the size of the United States. On its largest single day, Sept. 18, the hole reached 10.5 million square miles (27.1 million square kilometers).

The apparent contradiction is the story — and the key to reading it. The size of the hole in any one year is driven largely by weather high above Antarctica, while its long-term trend is driven by the amount of chlorine and bromine left in the atmosphere. Both signals showed up clearly in 2026.

A Stubborn Polar Vortex, Not a Treaty Failure

Each Southern Hemisphere winter, a ring of strong winds called the polar vortex traps frigid air over Antarctica. The cold allows polar stratospheric clouds to form, and on their icy surfaces, chlorine from old human-made chemicals is converted into reactive forms. When sunlight returns in late August and September, that chlorine rapidly destroys ozone.

This year the vortex was unusually large. Amy Butler, a research scientist at the NOAA Chemical Sciences Laboratory, said "Planetary-scale weather systems that typically well up from the lower atmosphere and break up the vortex were especially weak in August and September." That weak wave activity kept the strongest vortex winds several degrees of latitude away from the South Pole, widening the area where ozone loss could occur.

Europe's Copernicus Atmosphere Monitoring Service flagged the early growth in September, reporting that the hole had expanded to about 25 million square kilometers by Sept. 12. That was roughly 5 million square kilometers above the seasonal norm for that point in the year.

From Fifth Smallest to Seventh Largest in One Year

The swing from 2025 shows how much weather matters. Last year, NASA and NOAA ranked the hole as the fifth smallest since 1992, with a peak of 8.83 million square miles on Sept. 9, 2025. A weaker-than-normal polar vortex that August kept stratospheric temperatures above average and limited ozone loss.

This year's peak was about 90% as large as the largest single-day ozone hole on record, set in September 2000, ABC News reported. But size is only one measure. Depth matters too, and on that score 2026 was milder than the worst years.

Balloon-borne instruments launched from NOAA's South Pole observatory recorded a low of 105 Dobson units directly overhead on Oct. 17. The record low at that site is 92 Dobson units, measured in October 2006. Scientists define the ozone hole as the area where total ozone falls below 220 Dobson units, so this year's hole was wide but not as deep as several record-setting predecessors.

The Counterfactual Scientists Say Matters Most

The most important number in the report describes a hole that did not happen. Paul Newman, a senior scientist at the University of Maryland, Baltimore County, and longtime leader of NASA's ozone research team at Goddard Space Flight Center, said: "The 2026 ozone hole is large but could have been 1 to 2 million square miles (3.5 million square kilometers) larger if ozone-depleting substances were as prevalent in the atmosphere today as they were at their year 2000 peak."

That estimate is based on modeling, not direct observation, but it rests on measured chemistry. NOAA Global Monitoring Laboratory senior scientist Stephen Montzka said chlorine from ozone-depleting substances in the Antarctic stratosphere has fallen by about a third from its peak around 2000. Those reductions trace back to the 1987 Montreal Protocol, which phased out chlorofluorocarbons and related compounds.

Timing offers a second sign of progress. "The annual ozone hole has been forming later in the season in recent years, which tells us the ozone layer is healing," Newman said. NASA and NOAA estimate the Antarctic hole will recover by the 2070s as long-lived legacy chemicals slowly break down. Projections for full recovery vary somewhat between assessments depending on the measure used.

UV Exposure and the Long Watch Ahead

When ozone thins, more ultraviolet radiation reaches the surface, raising risks of skin cancer, cataracts and crop damage. The greatest exposure from the Antarctic hole falls on Antarctica itself, the Southern Ocean and its wildlife, with occasional effects at the southern tip of South America when the edge of the vortex drifts north.

The Antarctic ozone hole does not directly raise UV levels over the continental United States. For U.S. readers, the practical takeaway is unchanged: check the daily UV Index, especially in summer, at high elevations and near water or snow, and use shade, protective clothing and sunscreen when the index is high.

The broader lesson is about policy durability — whether a treaty signed in 1987 still delivers decades later. The ozone hole is one of the clearest tests of whether a global environmental treaty can deliver measurable results, and the 2026 data suggest it is working even in a bad weather year. Scientists also stress that recovery depends on continued compliance, since any rise in illegal emissions would slow the decline of chlorine.

The hole typically closes in November or December as the vortex breaks down. NASA and NOAA will continue tracking it with instruments on NASA's Aura satellite, the NOAA-20, NOAA-21 and Suomi NPP satellites, and balloon launches from the South Pole. NASA's Scientific Visualization Studio has published maps of this year's hole for readers who want to see its shape.

What Readers Want to Know

How big was the 2026 ozone hole? It averaged about 9.6 million square miles from Sept. 7 to Oct. 13 and peaked at 10.5 million square miles on Sept. 18, ranking seventh largest by area in 48 years.

Does a larger hole mean the Montreal Protocol is failing? No. NASA and NOAA say an unusually large and persistent polar vortex drove this year's size, and that the hole would have been 1 to 2 million square miles larger without the chemical phaseout.

Why was 2025's ozone hole so much smaller? A weaker-than-normal polar vortex in August 2025 kept temperatures higher, limiting ozone loss. That hole ranked fifth smallest since 1992.

Does the ozone hole affect UV levels in the United States? Not directly. The hole forms over Antarctica. U.S. residents should continue to follow the local UV Index for sun protection.

When will the ozone hole fully recover? NASA and NOAA estimate recovery by the 2070s as long-lived ozone-depleting chemicals break down.

When does the ozone hole close each year? It usually breaks up in November or December as the Antarctic polar vortex weakens.