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In the recent past, the evolution of stratospheric ozone (O3) wasaffected by both increasing ozone depleting substances (ODSs) and greenhousegases (GHGs). The impact of the single forcings on O3 is well known.Interactions between the simultaneously increased GHG and ODS concentrations,however, can occur and lead to nonlinear O3 changes. In this study, weinvestigate if nonlinear processes have affected O3 changes between 1960and 2000. This is done with an idealised set of time slice simulations withthe chemistry-climate model EMAC. Due to nonlinearity the past ozoneloss is diminished throughout the stratosphere, with a maximum reduction of1.2 % at 3 hPa. The total ozone column loss between 1960 and 2000 that ismainly attributed to the ODS increase is mitigated in the extra-polar regionsby up to 1.1 % due to nonlinear processes. A separation of the O3changes into the contribution from chemistry and transport shows thatnonlinear interactions occur in both. In the upper stratosphere a reducedefficiency of the ClOx-catalysed O3 loss chiefly causes thenonlinear O3 increase. An enhanced formation of halogen reservoirspecies through the reaction with methane (CH4) reduces the abundance ofhalogen radicals significantly. The temperature-induced deceleration of theO3 loss reaction rate in the Chapman cycle is reduced, which leads to anonlinear O3 decrease and counteracts the increase due to ClOx.Nonlinear effects on the NOx abundance cause hemispheric asymmetricnonlinear changes of the O3 loss. Nonlinear changes in O3 transportoccur in particular in the Southern Hemisphere (SH) during the monthsSeptember to November. Here, the residual circulation is weakened in thelower stratosphere, which goes along with a reduced O3 transport fromthe tropics to high latitudes. Thus, O3 decreases in the SH polarregion but increases in the SH midlatitudes. The existence of nonlinearitiesimplies that future ozone change due to ODS decline slightly depends on theprevailing GHG concentrations. Therefore the future ozone evolution will notsimply be a reversal of the past

Nonlinear response of modelled stratospheric ozone to changes in greenhouse gases and ozone depleting substances in the recent past