“PERHAPS the single most successful international agreement to date has been the Montreal protocol,” declared Kofi Annan, then head of the United Nations, back in 2003. Agreed 16 years earlier, the mechanism sought to limit damage to the stratospheric ozone layer that protects the planet from harmful ultraviolet radiation. The protocol phased out substances such as chlorofluorocarbons (CFCs)—coolants in devices ranging from air conditioners to refrigerators—which deplete the ozone. They also contribute to global warming. To date, the agreement has averted the equivalent of more than 135 billion tonnes of carbon-dioxide (CO 2 ) emissions. One of its most important architects, Mostafa Tolba, an Egyptian scientist, died on March 28th, aged 93.

Educated in Cairo and London, Dr Tolba helped found the institutions of modern climate diplomacy. In 1972 he led his country’s delegation to the Stockholm conference from which the United Nations Environment Programme emerged. After a short stint as its deputy executive director, he took full charge from 1975 until 1992. The Vienna Convention for the Protection of the Ozone Layer, the UN agreement accompanying the Montreal protocol, was a personal and public victory for Dr Tolba. It has since been ratified by 197 countries and by the European Union.

No other environmental initiative has achieved as much. Ozone-depleting chemicals in the atmosphere reached their zenith in 1994. The concentration of ozone in the atmosphere should be back to what it was before 1980 by the middle of this century, studies suggest. America’s Environmental Protection Agency estimates that up to 2m cases of skin cancer may be prevented globally each year until 2030 because of it.

The UN climate deal struck last year in Paris to limit global warming to “well below” 2°C above pre-industrial temperatures also owes much to the earlier agreement. Even after the failings of the Kyoto protocol, hashed out to curb CO 2 emissions in 1997, and of UN climate negotiations in Copenhagen in 2009, leaders could still point to Montreal. It provided proof of the efficacy of international agreements and of the predictable regulatory environments they can create.

More gains may yet accrue if a new deal is reached to limit other nasties under the protocol’s authority. Even if countries make good on all the pledges they made last year to slash emissions, the Paris agreement may still lead to global warming of 3°C. Curbing short-lived pollutants such as hydrofluorocarbons (HFCs), methane and black carbon could be one way to limit warming further. They are between 25 and 4,000 times more potent than CO 2 and linger in the atmosphere for a far shorter time—20 years rather than up to 500—so cutting back now could bring benefits quickly. According to the Arctic Monitoring and Assessment Programme, an intergovernmental group, curbing the emissions of short-lived pollutants could skim 0.2°C off expected global warming by mid-century.