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PublishedJournal ArticleWe investigate the performance of the newest generation multi-model ensemble (MME) from the Coupled Model Intercomparison Project (CMIP5). We compare the ensemble to the previous generation models (CMIP3) as well as several single model ensembles (SMEs), which are constructed by varying components of single models. These SMEs range from ensembles where parameter uncertainties are sampled (perturbed physics ensembles) through to an ensemble where a number of the physical schemes are switched (multi-physics ensemble). We focus on assessing reliability against present-day climatology with rank histograms, but also investigate the effective degrees of freedom (EDoF) of the fields of variables which makes the statistical test of reliability more rigorous, and consider the distances between the observation and ensemble members. We find that the features of the CMIP5 rank histograms, of general reliability on broad scales, are consistent with those of CMIP3, suggesting a similar level of performance for present-day climatology. The spread of MMEs tends towards being "over-dispersed" rather than "under-dispersed". In general, the SMEs examined tend towards insufficient dispersion and the rank histogram analysis identifies them as being statistically distinguishable from many of the observations. The EDoFs of the MMEs are generally greater than those of SMEs, suggesting that structural changes lead to a characteristically richer range of model behaviours than is obtained with parametric/physical-scheme-switching ensembles. For distance measures, the observations and models ensemble members are similarly spaced from each other for MMEs, whereas for the SMEs, the observations are generally well outside the ensemble. We suggest that multi-model ensembles should represent an important component of uncertainty analysis. © 2013 The Author(s).We  acknowledge  the  World  Climate  Research Programme’s   Working   Group   on   Coupled   Modelling,   which   is responsible  for  CMIP. For  CMIP the  US Department  of Energy’s Pro-\udgram  for  Climate  Model  Diagnosis  and  Intercomparison  provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. M.C. was partially supported by funding from NERC grants NE/I006524/1  and  NE/I022841/1.  MW  is  supported  by  the  Joint DECC/Defra    Met    Office    Hadley    Centre    Climate    Programme (GA01101). T.Y., J.D.A, H.S., S.E., M.Y., J.C.H. were supported by\udthe Global Environment Research Fund of the Ministry of the Environment  of  Japan  (S-10,  Integrated  Climate  Assessment  –  Risks,Uncertainties    and    Society,    ICA-RUS)

Reliability and importance of structural diversity of climate model ensembles