text: In the absence of model verification or falsification, Stainforth et al. (2007) describes the challenges of building confidence in climate models. Owing to the long time scales present, particularly in the ocean component, there is no possibility of atrue cycle of model improvement and confirmation, particularly since the life cycle of an individual model version (order of a few years) is substantially less than the simulation period (order of centuries). Model projections of future climate states relate to a state of the system that has not been previously experienced. Hence it is impossible to calibrate the model for the future climate state or confirm the usefulness of the forecast.