The equilibrium global warming (i.e., dTs) produced by 18 different CO2 doubling simulations 1 is summarized in Figure 13.1. The simulation numbers are in order of increasing projection of global warming. Multiple simulations have been performed by five of the seven involved GCMs; these serve as sensitivity studies for a specific model. As an example, simulation numbers 4 and 5 (UKMO), and 8 through 10 (GFDL), respectively, proceed to a finer horizontal resolution. Note that neither model indicates a significant influence of horizontal resolution on the model-predicted global warming. The UKMO GCM produced both the greatest (5.2°C (9.4°F)) and the smallest (1.9°C (3.4°F)) global warming, and this notable variation is the consequence of differences in assumptions about cloud parameters (Mitchell et al., 1989).
The horizontal solution technique used in the seven GCMs is either finite difference (UKMO, GISS, OSU) or spectral (GFDL, NCAR, CCC, BMR). The spectral models are in much better agreement (dTs= 3.5° to 4.0°C (6.3° to 7.2°F)) than the finite difference models (dTs = 1.9° to 5.2°C (3.4° to 9.4°F)). This is probably coincidental. Of the 19 models in Figure 12.2, eight are finite difference and eleven are spectral, and here neither group is