IN 1985 a gaping hole was found in the ozone layer above Antarctica. Two years later leaders from around the world signed the Montreal Protocol, a treaty to phase out the substances causing it, known as chlorofluorocarbons (CFCs), which were used in refrigeration and as propellants in products such as hairsprays and deodorants. Scientists had discovered more than a decade earlier that CFCs release chlorine into the stratosphere as they decompose—depleting ozone—and are also powerful greenhouse gases. Thanks to the treaty, the equivalent of some 135 billion tonnes of carbon-dioxide emissions were avoided, saving the ozone layer from complete collapse by the middle of this century. This week officials from around the world, led by America and China, are meeting in Rwanda to make a deal that would extend the Montreal Protocol to cover hydrofluorocarbons (HFCs), which were introduced to replace CFCs. Why?

HFCs don’t deplete the ozone layer but they still contribute hugely to global warming, as scientists discovered in the decades after their introduction. The average atmospheric lifetime for most commercially used HFCs is 15 years or less whereas carbon dioxide can stay in the atmosphere for more than five centuries. But, like CFCs, HFCs cause a greenhouse effect between hundreds and thousands of times as powerful as carbon dioxide. Total emissions of HFCs are still relatively low.

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