Until recently, GCMs required artifical fluxes of heat and moisture to prevent large drifts in the simulation of present climate. Some have questioned the validity of climate change simulated using such models. A new model has been developed at the Hadley Centre which does not use flux adjustments. The new model has been run for seven hundred years and produces negligible drift in surface climate as well as maintaining the main features of the deep ocean circulation. This has been achieved through improvements in surface fluxes and increased resolution in the ocean.
Aspects of model validation of the control climate against observations will be presented. The response to increases in greenhouses gases will be discussed and compared with the reponse in an earlier version of the model without flux adjustments.