Clock Above the Ice

The Halley Drop

Eli: McMurdo Station, Antarctica, 1986. Susan Solomon and an expedition team are measuring trace gases high in the stratosphere. In her own account, she'd put forward a hypothesis: 'an unanticipated chemistry involving chlorine,' running on the surfaces of clouds that form in the frigid air over Antarctica. Then the instruments pick up chlorine dioxide. Chlorine chemistry, caught at work. Later measurements showed chlorine monoxide climbing as ozone fell.

Mara: And that's the heart of what one listener asked us: what made the hole in the ozone layer, and did we really fix it? Today's angle is the endurance test between human environmental policy and atmospheric chemistry.

Eli: Solomon's chlorine answers the first half. The second half is still being tested over the same ice: whether a treaty can mend a tear that size, or whether the polar sky keeps ripping it back open.

Mara: Before anyone went hunting for chlorine, though, someone had to notice the hole.

Eli: That was a British Antarctic Survey team at Halley, on the Antarctic coast. They'd been reading ozone there for years with a Dobson spectrophotometer, a ground instrument that measures how much of the sun's ultraviolet light gets soaked up on its way down.

Mara: So ozone is whatever goes missing.

Eli: Exactly, and in 1982 a lot went missing. The reading dipped around 40 percent. The scientist running the program wasn't alarmed. He was skeptical, and figured the instrument had malfunctioned. The next year, the drop was still there. So he dug up his old data, and found the decline had really started back in 1977.

Mara: Five years of loss, sitting quietly in the archive.

Eli: He checked it, then posted the findings to Nature on Christmas Eve. The paper came out in 1985, from him and two Survey colleagues, after earlier reports from the Survey and from the Japan Meteorological Agency. Then came the awkward part. NASA's scientists realized their satellites had recorded the loss too. They'd overlooked it, treating the readings as an instrument problem.

Mara: Same data, two conclusions. What split them was how each group analysed it.

Eli: Once NASA looked properly, its satellite maps showed the hole growing through the 1980s to about the size of the continental United States.

Mara: And the chlorine behind it came from chlorofluorocarbons, CFCs, the gases in spray cans and refrigerators. Chemists had theorized that the same ultraviolet light that splits oxygen high up could split CFCs too, freeing chlorine atoms. Each atom then runs a catalytic cycle, a loop where it breaks apart an ozone molecule, comes out unchanged, and goes round again. It starts the reaction and is never used up, so one atom can wreck many thousands of ozone molecules.

Eli: And less ozone overhead lets more ultraviolet through. So chlorine was the suspect. Why it piled up over Antarctica instead of everywhere is Mara's to untangle.

A Candle in the Cold

Mara: The untangling starts nowhere near Antarctica. It starts with why anyone wanted these chemicals in the first place.

Eli: Early refrigerators leaked methyl chloride, and people died. So refrigeration and chemical companies went looking for something safer, and a team came up with R-12, a chlorofluorocarbon. Then, at an American Chemical Society meeting in 1930, the chemist who led that team made his case in person. He breathed in R-12 and blew out a candle with it. The EPA's history of the episode calls it dramatic, and adds it 'would be a clear violation of safe handling practices today.'

Mara: Which brings it back to Solomon's clouds.

Eli: Antarctica is mountains ringed by open ocean, and together they drop the ozone layer below minus 78 Celsius in winter. That cold makes polar stratospheric clouds: thin ice clouds high up, whose particles give chemistry a surface to happen on.

Mara: Paul Crutzen, studying them in the 1980s, explained that there, chlorine and bromine become 'highly reactive, catalytically active forms that subsequently destroy ozone molecules.' The air sits sealed in a vortex all winter, the chlorine primed. When sunlight returns in spring, the cycle runs flat out.

Eli: That's the chemistry Solomon's team caught in the act at McMurdo.

Mara: And the fuel lasts. CFC-11, one of the big ones, stays in the air about 52 years on average, and even that estimate carries real uncertainty. Almost all of it is finally broken apart by ultraviolet light, high in the stratosphere.

Eli: Half a century. That candle trick has a very long tail.

Mara: Governments did act. The Montreal Protocol is the international treaty that sets deadlines for phasing these chemicals out, and countries kept tightening it with amendments.

Eli: Twenty-seven nations signed it in 1987, first pledging to halve CFC production. R-12 included.

Mara: Susan Solomon is among those who point to where it led. By their figure, the treaty and its amendments have cut production and consumption of ozone-depleting substances by 99 percent, helping put the ozone layer on a path to recovery.

Eli: That's the number behind the belief the ozone layer's already fixed.

Mara: What no treaty could promise is how fast the sky follows. A cut on the ground still has to wait out a half-century lifetime up high, and the Antarctic spring is where that wait gets tested.

The September Surge

Eli: For a while, the springs looked like proof the fix had worked. By most metrics, the 2024 and 2025 holes were smaller. The 2025 hole ranked fourth or fifth smallest since 1992. And the World Meteorological Organization reported it closed on December 1st, the earliest finish since 2019.

Mara: Small, and gone early. The shape people expect a recovery to take.

Eli: Then 2026. Ozone's counted in Dobson units, roughly how thick the layer would be if it were squeezed down to sea-level pressure, and the hole is the patch where those numbers run thin. Through late August that patch kept spreading. By mid-September, it covered 25 million square kilometres, well above normal for the date. Ozone researchers in New Zealand called it the largest September hole in 20 years, driven by a very strong polar vortex, a swirling whirlpool of winds that traps freezing air, and cold over the pole. One report wondered whether its size signalled a change in how the hole behaves.

Mara: Antarctica itself is about 14 million. The hole dwarfed the continent beneath it.

Eli: The worst years came around 2000 and 2006. Those holes reached nearly 30 million square kilometres, more than three and a half Australias. So 2026 isn't a record. What makes it jarring is the timing: it opened after the 99 percent cut. But that cut stopped new supply. It couldn't touch the chlorine already up there, on its half-century clock. Every winter the cold returns, the clouds form, and the old chlorine goes back to work. A strong, cold vortex just gives it more room.

Mara: Full recovery will lag decades behind the phaseout.

Eli: So one of the smallest holes in decades and the largest September hole in twenty years sit twelve months apart. Sorting chemistry from weather in that swing is what Mara's been digging into.

Decades to Clear

Mara: Susan Solomon, the same scientist who went chasing chlorine at McMurdo in 1986, came back to it decades later. She and colleagues used atmospheric fingerprinting, a method borrowed from climate science: simulate the atmosphere with and without ozone-depleting substances and greenhouse gases, then see which pattern the real ozone matches. It matched the drop in ozone-depleting substances. Solomon's summary: 'The conclusion is, with 95 percent confidence, it is recovering. Which is awesome. And it shows we can actually solve environmental problems.'

Eli: Recovering, in the same decade that produced the 2026 hole.

Mara: Both can hold, because the hole's size each year is mostly set by weather up high: planetary waves, and how strong the vortex gets. The big, lingering holes of 2020 and 2021 got that reading from the treaty's scientific assessment panel. It called them largely dynamically driven, a pattern that 'does not challenge the evidence for the emergence of recovery.'

Eli: The chlorine count moves slowly too. By the assessment numbers Solomon and Saulo point to, total chlorine entering the stratosphere peaked in 1993. By 2024, it was down about 14 percent. That's the 52-year lifetime showing itself, three decades on.

Mara: Down, but most of that peak is still up there.

Eli: And there are jolts. Smoke from Australia's 2019 to 2020 bushfires climbed into the stratosphere. Solomon and colleagues showed smoke like that can make nitrogen dioxide plummet, kicking off a chain that ends in chlorine destroying ozone. The 2020 hole grew about 10 percent.

Mara: Same old chlorine, handed a fresh opening by fire.

Eli: Then a rogue source. Monitors caught unexpected emissions of CFC-11, a banned gas. The treaty's technology panel concluded old production and existing foam were unlikely to explain it. It was traced to illegal use in polyurethane foam insulation in China, and the government there moved on monitoring and enforcement. By early 2021, two Nature papers suggested emissions were heading down again, and the science panel said recovery wouldn't be significantly delayed. The Environmental Investigation Agency, which tracked the case, still flagged critical gaps in data and understanding.

Mara: That's where the readings split, and it's a real split. Celeste Saulo, who heads the WMO, put it this way: 'This is an environmental success story.' She says the layer has been recovering since 2000, with full recovery expected within several decades. She, Solomon, Laurence Rouil and António Guterres read it as a problem halted at the source, on a path that won't turn back.

Eli: Richard Querel and Anna Jones, with the Environmental Investigation Agency and the International Ozone Commission, read the same record as unfinished: recovery incomplete, exposed to illegal emissions, and dependent on decades more of countries keeping the rules. Their yardstick is the projection itself: springtime Antarctic ozone back to 1980 values during the 2060s.

Mara: So what made the hole is chlorine from CFCs, turned loose on Antarctica's winter clouds. And the fix is real and unfinished at once. The ban targeted new damage, but the sky clears on its own slow schedule, with the 2060s as the projected finish. What's left of the chlorine Solomon's team caught at work in 1986 drains out one spring at a time.

Eli: Two signals to watch. Each fire season and volcanic plume that lofts smoke into the stratosphere, and whether it pushes back that mid-century healing. And the global air-sampling network, for any unexpected spike in halogens that would point to a rogue factory or a leaking stockpile of old chemicals.

Mara: Polar stratospheric clouds offer a place to dig deeper into the chemistry.

Eli: Follow climate with us as the atmosphere continues its slow recovery.

Mara: And that's the story. I'm Mara.

Eli: And I'm Eli. Thanks for listening, and until next time.