Climate simulations play an important role in climate science.* These simulations involve mathematical models that are implemented on computers and imitate processes in the climate system. Like the history of numerical weather prediction, the history of climate science is strongly related to the history of the computer. There are two main reasons why simulation is so important in climate science. First, computers removed a barrier in meteorological practice: The speed with which calculations could be done has been enhanced tremendously. We cannot practically do the calculations in climate simulations without the use of computers. Second, simulation is an important ingredient of climate science because real experiments with the climate as a whole are impossible. If we want to ‘experimentally’ manipulate climate, we need to perform such manipulations on a representation of the climate system.