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Fortunately this error was quickly corrected. The most important leap was made just a year after Lovelock’s report, in another Nature paper, this time written by Mario Molina and F. Sherwood (Sherry) Rowland—perhaps one of the most environmentally important scientific publications of all time and one that was to gain the two men a Nobel Prize.4 In June 1974 Molina and Rowland proposed that CFCs in the stratosphere could be split apart by ultraviolet light, leading to dangerously reactive free chlorine atoms circulating high in the atmosphere. These atoms could destroy stratospheric ozone on a scale that no natural process had so far achieved, seriously endangering life at the Earth’s surface. Their work was taken further by Paul Crutzen, the Dutch-born atmospheric chemist who is today also a member of the planetary boundaries expert group (and mentioned in an earlier chapter for his proposals on geoengineering). Crutzen calculated that up to 40 percent of ozone could be destroyed in the highest regions of the stratosphere by chlorine-containing CFCs, with the most important ozone-destroying reactions taking place in extremely cold, thin ice clouds high in the springtime polar stratosphere.

And so it proved: In 1985 scientists based at the British Antarctic Survey’s Halley Base noticed that stratospheric ozone concentrations were plummeting in the frigid air of the Antarctic spring—with up to 40 percent of the ozone layer depleted in a phenomenon quickly dubbed the “ozone hole.” NASA, which had also spotted the hole but first assumed it to be an instrumental error, quickly confirmed the thesis with worldwide satellite measurements. Theory and observation aligned, and Midgley’s CFCs were generally accepted as being the main culprits. Further damning evidence came when falling ozone levels were also identified in the Northern Hemisphere, raising the perils of skin cancer and eye cataracts for large populations in countries like the United States.

HUMANITY’S FINEST HOUR

The Vienna Convention for the Protection of the Ozone Layer, signed in 1985, is often cited as the most successful achievement ever in multilateral environmental policymaking. Actually the Convention was mostly symbolic: The real breakthrough came two years later in Montreal, with the signing of a Protocol among the major industrialised countries that began the phasing out of CFC production. The story is worth examining in detail, not only because it shows that humanity can successfully navigate away from a breach of a planetary boundary, but because studying the way in which policies were developed and agreed internationally can potentially help us tackle even thornier issues—starting with climate change.

There is no denying that the international ozone regulation regime has been startlingly successful. Humanity was over the planetary boundary—suggested by the expert group at 5 percent loss of stratospheric ozone below its pre-1980 level—for just five years, between 1992 and 1998. We are now well within the ozone boundary: Recovery has been slow, but losses currently average around 3 and 4 percent, and the situation is expected to gradually improve.5 The Antarctic ozone hole stopped growing a decade ago: Its all-time record was set on September 9, 2000, at 11.5 million square miles (29.9 million square km).6 Looking forward, experts project that the Earth’s ozone layer will recover back to its pre-1980 level by sometime between 2060 and 2075.

In navigating back within the ozone boundary, humanity certainly avoided a hellish future. One NASA study suggests that without the Montreal Protocol two-thirds of the Earth’s ozone layer would have disappeared by 2065.7 The ozone hole over Antarctica would be a year-round feature, while another hole would have opened up over the North Pole too. UV radiation falling on mid-latitude cities like Washington or London would be intense enough to cause sunburn in as little as five minutes. Ecological effects would have been devastating, and additional human skin cancer rates would be in the hundreds of thousands or even