Feedback Analyses of Equilibrium Climate Change Simulations

Abstract

Performing equilibrium climate change simulations is a standard method to study the forcing-response relationship within the climate system. An important finding is th different climate models simulate a considerably different global mean response to the same kind of forcing, and previous studies have focussed to explore the respective reason. Radiative feedbacks are essential in controlling the global response and a model's distinctive climate sensitivity. The cloud feedback has been identified to be of particular importance. Respective evidence in equilibrium simulations is valid for transient model simulations as well. Another key issue is to explain the distinctive global mean response of one and the same model to the same amount of forcing induced by different forcings mechanisms. This so-called efficacy characterising a certain forcing can also be related to the specific acting of feedbacks, and not necessarily to the cloud feedback only. Several ways to explore different efficacy of different forcings have been developed, and it is not clear which method is best suited for the purpose. In this talk we will point out some merits and shortcomings of the "partial radiative perturbation method". It has been applied to equilibrium climate change simulations with a coupled chemistry-climate model. The particular focus will be on CO2 increase simulations.