CMIP3 ensemble spread, model similarity, and climate prediction uncertainty

The CMIP3 multi-model ensemble spread most likely underestimates the real model uncertainty in future climate predictions because of the similarity, and shared defects, of the models in the ensemble. To generate an appropriate level of uncertainty, the spread needs inflating. We derive the mathematical connection between an assumed level of correlation between the model output and the necessary inflation of the spread, and illustrate the connection by making temperature predictions for the UK for the 21st century using four different correlation scenarios.