The last chapter showed that the most effective tool we possess for the prediction of future climate change due to human activities is the climate model. This chapter will describe the predictions of models for likely climate change next century. It will also consider other factors which might lead to climate change and assess their importance relative to the effect of greenhouse gases.

Model projections

A principal reason for the development of climate models is to learn about some of the detail of the likely climate change next century and beyond. Because model simulations into the future depend on assumptions regarding the future anthropogenic emissions of greenhouse gases, which in turn depend on assumptions about many factors involving human behaviour, it has been thought inappropriate and possibly misleading to call the simulations of future climate so far into the future ‘predictions’. They are therefore generally called ‘projections’ to emphasize that what is being done is to explore the likely future climates which will arise from a range of assumptions regarding human activities.

Results which come from the most sophisticated coupled atmosphere-ocean models of the kind described in the last chapter provide fundamental information on which to base climate projections. However, because they are so demanding on computer time only a limited number of results from such models are available. Many studies have also therefore been carried out with simpler models.