The ozone hole: the time the world fixed one

In the 1970s, two chemists worked out something alarming on paper. A family of chemicals called CFCs, used in fridges, air conditioners and spray cans precisely because they were so stable and safe near people, would drift up to the stratosphere, break apart under harsh sunlight, and release chlorine that destroys ozone. The ozone layer is the planet's sunscreen: it absorbs the ultraviolet radiation that causes skin cancer and eye damage, something Australians have particular reason to care about. The chemical industry called the theory speculative and pointed out that nobody had measured any damage.

One detail explains why so small an amount of chemical could do so much harm. When ultraviolet light frees a chlorine atom from a CFC molecule, that atom does not destroy one ozone molecule and stop. It acts as a catalyst, meaning it breaks an ozone molecule apart, comes out unchanged, and goes on to break another, and another, thousands of times over before it is finally removed from the stratosphere. So a trace of a chemical, measured in parts per billion, could thin a layer of the sky across a whole continent. Understanding that catalytic cycle is what turned a paper theory into a genuine alarm, because it meant even small releases mattered enormously.

Then, in 1985, scientists with the British Antarctic Survey published measurements showing a massive seasonal hole in the ozone layer over Antarctica. It was so severe that when satellites had recorded the same readings earlier, the numbers had been set aside as too extreme to be right. The theory was no longer theoretical. The image of a hole in the sky, over the pole nearest Australia, shocked the public in a way the chemistry papers never had.

And why a hole over Antarctica, when the chemicals were released mostly in the rich countries of the north? The answer is the extreme cold of the Antarctic winter, which forms high icy clouds in the stratosphere. On the surface of those cloud particles the chlorine is converted into its most ozone destroying forms and held there through the long polar dark, so that when the sun returns in the southern spring the destruction happens all at once. That is why the hole opens each spring, and why it appeared first and worst at the bottom of the world, closest to Australia and New Zealand.

Industry's first response followed a familiar script: the science was uncertain, alternatives did not exist, and phasing out CFCs would cripple refrigeration and cost enormous sums. But two things undermined the resistance. The science kept getting stronger, with an expedition to Antarctica directly linking the hole to CFC chemistry. And the chemical companies discovered they could actually make substitutes, at which point some of them realised that a phase out they were ready for was a commercial advantage over competitors who were not. Opposition softened into competition.

In 1987, nations signed the Montreal Protocol, agreeing to cut and then eliminate ozone destroying chemicals on a schedule. It had teeth: trade rules discouraged countries from staying outside it, a fund helped developing countries afford the switch, and the schedule could be tightened as the science firmed up, which it repeatedly was. Over the following years it became the first treaty of any kind ratified by every country on Earth.

It worked. The chemicals were replaced, in some cases by substitutes that turned out to have their own problems and were then dealt with under later amendments to the same treaty, which is itself a lesson: agreements that can learn keep working. Atmospheric measurements show the destroyer chemicals declining, and scientific assessments now project that the ozone layer will substantially heal over the coming decades of this century, with the Antarctic hole recovering last. Millions of skin cancer cases will simply never happen. Hardly anyone under forty ever thinks about it, which is what complete success looks like.

Why did Montreal work? Four ingredients lined up. The science was clear, with a visible hole and a direct chemical culprit. Substitutes were available, so saying yes did not mean giving up refrigeration, just changing what was inside it. The agreement was binding, with schedules, money and trade rules rather than polite aspirations. And the problem was tractable: CFCs came from a small number of factories owned by a handful of companies, making the phase out enforceable.

It is worth sitting with that last point, because it is where the comparison with climate gets uncomfortable. A rule is only as good as its enforceability, and CFCs were a gift in that respect: trace the supply back and you reach a short list of chemical plants, so a country that agreed to the treaty could actually deliver on it. Carbon dioxide has no such choke point. It pours from power stations, cars, farms, furnaces and stoves in every town on Earth, which is why the same four ingredients, real as they are, have to work far harder on climate than they did on ozone.

The honest comparison with climate matters. Carbon dioxide is much harder than CFCs: it comes from billions of sources woven through every economy on Earth, not a few factories, and the substitute is an entire energy system rather than a different gas in the same fridge. Anyone who says climate is just another Montreal is oversimplifying. But anyone who says global cooperation on the atmosphere is impossible has to explain the treaty every country on Earth signed, the industry that innovated once the rules were set, and the layer of the sky that is now healing because of it. The playbook is real. The job is bigger.