Understanding the CMIP3 multi-model ensemble

The CMIP3 multi-model ensemble has been widely utilised for climate research and prediction, but the properties and behavior of the ensemble are not yet fully understood. Here we present some investigations into various aspects of the ensemble’s behaviour, in particular focussing on the performance of the multi-model mean. We present an explanation of this phenomenon in the context of the statistically indistinguishable paradigm, and also provide a quantitative analysis of the main factors which which control how likely the mean is to out-perform the models in the ensemble, both individually and collectively. Our analyses lend further support to the usage of the paradigm of a statistically indistinguishable ensemble, and indicate that the current ensemble size is too small to adequately sample the space from which the models are drawn.