8.6.2 Interpreting the Range of Climate Sensitivity Estimates Among General Circulation Models

8.6.2.1 Definition of Climate Sensitivity

As defined in previous assessments (Cubasch et al., 2001) and in the Glossary, the global annual mean surface air temperature change experienced by the climate system after it has attained a new equilibrium in response to a doubling of atmospheric CO₂ concentration is referred to as the ‘equilibrium climate sensitivity’ (unit is °C), and is often simply termed the ‘climate sensitivity’. It has long been estimated from numerical experiments in which an AGCM is coupled to a simple non-dynamic model of the upper ocean with prescribed ocean heat transports (usually referred to as ‘mixed-layer’ or ‘slab’ ocean models) and the atmospheric CO₂ concentration is doubled. In AOGCMs and non-steady-state (or transient) simulations, the ‘transient climate response’ (TCR; Cubasch et al., 2001) is defined as the global annual mean surface air temperature change (with respect to a ‘control’ run) averaged over a 20-year period centred at the time of CO₂ doubling in a 1% yr^–1 compound CO₂ increase scenario. That response depends both on the sensitivity and on the ocean heat uptake. An estimate of the equilibrium climate sensitivity in transient climate change integrations is obtained from the ‘effective climate sensitivity’ (Murphy, 1995). It corresponds to the global temperature response that would occur if the AOGCM was run to equilibrium with feedback strengths held fixed at the values diagnosed at some point of the transient climate evolution. It is computed from the oceanic heat storage, the radiative forcing and the surface temperature change (Cubasch et al., 2001; Gregory et al., 2002).

The climate sensitivity depends on the type of forcing agents applied to the climate system and on their geographical and vertical distributions (Allen and Ingram, 2002; Sausen et al., 2002; Joshi et al., 2003). As it is influenced by the nature and the magnitude of the feedbacks at work in the climate response, it also depends on the mean climate state (Boer and Yu, 2003). Some differences in climate sensitivity will also result simply from differences in the particular radiative forcing calculated by different radiation codes (see Sections 10.2.1 and 8.6.2.3). The global annual mean surface temperature change thus presents limitations regarding the description and the understanding of the climate response to an external forcing. Indeed, the regional temperature response to a uniform forcing (and even more to a vertically or geographically distributed forcing) is highly inhomogeneous. In addition, climate sensitivity only considers the surface mean temperature and gives no indication of the occurrence of abrupt changes or extreme events. Despite its limitations, however, the climate sensitivity remains a useful concept because many aspects of a climate model scale well with global average temperature (although not necessarily across models), because the global mean temperature of the Earth is fairly well measured, and because it provides a simple way to quantify and compare the climate response simulated by different models to a specified perturbation. By focusing on the global scale, climate sensitivity can also help separate the climate response from regional variability.