Abstract

2017 marked the 30th anniversary of the Montreal Protocol, which was implemented to protect the stratospheric ozone layer from the harmful effects of ozone depleting substances (ODS). Controls introduced under the Montreal Protocol and its subsequent amendments first slowed the rate of accumulation of these halogenated ODS in the atmosphere, and since the late 1990s their atmospheric concentrations have begun to decline. A reduction in stratospheric chlorine concentrations should lead to an increase in atmospheric ozone as the strength of the halogen catalyzed ozone destruction cycles declines. However, detecting recovery of the stratospheric ozone layer is complicated by a number of additional factors which affect the year to year variability of total column ozone values (e.g. volcanic eruptions, the solar cycle, the quasi-biennial oscillation and the El Niño-Southern Oscillation). As a result of these factors, identifying robust recovery of total column ozone and ascribing that recovery to a decline in stratospheric halogen species is a complex issue.

In this study, total column ozone values from an ensemble of UM-UKCA model simulations are examined to investigate different definitions of progress on the road to ozone recovery: (i) a slowed rate of decline and the date of minimum column ozone; (ii) the identification of significant positive trends; and (iii) a return to historic values. Focus is then moved to the tropics, where ozone recovery resulting from declining stratospheric chlorine concentrations may be offset by circulation changes resulting from increasing greenhouse gas concentrations.