ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-10-9647-2010 Contributions to stratospheric ozone changes from ozone depleting substances and greenhouse gases Plummer D. A. 1 Scinocca J. F. 1 Shepherd T. G. 2 Reader M. C. 1 Jonsson A. I. 2 Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, B. C., Canada Department of Physics, University of Toronto, Toronto, Canada 15 04 2010 10 4 9647 9694 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/10/9647/2010/acpd-10-9647-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/10/9647/2010/acpd-10-9647-2010.pdf

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 mechanisms 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 stratosphere by CO<sub>2</sub>. 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 approximately 30% larger than the actual decrease in ozone due to the offsetting effects of cooling by increased CO<sub>2</sub>. Over the 21st century, as ODSs decrease, continued cooling from CO<sub>2</sub> 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 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 halogen concentrations within statistical uncertainty, though the lower polar stratosphere of the Southern Hemisphere is an exception with ozone concentrations recovering more slowly than indicated by the halogen concentrations. These results also reveal the degree to which climate change, and stratospheric CO<sub>2</sub> cooling in particular, mutes the chemical effects of N<sub>2</sub>O on ozone in the standard future scenario used for the WMO Ozone Assessment. Increases in the residual circulation of the atmosphere and chemical effects from CO<sub>2</sub> cooling more than halve the increase in reactive nitrogen in the mid to upper stratosphere that results from the specified increase in N<sub>2</sub>O between 1950 and 2100.

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