Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 9 21 2009 10.5194/acp-9-8447-2009 http://www.atmos-chem-phys.net/9/8447/2009/ http://www.atmos-chem-phys.net/9/8447/2009/acp-9-8447-2009.html http://www.atmos-chem-phys.net/9/8447/2009/acp-9-8447-2009.pdf 8447 8452 2009-11-05 The effect of nonlinearity in CO₂ heating rates on the attribution of stratospheric ozone and temperature changes 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, Toronto, Ontario, Canada An analysis of the attribution of past and future changes in stratospheric ozone and temperature to anthropogenic forcings is presented. The analysis is an extension of the study of Shepherd and Jonsson (2008) who analyzed chemistry-climate simulations from the Canadian Middle Atmosphere Model (CMAM) and attributed both past and future changes to changes in the external forcings, i.e. the abundances of ozone-depleting substances (ODS) and well-mixed greenhouse gases. The current study is based on a new CMAM dataset and includes two important changes. First, we account for the nonlinear radiative response to changes in CO₂. It is shown that over centennial time scales the radiative response in the upper stratosphere to CO₂ changes is significantly nonlinear and that failure to account for this effect leads to a significant error in the attribution. To our knowledge this nonlinearity has not been considered before in attribution analysis, including multiple linear regression studies. For the regression analysis presented here the nonlinearity was taken into account by using CO₂ heating rate, rather than CO₂ abundance, as the explanatory variable. This approach yields considerable corrections to the results of the previous study and can be recommended to other researchers. Second, an error in the way the CO₂ forcing changes are implemented in the CMAM was 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. 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, 26475–26491, 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., Butchart, N., Waugh, D. W., et al.: Assessment of temperature, trace species and ozone in chemistry-climate model simulations of the recent past, J. Geophys. Res., 111, D22308, doi:10.1029/2006JD007327, 2006. Fomichev, V. I., Blanchet, J. -P., and Turner, D. S.: Matrix parameterization of the 15 μm CO₂ band cooling in the middle and upper atmosphere for variable CO₂ concentration, J. Geophys. Res., 103, 11505–11528, 1998. Forster, P. M. and Joshi, M.: The role of halocarbons in the climate change of the troposphere and stratosphere, Climatic Change, 71, 249–266, 2005. Fomichev, V. I., Fu, C., de Grandpré, J., Beagley, S. R., Ogibalov, V. P., and McConnell, J. C.: Model thermal response to minor radiative energy sources and sinks in the middle atmosphere, J. Geophys. Res., 109, D19107, doi:10.1029/2004JD004892, 2004. Fomichev, V. I., Jonsson, A. I., de Grandpré, J., Beagley, S. R., McLandress, C., Semeniuk, K., and Shepherd, T. G.: Response of the middle atmosphere to CO₂ doubling: Results from the Canadian Middle Atmosphere Model, J. Climate, 20, 1121–1144, doi:10.1175/JCLI4030.1, 2007. Ramaswamy, V., Chanin, M. -L., Angell, J., et al.: Stratospheric temperature trends: Observations and model simulations, Rev. Geophys., 39, 71–122, 2001. Randel, W. J., Shine, K. P., Austin, J., et al.: An update of observed stratospheric temperature trends, J. Geophys. Res., 114, D02107, doi:10.1029/2008JD010421, 2009. Scinocca, J. F., McFarlane, N. A., Lazare, M., Li, J., and Plummer, D.: Technical Note: The CCCma third generation AGCM and its extension into the middle atmosphere, Atmos. Chem. Phys., 8, 7055–7074, 2008. Shepherd, T. G. and Jonsson, A. I.: On the attribution of stratospheric ozone and temperature changes to changes in ozone-depleting substances and well-mixed greenhouse gases, Atmos. Chem. Phys., 8, 1435–1444, 2008. Shine, K. P., Bourqui, M. S., Forster, P. M., et al.: A comparison of model-simulated trends in stratospheric temperatures, Q. J. Roy. Meteorol. Soc., 129, 1565–1588, 2003. Shine, K. P., Barnett, J. J., and Randel, W. J.: Temperature trends derived from Stratospheric Sounding Unit radiances: The effect of increasing CO₂ on the weighting function, Geophys. Res. Lett., 35, L02710, doi:10.1029/2007GL032218, 2008. Waugh, D. W. and Eyring, V.: Quantitative performance metrics for stratospheric-resolving chemistry-climate models, Atmos. Chem. Phys., 8, 5699–5713, 2008. WMO (World Meteorological Organization): Scientific Assessment of Ozone Depletion: 2006, Global Ozone Research and Monitoring Project-Report, No 50, Geneva, Switzerland, 572~pp., 2007.