A large number of perturbed-physics simulations of the HadAM3 atmospheric model were compared with the CERES (Clouds and Earth's Radiant Energy System) estimates of Outgoing Longwave Radiation (OLR) and Reflected Shortwave Radiation (RSR) as well as OLR and RSR from the earlier ERBE (Earth Radiation Budget Experiment) estimates. The model configurations were produced from several independent optimisation experiments in which four parameters were adjusted. Model-observation uncertainty was estimated by combining uncertainty arising from: satellite measurements, observational radiation imbalance, total solar irradiance, radiative forcing, natural aerosol, internal climate variability, Sea Surface Temperature and that arising from untuned parameters. Using an emulator built from 14,000 "slab" model evaluations carried out using the climateprediction.net ensemble the climate sensitivity for each configuration was estimated. Combining different prior probabilities for model configurations with the likelihood for each configuration, and taking account of uncertainty in the emulated climate sensitivity gives, for the HadAM3 model, a 2.5-97.5% range for climate sensitivity of 2.7-4.2 K if the CERES observations are correct. If the ERBE observations are correct then they suggest a larger range, for HadAM3, of 2.8-5.6 K. Amplifying the CERES observational covariance estimate by a factor of 20 brings CERES and ERBE estimates into agreement. In this case the climate sensitivity range is 2.7-5.4 K. Our results rule out, at the 2.5 % level, for HadAM3 and several different prior assumptions climate sensitivies greater than 5.6 K.
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