Former UN Secretary-General Kofi Annan once stated that perhaps the single most successful international environmental agreement to date has been the

Kofi Annan hailed the Montreal Protocol as perhaps the single most successful environmental agreement.

The Rio Summit set broad frameworks but is not the treaty he singled out.

The Kyoto Protocol on emissions had limited reach and did not earn that praise.

The Paris Agreement came later and was not the accord he described.

The Montreal Protocol phased out ozone-depleting substances worldwide.

Its near-universal acceptance is putting the ozone layer on the path to recovery.