A new fast stratospheric ozone chemistry scheme in an intermediate general-circulation model. II: Application to effects of future increases in greenhouse gases

While chlorine loadings in the atmosphere are expected to decrease towards pre-ozone-hole levels by 20601 the greenhouse gases (GHGs) burden is rising continuously and may in turn significantly modify the stratospheric ozone layer. In this study, use is made of the new interactively coupled chemistry-general-circulation model IGCMFASTOC to investigate in detail the response of stratospheric ozone to a specified increase in GHG concentrations between 1979 and 2060. The Intermediate General Circulation Model (IGCM) is a relatively fast general-circulation model with parametrizations of an intermediate level of complexity, and the new chemistry scheme Fast STratospheric Ozone Chemistry (FASTOC) is an efficient input-output model composed of pre-computed nonlinear functions. A total of seven 14-year time-slice simulations are performed to allow for the separation of individual processes and the analysis of sensitivity to methane and stratospheric water-vapour scenarios. In a basic scenario where all GHG concentrations increase (except stratospheric H2O) the total ozone column is found to increase at a global rate of 0.27 DU decade(-1) with maxima in late winter-spring poles. This increase is merely the result of two opposing effects: (i) An increase of ozone concentration in the summer and tropical stratospheric region near 30-40 km which is passively advected into the polar-night vortex between 40 and 20 kin. This ozone increase is due to the slowing down of the NO, catalytic cycle caused by the cooling of the stratosphere resulting from increased GHG concentrations in the stratosphere. (ii) A decrease of ozone concentration in the same region which is also advected into the polar-night vortex, due to enhanced NOx concentrations caused by the increased N2O concentrations. These two opposing effects are additive to a large degree, i.e. the sum of their individual effects is a good approximation of their total effect when the processes are acting together. A significant deviation from this additivity occurs inside the northern polar-night vortex and to a lesser extent in the southern polar-night vortex. Our analysis suggests that methane is mainly responsible for this feature. The impact of methane increase on ozone is confined to the high latitudes and is found to have a negative sign in the northern polar vortex when the stratospheric cooling is taken into account, and a positive sign when no stratospheric cooling is taken into account. The scenario where methane is kept at its 1979 level produces a global rate of increase of the ozone column of 9% above the basic scenario. The scenario where stratospheric H2O is increased by 50% leads to a global rate of increase of the ozone column of 70% above the basic scenario. Finally, in a scenario where CO2 is reduced to its 1979 level and the other GHGs involved in the ozone chemistry are increased as in the basic scenario, the global effect of GHGs on the ozone layer is to decrease it at a rate of 0.37 DU decade(-1).