text: Given the inadequacies of current climate models, how should we interpret the multi-model ensemble simulations of the 21 st century climate used in the IPCC assessment reports? This ensemble-of-opportunity is comprised of models with generally similar structures but different parameter choices and calibration histories. McWilliams (2007) and Parker (2010) argue that current climate model ensembles are not designed to sample representational uncertainty in a thorough or strategic way. Stainforth et al. (2007) argue that model inadequacy and an inadequate number of simulations in the ensemble preclude producing meaningful probability density functions (PDFs) from the frequency of model outcomes of future climate. Nevertheless, as summarized by Parker (2010), it is becoming increasingly common for results from individual multi-model and perturbed-physics simulations to be transformed into probabilistic projections of future climate, using Bayesian and other techniques. Parker argues that the reliability of these probabilistic projections is unknown, and in many cases they lack robustness. Knutti et al. (2008) argues that the real challenge lies more in how to interpret the PDFs rather whether they should be constructed in the first place. Stainforth et al. (2007) warns against over interpreting current model results since they could be contradicted by the next generation of models, undermining the credibility of the new generation of model simulations.