Modelling the potential impacts of the recent, unexpected increase in CFC-11 emissions on total column ozone recovery

Abstract. The temporal evolution of long-lived, anthropogenic chlorofluorocarbons is a key control on the timing of total column ozone (TCO) recovery. Recent observations have shown that the atmospheric mixing ratio of CFC-11 is not declining as expected under complete compliance with the Montreal Protocol, and indicate a new source of CFC-11. In this study, the impact of a number of potential future CFC-11 emissions scenarios on TCO recovery is investigated using the UM-UKCA model. Key uncertainties related to this new CFC-11 source and their impact on the timing of TCO recovery are explored, including: the duration of new CFC-11 emissions/production; the impact of any newly created bank; and the effects of co-production of CFC-12. Scenario-independent relationships are identified between cumulative CFC emissions and the timing of ozone recovery, which can be used to establish the impact of future CFC emissions pathways on ozone recovery in the real world. It is found that, for every 200 Gg Cl emitted, the timing of global TCO recovery is delayed by ~ 0.56 years. However, a marked hemispheric asymmetry in the latitudinal impacts of cumulative Cl emissions on the timing of TCO recovery is identified, with longer delays in the southern hemisphere than the northern hemisphere for the same emission. Together, these results indicate that, if rapid action is taken to curb recently identified CFC-11 production then no significant delay in the timing of TCO recovery is expected, highlighting the importance of ongoing, long-term measurement efforts to inform the accountability phase of the Montreal Protocol. However, if the emissions are allowed to continue into the future, and are associated with the creation of large banks, then significant delays in the timing of TCO recovery may occur.