Estimating and Representing the Recovery of the Stratospheric Ozone Layer

Abstract:

The stratospheric ozone layer is now recovering from the effects of halogenated Ozone Depleting Substances (ODSs). These ODSs release chlorine and bromine in the stratosphere which catalyse ozone loss. Due to action under the Montreal Protocol the total stratospheric abundance of these halogens peaked around two decades ago and is slowly declining. However, it will take until around 2050 for the total halogen loading to return the 1980 levels, which is often taken as a reference point for the onset of ozone depletion. Similarly, model predictions of the date at which the ozone layer will return to 1980 values (in different regions) is used as essentially the only metric for ozone recovery.

In this talk I will discuss how well we can quantify and present the evolving extent of ozone recovery. There is some uncertainty related to the expected evolution of stratospheric halogens due to the release of compounds not controlled by the Montreal Protocol, such as very short-lived substances (VSLS) or rogue production of controlled substances such as CFC-11. Then there is uncertainty of future climate scenarios which feedback onto stratospheric ozone through chemical, radiative and dynamical effects. In addition to these factors that will affect the future evolution of the ozone layer, there are issues in how we represent the time-dependent extent of recovery. The use of a single ‘return to 1980’ date is simple but has many shortcomings. It ignores atmospheric variability which can mean, for example, that 2020 was a year of record Arctic depletion despite many years of decreasing chlorine and bromine. More importantly, the simple return metric ignores the pathway that recovery takes and does not give a measure of the extent at any time prior to complete return. Some of this work is taken from the recent paper by Dhomse et al. (Nature Communications, 10, 5781, doi:10.1038/s41467-019-13717-x, 2019).