the Montreal protocol has paused the southward movement of the jet stream since the turn of the century and may even be starting to reverse it as the ozone hole begins to close. Last September, satellite images revealed the ozone hole annual peak had shrunk to 16.4m sq km, the smallest extent since 1982.

the ozone hole might close by 2070. Here’s how that happened.

The success of the Vienna Convention lay in its increasing ambition over time. Regulations became tighter as more evidence emerged of the depletion of the ozone layer and the gases that were causing it. The deadline to phase out the production of ozone-depleting gases continued to be brought forward. More countries joined. By the turn of the millennium, 174 parties had signed on. In 2009, it became the first of any convention to achieve universal ratification. The success of this international effort was truly stunning. Before the first protocol came into action in 1989, the use of ozone depleting substances continued to increase. But the phaseout that followed was rapid. Within 1 year, consumption fell 25% below its 1986 levels. Within 10 years the levels had fallen by almost 80% (far beyond the initial target of the Montreal Protocol of a 50% reduction). As of today, their use has fallen by 99.7% compared to 1986.

this is probably the most successful collective action ever. a good amount of luck and timing seemed to have been involved, unclear how much mechanism design mattered.