1 2 3 4 5 6 7 8 On the Effective Number of Climate Models

31 Climate models are essential tools for assessing future climate change. In making predictions, it 32 is beneficial to examine simulations from various models that are developed at centers around the 33 world. The simple average over an ensemble of such models is often taken as the optimal 34 prediction, which in studies of current climate is demonstrated as being more accurate than 35 relying on any one individual model realization. However, this is only true to the extent that 36 different models provide statistically independent information. Here, we examine the ability of 37 current-generation models in simulating the observed present-day mean climate and show that 38 similarities in model implementation play an important role in ensemble estimation. We 39 demonstrate that the effective number of models is considerably smaller than the actual number 40 comprising the ensemble. Our results suggest that the common practice of taking simple 41 ensemble averages needs to be reconsidered. 42