The climate conditions that we experience are the result of complex interactions between processes occurring in the atmosphere and in the oceans. These processes operate at global and local scales and are influenced by other factors, including the land surface, polar ice sheets and the sun. This is why different parts of the world experience different climates. Global Climate Models (GCMs) are computer models that attempt to capture and simulate all these processes, based on our current knowledge.
Global Climate Models are run on supercomputers at a number of centres around the world, including the Max Planck Institute in Germany, the UK Met Office Hadley Centre, and the National Oceanic and Atmospheric Administration in the USA. The models use physical laws and mathematical equations that reflect our understanding of atmospheric and oceanic processes.
The nature of planning decisions that are made over the medium term is different from those that are made in the short term. So what is required from climate projections is different from what is needed from shorter-term weather predictions. The resolution provided by GCMs is useful to inform medium- to long-term planning decisions. The poor availability of historical weather observations in some parts of Africa for example limits understanding of how reliable these models are.