Last year marked 30 years since the Montreal Protocol, an international agreement signed by 197 countries aimed at healing the ozone layer . Heralded as a landmark environmental treaty – the first in UN history to achieve universal ratification – its effectiveness has been a beacon for international progress on so-called ‘wicked’ problems .

But there has been a surprising rise in emissions of one of the most damaging ozone-depleting chemicals banned by the treaty, a new study has found. Its apparent source: …