Analysis of feedbacks in global climate simulations driven by different radiative forcings

Kurzfassung

If radiatively active tracers other than CO2 contribute to the total climate effect of some emission sector (e.g., road traffic, aviation, etc.), knowledge of the various radiative forcings may be insufficient to rank the relative importance of the components. In this case the efficacy of each component, i.e., its impact on surface temperature per unit radiative forcing, ought to be known as well. Such knowledge can only be created via dedicated climate model simulations for each component forcing. However, in case of significantly different efficacies, it is also important to know the respective physical origin, as otherwise the acceptance of the calculated efficacy parameters for use in assessment studies will remain low. By means of an analysis of global radiative feedbacks, the efficacy differences may be attributed to the responsible feedback processes. In this talk, results from a set of climate model simulations (using setups of the EMAC model either including or neglecting interactive chemistry) will be presented. The radiative forcing is either produced by enhanced CO2 concentration or by enhanced NOx and CO surface emissions. Radiative feedbacks turn out to differ significantly, offering an explanation for differences in the climate sensitivity parameters. The specific role of chemical feedbacks will be addressed for either type of forcing. It will be shown that the radiative forcing has to be sufficiently large in view of statistical detection problems with respect to feedback and response differences. Conceptual inconsistencies between the common ways to determine efficacy parameters and to perform feedback analysis will be pointed out, together with some ideas to improve the analysis framework.