General Circulation Models

GCMs are complex, three-dimensional computer-based models of the atmospheric circulation. Uncertainties in our understanding of climate processes, the natural variability of the climate, and limitations of the GCMs mean that their results are not definite predictions of future climate.

Until recently, the standard approach has been to perform equilibrium experiments with GCMs. These require two model runs: first,with the "pre- industrial" atmospheric carbon dioxide concentration; second, with carbon dioxide concentrations doubled over pre-industrial levels. The results from the two experiments are then compared to determine the change in climate.

Equilibrium experiments do not take account of time-dependent effects, such as the thermal inertia of the oceans, or the transient nature of greenhouse gas forcing, but are less computationally demanding. Advances in computing power are enabling transient-response experiments to be performed. In these, atmospheric carbon dioxide concentrations increase gradually through the perturbed simulation. These experiments require a fully-coupled atmosphere-ocean model, and results are only gradually becoming available for analysis and scenario construction.

The results from transient experiments can be difficult to interprete. One reason is that it is difficult to separate out the signal (the effects of global warming) from the noise (natural variability). Equilibrium experiments have the advantage that, although they are less 'realistic', the signal of the enhanced greenhouse effect should be much clearer.