ABSTRACT
In order to project the impact of human perturbations on the climate system, it is necessary to calculate the effects of all the key processes operating in the climate system. These processes can be represented in mathematical terms, but the complexity of the system means that the calculations can be performed in practice only on a computer. The mathematical formulation is therefore implemented in a computer program, which is referred to as a "model". If the model includes enough of the components of the climate system to be useful for simulating the climate (at a minimum, the atmosphere and ocean), it is commonly called a "climate model". A climate model that explicitly included all our current understanding of the climate system would be too complex to run on any existing computer. Instead, simplifications are made to a varying extent. The more that nature is simplified in constructing a model, the faster the model can be run or the less powerful the computer that is needed because fewer calculations are performed. In this chapter the major types of models and the simplifications used to create them are outlined for each of the major steps involved in simulating the climate and sea level response to anthropogenic emissions of GHGs and aerosols. The strengths and weaknesses of the major types of models are also outlined in this chapter. The discussion found in this chapter draws heavily upon Harvey et al. (1997).
