Abstract
Abstract. 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.
Highlights
There has been considerable interest in global-mean stratospheric cooling as a fingerprint of ozone depletion, since in the stratosphere, global-mean temperature is radiatively controlled
The traditional approach (e.g. Ramaswamy et al, 2001; Shine et al, 2003) has been to attribute the stratospheric cooling observed in the recent past to a combination of CO2 increases and ozone depletion, the latter being attributed to the increase in the stratospheric abundance of ozone-depleting substances (ODS) up to about 1998
SJ08 performed a theoretical analysis which untangled the ozone-temperature feedback and showed that the attribution of temperature changes to CO2 and ODS changes could be quite different from that due to CO2 and ozone changes. They illustrated their point by analyzing chemistry-climate simulations from the Canadian Middle Atmosphere Model (CMAM), which exhibited distinct linear trends in both temperature and ozone over 1975–1995 and 2010–2040, these two time periods representing the periods of approximately linear ODS buildup and ODS decline, respectively
Summary
There has been considerable interest in global-mean stratospheric cooling as a fingerprint of ozone depletion (see e.g. Chap. 5 of WMO, 2007), since in the stratosphere, global-mean temperature is radiatively controlled. SJ08 found that while about 10% of the ODS-induced upper stratospheric ozone depletion over the period of rapid ODS increases (1975–1995) was masked by CO2-induced cooling, the traditional approach towards temperature attribution is largely valid over this period. The analysis is expanded to account for the nonlinear relation between CO2 abundance and heating rates, which results in a nonlinear response of temperature to CO2 changes over centennial time scales To our knowledge this effect has not previously been taken into account in statistical analysis of middle atmosphere trends (such as analysis using Multiple Linear Regression). The consequences of this nonlinearity for the attribution of upper stratospheric ozone and temperature changes are analysed
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