We have reached a tipping point over the Earth’s future. If we don’t act now it will be too late | Johan Rockström

World leaders must come up with a plan at this month's climate change conference in Paris

We Homo sapiens got lucky. Very lucky. Back in the 1920s, when looking for a "safe" gas to use in refrigerators, chlorine was the element of choice in a new family of manmade chemical compounds - chlorofluorocarbons. In the 1970s, Paul Crutzen, Mario Molina and Sherwood Rowland discovered that while it was safe in our fridges, it was destroying the ozone layer, which is essential to protect all life on land.

Luck struck twice. Nasa scientists measuring ozone above Antarctica in the 1980s never saw the ozone hole in their data. Their computers were programmed to ignore any figures deemed "impossible". Luckily, the British Antarctic Survey had no such technology and sounded the alarm. In 1997, nations signed the Montreal Protocol outlawing CFCs.

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