On September 16, 1987, delegates from 24 countries and the European Community signed the Montreal Protocol on Substances that Deplete the Ozone Layer, agreeing to freeze and then cut production of chlorofluorocarbons and halons. The ozone hole over Antarctica had been reported two years earlier. Nobody had expected the science to move this fast or the diplomats to move faster.
The mechanism had been known since 1974, when Sherwood Rowland and Mario Molina at Irvine showed that CFCs were inert enough to drift into the stratosphere, where ultraviolet light breaks them and releases chlorine, and that a single chlorine atom can destroy something like 100,000 ozone molecules before it is removed. Industry disputed it. Then Joe Farman and his colleagues at the British Antarctic Survey measured ozone over Halley Bay and found the springtime column had fallen by about a third since the 1950s. Satellite data confirmed it. Nobody had a competing explanation.
CFCs were everywhere in 1987: refrigerants, foam blowing, aerosol propellants, and cleaning solvents for electronics, and the industry was worth billions. DuPont, the largest producer, had said in the 1970s that the theory was a load of rubbish and in 1986, after the hole, announced it would develop substitutes. That turn removed the main obstacle. The treaty's real design was what made it work: it set a schedule, it bound countries that had not yet used CFCs to no growth, it banned trade in controlled substances with non-parties, and it had a fund to pay the developing world's costs.
The schedule was tightened repeatedly. The London amendment in 1990 and the Copenhagen amendment in 1992 moved to a complete phase-out, and the Kigali amendment in 2016 began cutting the hydrofluorocarbons that had replaced CFCs and turned out to be powerful greenhouse gases. Consumption of the controlled substances is down by more than 98 per cent from the peak.
The ozone layer is recovering, slowly. The Antarctic hole still opens every spring and is expected to close around the middle of the century, and the protocol is the reason: it is the only United Nations environmental treaty every country on earth has ratified, and it avoided an estimated tens of millions of skin cancer cases. Rowland and Molina shared the 1995 Nobel Prize in Chemistry.