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  • Williams, K.D., W.J. Ingram and J.M. Gregory, 2008: Time variation of climate sensitivity in GCMs. J. Climate, 10.1175/2008JCLI2371.1. In press.

Effective climate sensitivity is often assumed to be constant, but previous studies of General Circulation Model (GCM) simulations have found it varying as the simulation progresses. This complicates the fitting of simple models to such simulations, as well as having implications for the estimation of climate sensitivity from observations. This study examines the evolution of the feedbacks determining the climate sensitivity in GCMs submitted to the Coupled Model Intercomparison Project. Apparent centennial timescale variations of climate sensitivity during stabilisation to a forcing can be considered an artifact of using conventional forcings, allowing only for instantaneous effects and stratosphere-adjustment. If the forcing is adjusted for processes occurring on timescales which are short compared to the climate stabilisation timescale then there is little centennial timescale evolution of climate sensitivity in any of the GCMs. We suggest that much of the apparent variation in climate sensitivity identified in previous studies is actually due to the comparatively fast forcing adjustment. Persistent differences are found in the strength of the feedbacks between the coupled atmosphere--ocean (AO) versions and their atmosphere-mixed-layer ocean (AML) counterparts, (the latter are often assumed to give the climate sensitivity of the AOGCM). This indicates that the response of the dynamic ocean influences the atmospheric feedbacks in some GCMs and that the AML model may not provide a good estimate of the equilibrium climate sensitivity for the parallel AO version. The adjustment to the forcing to account for comparatively fast processes varies in magnitude and sign between GCMs, as well as differing between AO and AML versions of the same model. There is evidence from one AOGCM that the forcing adjustment may take a couple of decades, with implications for observationally based estimates of equilibrium climate sensitivity. We suggest that at least some of the spread in 21st century global temperature predictions between GCMs is due to differing adjustment processes, hence work to understand these differences should be a priority.

Last Updated: 2008-08-22

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