The Montreal Protocol has delivered, but it must transform to deal with new threats to the planet.

Four decades ago, the world did something remarkable. It discovered that refrigerants once celebrated as technological miracles were destroying the ozone layer, and it responded by negotiating a global treaty to phase them out. Crucially, the agreement recognised differentiated responsibilities: developed countries committed to funding the transition in developing countries.

The result has been extraordinary. The universally ratified Montreal Protocol has eliminated about 99% of controlled ozone-depleting substances (ODS). The ozone layer is healing and is expected to return to 1980 levels by around 2040 across most of the world.