Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 4 2007 10.5194/acpd-7-12327-2007 http://www.atmos-chem-phys-discuss.net/7/12327/2007/ http://www.atmos-chem-phys-discuss.net/7/12327/2007/acpd-7-12327-2007.html http://www.atmos-chem-phys-discuss.net/7/12327/2007/acpd-7-12327-2007.pdf 12327 12347 2007-08-21 On the attribution of stratospheric ozone and temperature changes to changes in ozone-depleting substances and well-mixed greenhouse gases T. G. Shepherd tgs@atmosp.physics.utoronto.ca A. I. Jonsson Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 1A7, Canada The vertical profile of global-mean stratospheric temperature changes has traditionally represented an important diagnostic for the attribution of the cooling effects of stratospheric ozone depletion and CO₂ increases. However, CO₂-induced cooling alters ozone abundance by perturbing ozone chemistry, thereby coupling the stratospheric ozone-temperature response to changes in CO₂ and ozone-depleting substances (ODSs). Here we untangle the ozone-temperature coupling and show that the attribution of global-mean stratospheric temperature changes to CO₂ and ODS changes (which are the true anthropogenic forcing agents) can be quite different from the traditional attribution to CO₂ and ozone changes. The significance of these effects is quantified empirically using simulations from a three-dimensional chemistry-climate model. The results confirm the essential validity of the traditional approach in attributing changes during the past period of rapid ODS increases, although we find that about 10% of the upper stratospheric ozone decrease from ODS increases over the period 1975–1995 was offset by the increase in CO₂, and the CO₂-induced cooling in the upper stratosphere has been somewhat overestimated. When considering ozone recovery, however, the ozone-temperature coupling is a first-order effect; fully 2/5 of the upper stratospheric ozone increase projected to occur from 2010–2040 is attributable to CO₂ increases. Thus, it has now become necessary to base attribution of global-mean stratospheric temperature changes on CO₂ and ODS changes rather than on CO₂ and ozone changes. de Grandpré, J., Beagley, S. R., Fomichev, V. I., Griffioen, E., McConnell, J. C., Medvedev, A. S. and Shepherd, T. G.: Ozone climatology using interactive chemistry: Results from the Canadian Middle Atmosphere Model, J. Geophys. Res., 105, 26 475–26 491, 2000. Eyring, V., Kinnison, D. E., and Shepherd, T. G.: Overview of planned coupled chemistry-climate simulations to support upcoming ozone and climate assessments, SPARC Newsletter, 25, 11–17, 2005. Eyring, V., Waugh, D. W., Bodeker, G. E., et al.: Multi-model projections of stratospheric ozone in the 21st century, J. Geophys. Res., 112, in press, doi:10.1029/2006JD008332, 2007. Fomichev, V. I., Ward, W. E., Beagley, S. R., McLandress, C., McConnell, J. C., McFarlane, N. A., and Shepherd, T. G.: The extended Canadian Middle Atmosphere Model: Zonal-mean climatology and physical parameterisations, J. Geophys. Res., 107(D10), 4087, doi:10.1029/2001JD000479, 2002. Haigh, J. D. and Pyle, J. A.: Ozone perturbation in a two-dimensional circulation model, Q. J. R. Meteorol. Soc., 108, 551–574, 1982. IPCC/TEAP (Inter-Governmental Panel on Climate Change/Technology and Economic Assessment Panel), 2005: IPCC/TEAP Special Report on Safeguarding the Ozone Layer and the Global Climate System: Issues Related to Hydrofluorocarbons and Perfluorocarbons, 488 pp., Cambridge University Press. Jonsson, A. I., de Grandpré, J., Fomichev, V. I., McConnell, J. C., and Beagley, S. R.: Doubled CO₂-induced cooling in the middle atmosphere: Photochemical analysis of the ozone radiative feedback, J. Geophys. Res., 109, D24103, doi:10.1029/2004JD005093, 2004. Ramaswamy, V., Chanin, M.-L., Angell, J. , et al.: Stratospheric temperature trends: Observations and model simulations, Rev. Geophys., 39, 71–122, 2001. Shine, K. P., Bourqui, M. S., Forster, P. M., et al.: A comparison of model-simulated trends in stratospheric temperatures, Quart. J. Roy. Meteorol. Soc., 129, 1565–1588, 2003. WMO (World Meteorological Organization): Scientific Assessment of Ozone Depletion: 2006, Global Ozone Research and Monitoring Project–Report No. 50, 572 pp., Geneva, Switzerland, 2007.