The effect of climate change on ozone depletion through changes in stratospheric water vapour

Several studies have predicted substantial increases in Arctic ozone deplet ion due to the stratospheric cooling induced by increasing atmospheric CO2 concentrations(1,2). But climate change may additionally influence Arctic o zone depletion through changes in the water vapour cycle. Here we investiga te this possibility by combining predictions of tropical tropopause tempera tures from a general circulation model with results from a one-dimensional radiative convective model, recent progress in understanding the stratosphe ric water vapour budget, modelling of heterogeneous reaction rates and the results of a general circulation model on the radiative effect of increased water vapour(3). Whereas most of the stratosphere will cool as greenhouse- gas concentrations increase, the tropical tropopause may become warmer, res ulting in an increase of the mean saturation mixing ratio of water vapour a nd hence an increased transport of water vapour from the troposphere to the stratosphere. Stratospheric water vapour concentration in the polar region s determines both the critical temperature below which heterogeneous reacti ons on cold aerosols become important (the mechanism driving enhanced ozone depletion) and the temperature of the Arctic vortex itself. Our results in dicate that ozone loss in the later winter and spring Arctic vortex depends critically on water vapour variations which are forced by sea surface temp erature changes in the tropics. This potentially important effect has not b een taken into account in previous scenarios of Arctic ozone loss under cli mate change conditions.