As discussed in Sections 3.3.1 and 3.4.1, the prognosis for the future of the stratospheric ozone layer over the next half century is one of continuing recovery. Over that period, GHGs (notably carbon dioxide, methane and nitrous oxide) that are not controlled under the Montreal Protocol are expected to significantly affect future stratospheric ozone levels.104,156
Unlike carbon dioxide and methane—whose net effects are likely to be positive for the eventual recovery of the ozone layer—human-sourced emissions of nitrous oxide will have a negative impact. (In terms of their weighted ozone depleting potential, nitrous oxide emissions are larger than any of the ODSs controlled under the Montreal Protocol and are growing.) Consequently, as Ravishankara et al.156 noted, ‘increases in anthropogenic N2O [nitrous oxide] emissions or decreases due to abatement strategies would … affect the date for the recovery of the ozone layer’. A delay in the recovery date could delay realisation of certain health benefits (principally avoided cases of skin cancer) that are expected to accompany recovery. On the other hand, there is also the potential for reducing the recovery period through effective action to reduce nitrous oxide emissions.