Slide 27: GCM model responses
All GCMs are tested to ensure that they correctly model previous palaeoclimatological conditions to the present day.
However, although they often agree on general trends for a given scenario, they may predict moderately different responses over time.
Consequently, climate scientists tend to use several different models and scenarios for any given set of predictions or plans.
The IPCC TAR (Third Assessment Report) uses an average of as many as 20 model predictions when stipulating future climate trends, although as yet not all models have produced runs for all of the SRES future trend scenarios.