Modelling Global Warming
Global average temperature has increased by around 0.6°C during the 20th century. At the same time, greenhouse gas concentrations have increased substantially. To assess whether the two are associated involves the detection of a human "fingerprint" on 20th century climate change. Detection of this man-made influence requires the use of computer model simulations of the likely climatic effects of changing the atmospheric composition, and the comparison of the results with observations. Computers model the climate by expressing the basic physical processes which control weather and climate as a series of mathematical equations. The climate however, is a very complex system, and supercomputers are needed for the task.
The most complex climate models are called general circulation models or GCMs because they model the circulation of the atmosphere. Coupled ocean-atmosphere GCMs also model the climatic influences of the oceans, which store so much energy, and the ocean-atmosphere interactions. GCMs can simulate both global average temperature changes and inter-regional differences. In addition, the climatic influence of both greenhouse gases and aerosols can be incorporated. By taking into account the global cooling potential of man-made aerosols, GCMs have simulated a rise in temperature close to the observed 0.6°C. This provides strong but not conclusive evidence that mankind has contributed towards global warming.