Antarctic ozone hole spreads to 27.4 million square kilometres on a cold, strong polar vortex

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 ^[1][2]. 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 ^[19], 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 ^[14].

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 ^[15]. The authors link worse depletion in those years to smoke from Australia's 2019-20 wildfires and aerosols from the 2021 La Soufriere eruption ^[15]. The 2022 Hunga Tonga-Hunga Ha'apai eruption brought an early start to the 2023 season ^[15]. In recent years the hole has routinely persisted late into spring ^[18]. Full recovery of the Antarctic hole is still projected before 2090 ^[17].