Kicking the Can Down the Road in Ozone Recovery

Every four years the international scientific assessment of ozone depletion is prepared to support decisions made by the Parties to the Montreal Protocol. A key component of each assessment is an outlook of the ozone recovery timeline, which has been quantified using equivalent effective stratospheric chlorine (EESC), a metric that has been developed to relate surface level atmospheric abundance of ozone depleting substances (ODSs) to stratospheric ozone destruction. In each assessment, the year in which EESC values will return to below 1980 levels is estimated, given the best scientific understanding of atmospheric processes and assuming global compliance with the Protocol. However, since 2006, the expected EESC return date to below 1980 levels has been consistently delayed between assessments from an expected EESC return date of 2049 in the 2006 assessment to 2066 in the 2022 assessment, an ozone recovery delay of 17 years over a 16-year assessment period. Has this delay in expected ozone recovery been a result of consistently underestimating global production and emissions of ODSs or due to changes in the scientific understanding and representation of atmospheric processes? Here, we investigate this question by identifying the primary drivers that have delayed the expected ozone recovery date between each consecutive international ozone assessment from 2006 to 2022. We find that changes in the formulation of fractional release factors that underlie the calculation of EESC can only partially explain this delay, and that changes in 1) atmospheric lifetime assumptions 2) bank calculation methods 3) updated historical mole fraction estimates and 4) an under-estimate of the atmospheric release of CCl4 account for much of the remaining delay. Since some of these factors are amenable to future controls (i.e., capture of ODSs from banks and limitations on future atmospheric release of CCl4), it is important to understand the reasons for the delays in the expected ozone recovery date.