Quantifying the contributions to stratospheric ozone changes from ozone depleting substances and greenhouse gases
A state-of-the-art chemistry climate model coupled to a three-dimensional ocean model is used to produce three experiments, all seamlessly covering the period 1950–2100, forced by different combinations of long-lived Greenhouse Gases (GHGs) and Ozone Depleting Substances (ODSs). The experiments are designed to investigate the mech-5 anisms by which GHGs and ODSs affect the evolution of ozone, including changes in the Brewer-Dobson circulation of the stratosphere and cooling of the upper strato-sphere by CO 2 . Separating the effects of GHGs and ODSs on ozone, we find the decrease in upper stratospheric ozone from ODSs up to the year 2000 is approxi-mately 30% larger than the actual decrease in ozone due to the offsetting effects of 10 cooling by increased CO 2 . Over the 21st century, as ODSs decrease, continued cool-ing from CO 2 is projected to account for more than 50% of the projected increase in upper stratospheric ozone. Changes below 20 hPa show a redistribution of ozone from tropical to extra-tropical latitudes with an increase in the Brewer-Dobson circulation, while globally averaged the amount of ozone below 20 hPa decreases over the 21st 15 century. Further analysis by linear regression shows that changes associated with GHGs do not appreciably alter the recovery of stratospheric ozone from the effects of ODSs; over much of the stratosphere ozone recovery follows the decline of halo-gen concentrations within statistical uncertainty, though the lower polar stratosphere of the Southern Hemisphere is an exception with ozone concentrations recovering more 20 slowly than indicated by the halogen concentrations. These results also reveal the de-gree to which climate change, and stratospheric CO 2 cooling in particular, mutes the chemical effects of N 2 O on ozone in the standard future scenario used for the WMO Ozone Assessment. Increases in the residual circulation of the atmosphere and chem-ical effects from CO 2 cooling more than halve the increase in reactive nitrogen in the 25 mid to upper stratosphere that results from the specified increase in N 2 O between 1950 and 2100.