Slide 16: Global circulation models
GCMs enable us to get some picture of future climate responses to given situations. As the IPCC puts it, GCMs "...are the only credible tools currently available for simulating the response of the global climate system to increasing greenhouse gas concentrations". The first GCM was developed in the 1950's by Norman Philips. It was an incredibly simple model, comprising just two atmospheric layers, and two hemispheric sectors in which the model functions were run.
Other early GCMs involved up to five atmospheric layers, over a simple oceanic model. The model would be set with a given CO2 level, and then allowed to run to equilibrium, at which point the CO2 was raised to a new level (such as 600 ppm), and then run to equilibrium again. Needless to say, such simple models were no more than a sketch of the likely climatic response, and contemporary models are considerably more complex. Furthermore, recent models allow transient, or incremental, increases in gas concentrations to be modelled.
They are 3-D, and may comprise thousands of individual cells
IPCC-TGCIA (1999): Guidelines on the Use of Scenario Data for Climate Impact and Adaptation Assessment. Version 1. Prepared by Carter, T.R., Hulme, M. and Lal, M., Intergovernmental Panel on Climate Change, Task Group on Scenarios for Climate Impact Assessment, 69 pp.