1.Global scale general circulation models (GCM) ~an c~rrentl~ only provide a mugh_guide-1.o. the likely futur.e~co~uLlnsh_chmate m an enhanced greenhouse effeet-war-m€d-W..orld_ Such computer models use basic physical laws to describe changes in momentum, heat and water va_pour as a consequence of atmospheric motion and have evolved from elementary and rather crude constructs to complex and demanding affairs which require some of the most powerful computing platforms in existence. For the purposes of simulating the effects of a hypothetical change in climatic inputs, the atmosphere is divided into a number of vertical levels (up to 19) and spatially organised into a series of horizontal grid points (300-1,000km apart). For each of these layers, and each gnd point, a number of climatic parameters are calculated by running the model forwards in a series of simulated time steps (of the order of 30 minutes) starting from some initial condition. Normally equilibrium results are obtained after a few tens of years and comparisons between the equilibrium climate and a control run can be made.