text: 3) Chaotic Uncertainty. It is well understood that climate is a chaotic object and climate models reflect that property. The outcome of a particular model run (???simulation??) depends sensitively on the initial conditions; even minute changes can lead to greatly differing outcomes. For example, the five runs of a Japanese MRI model show temperature trends that differ by almost a factor of 10, an order of magnitude. (If more runs had been performed, the spread would have been even greater.) One can show that taking the mean of an ensemble of more than 10 runs leads to an asymptotic value for the trend. However, most modelers face constraints on time and money and are not able to carry out so many runs. For example, of the 22 models in the IPCC compilation of ???20 CEN?? there are 5 single run models, 5 two-run models, and only 7 models with four or more runs.