Reply on RC1

Abstract

To estimate equilibrium climate sensitivity, a common approach is to linearly extrapolate temperatures as a function of top of atmosphere energetic imbalance (Effective Climate Sensitivity). In this study, we consider an alternative approach for estimating equilibrium climate sensitivity through Bayesian calibration of a multiple timescale simple climate model. Results suggest potential biases in effective sensitivity estimates in the case of particular models where radiative tendencies imply energetic imbalances which differ between pre-industrial and quadrupled CO2 states. These biases imply the need for reconsideration of some model published values of climate sensitivity, and the presence of radiative imbalances in a number of CMIP5 and CMIP6 models underlines the urgent requirement for operational climate sensitivity experiments on millennial timescales to assess if such biases exist in estimates of climate sensitivity in the wider CMIP ensembles.