How do we go about estimating climate change in the future? This is done in a number of stages, shown in this figure. The first thing we need to know is what the emissions will be of greenhouse gases and other gases which affect climate change. These projections are deduced from separate models which take into account population growth, energy use, economics, technological developments, and so forth. We do not carry out this stage at the Hadley Centre, but we take future scenarios of these emissions from others, particularly IPCC.
Having obtained projections of how emissions will change, we then calculate how much remains in the atmosphere, ie what future concentrations will be. For CO2, this is done using a model of the carbon cycle, which simulates the transfer of carbon between sources (emissions) and sinks in the atmosphere, ocean and land (vegetation). For gases such as methane, we use models which simulate chemical reactions in the atmosphere.
Next we have to calculate the heating effect of the increased concentrations of greenhouse gases; this is often called climate forcing. This is done within the climate model, described shortly, which generates spatial patterns of changes in temperature, rainfall and sea level etc., across the surface of the earth and through the depth of the atmosphere and oceans.
Following on from the climate change prediction, the impacts of climate change, on socio-economic sectors such as as water resources, food supply, flooding, are calculated by other research groups. Hadley Centre model predictions are made freely available to the impacts community and well over 100 groups have now used the data.