Signal Montreal Protocol shows boundary transgression can be reversed

Summary

The report holds up the Montreal Protocol as an encouraging example of action that benefits several boundaries at once. Regulating ozone-depleting substances allowed the stratospheric ozone layer to recover. Because many of these substances are also powerful greenhouse gases, the controls prevented considerable additional warming as well. The ozone layer has largely recovered since the mid-1990s and, at 285.7 DU, now lies within its boundary of 277.4 DU. The report judges that this success proves the trend of boundary transgression can be reversed. The essential-use concept built into the protocol is now guiding the phase-out of PFAS, and the report sees scope to extend it to other hazardous substances.