We explore the consequences of an earlier phase-out of Ozone Depleting Substances, starting ten years before the Montreal Protocol. Atmospheric chemistry simulations verify the effectiveness of such an early-action scenario: stratospheric chlorine abundance remains below the level at which the ozone hole was discovered, even though countries are permanently allowed to continue using ODS to a non-negligible extent. A sectorally detailed technico-economic analysis finds that the additional cost of the earlier action scenario would have been moderate. We conclude that the Montreal Protocol was only partially successful at precaution: global atmospheric environmental problems could be regulated before surprising non-linearities occur.