Modelling and prediction

In order to make predictions of climate change, we have to calculate the effects of all the key processes operating in the climate system. Our knowledge about these processes can be represented in mathematical terms, but the complexity of the system means that the calculation of their effects can only be performed in practice using a computer. The mathematical formulation is therefore implemented in a computer program, which we refer to as a climate model. Various types of climate model are used for climate simulation and prediction. Usually the surface of the earth is divided up using a rectangular grid, and the ocean and atmosphere further sub-divided at different levels to make computation possible (see figure). Best results are obtained with a fine grid and many different levels, but this requires enormous computer power. The limitations of our knowledge and computing resources mean that the results of climate models are always subject to some uncertainty.