Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 4 2009 10.5194/acpd-9-14857-2009 http://www.atmos-chem-phys-discuss.net/9/14857/2009/ http://www.atmos-chem-phys-discuss.net/9/14857/2009/acpd-9-14857-2009.html http://www.atmos-chem-phys-discuss.net/9/14857/2009/acpd-9-14857-2009.pdf 14857 14872 2009-07-09 An updated analysis of the attribution of stratospheric ozone and temperature changes to changes in ozone-depleting substances and well-mixed greenhouse gases A. I. Jonsson andreas.jonsson@utoronto.ca V. I. Fomichev T. G. Shepherd Department of Physics, University of Toronto, Toronto, Ontario, Canada ESSE, York University, 4700 Keele St., Toronto, Ontario, Canada This paper presents an analysis of the attribution of past and future changes in stratospheric ozone and temperature to anthropogenic forcings. Recently, Shepherd and Jonsson (2008) argued that such an analysis needs to account for the ozone-temperature feedback, and that the failure to do so could potentially lead to very large errors. This point was illustrated by analyzing chemistry-climate simulations from the Canadian Middle Atmosphere Model (CMAM) and attributing both past and future changes to changes in the abundances of ozone-depleting substances (ODS) and well-mixed greenhouse gases. In the current paper, we have expanded the analysis to account for the nonlinear radiative response to changes in CO₂. It is shown that over centennial time scales the relationship between CO₂ abundance and radiative cooling in the upper stratosphere is significantly nonlinear. Failure to account for this effect in multiple linear regression analysis would lead to misleading results. In our attribution analysis the nonlinearity is taken into account by using CO₂ heating rate, rather than CO₂ abundance, as the explanatory variable. In addition, an error in the way the CO₂ forcing changes are implemented in the CMAM has been corrected, which significantly affects the results for the recent past. As the radiation scheme, based on Fomichev et al. (1998), is used in several other models we provide some description of the problem and how it was fixed. <br><br> The updated results are as follows. From 1975–1995, during the period of rapid ozone decline, ODS and CO₂ increases contributed roughly equally to upper stratospheric cooling, while the CO₂-induced cooling (which increases ozone) masked about 20% of the ODS-induced ozone depletion. From 2010–2040, during the period of most rapid ozone recovery, CO₂-induced cooling will dominate the upper stratospheric temperature trend and will contribute roughly equally with the ODS decline to ozone increases, effectively doubling the rate of ozone recovery. de Grandpré, J., Beagley, S. R., Fomichev, V. I., Griffioen, E., McConnell, J. C., Medvedev, A. S., and Shepherd, T. 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