8.6.2.2 Why Have the Model Estimates Changed Since the TAR?

The current generation of GCMs^[5] covers a range of equilibrium climate sensitivity from 2.1°C to 4.4°C (with a mean value of 3.2°C; see Table 8.2 and Box 10.2), which is quite similar to the TAR. Yet most climate models have undergone substantial developments since the TAR (probably more than between the Second Assessment Report and the TAR) that generally involve improved parametrizations of specific processes such as clouds, boundary layer or convection (see Section 8.2). In some cases, developments have also concerned numerics, dynamical cores or the coupling to new components (ocean, carbon cycle, etc.). Developing new versions of a model to improve the physical basis of parametrizations or the simulation of the current climate is at the heart of modelling group activities. The rationale for these changes is generally based upon a combination of process-level tests against observations or against cloud-resolving or large-eddy simulation models (see Section 8.2), and on the overall quality of the model simulation (see Sections 8.3 and 8.4). These developments can, and do, affect the climate sensitivity of models.

The equilibrium climate sensitivity estimates from the latest model version used by modelling groups have increased (e.g., CCSM3 vs CSM1.0, ECHAM5/MPI-OM vs ECHAM3/LSG, IPSL-CM4 vs IPSL-CM2, MRI-CGCM2.3.2 vs MRI2, UKMO-HadGEM1 vs UKMO-HadCM3), decreased (e.g., CSIRO-MK3.0 vs CSIRO-MK2, GFDL-CM2.0 vs GFDL_R30_c, GISS-EH and GISS-ER vs GISS2, MIROC3.2(hires) and MIROC3.2(medres) vs CCSR/NIES2) or remained roughly unchanged (e.g., CGCM3.1(T47) vs CGCM1, GFDL-CM2.1 vs GFDL_R30_c) compared to the TAR. In some models, changes in climate sensitivity are primarily ascribed to changes in the cloud parametrization or in the representation of cloud-radiative properties (e.g., CCSM3, MRI-CGCM2.3.2, MIROC3.2(medres) and MIROC3.2(hires)). However, in most models the change in climate sensitivity cannot be attributed to a specific change in the model. For instance, Johns et al. (2006) showed that most of the individual changes made during the development of HadGEM1 have a small impact on the climate sensitivity, and that the global effects of the individual changes largely cancel each other. In addition, the parametrization changes can interact nonlinearly with each other so that the sum of change A and change B does not produce the same as the change in A plus B (e.g., Stainforth et al., 2005). Finally, the interaction among the different parametrizations of a model explains why the influence on climate sensitivity of a given change is often model dependent (see Section 8.2). For instance, the introduction of the Lock boundary-layer scheme (Lock et al., 2000) to HadCM3 had a minimal impact on the climate sensitivity, in contrast to the introduction of the scheme to the GFDL atmospheric model (Soden et al., 2004; Johns et al., 2006).