The computer climate models used for the majority of the work at the Hadley Centre are detailed three-dimensional representations of major components of the climate system. They are mostly run on the Met Office's NEC SX-6 supercomputers. We use these models in various different configurations, which are explained below.

Simple models of the climate system have been developed and used both to gain physical insight into major features of the behaviour of the climate system, and to produce climate projections for a range of assumptions about emissions of carbon dioxide and other greenhouse gases. Such models are described, for instance, in An introduction to simple climate models used in the IPCC Second Assessment Report, IPCC Technical Paper II, published in 1997 by Working Group I of the Intergovernmental Panel on Climate Change.

(AGCMs) consist of a three-dimensional representation of the atmosphere coupled to the land surface and cryosphere. An AGCM is similar to a model used for numerical weather prediction (weather forecasting), but because it has to produce projections for decades or centuries rather than days it uses a coarser level of detail. The AGCM has to be provided with data for sea-surface temperatures and sea-ice coverage. Hence an AGCM by itself cannot be used for climate prediction, because it cannot indicate how conditions over the ocean will change. AGCMs are useful for studying atmospheric processes, the variability of climate and its response to changes in sea-surface temperature.

This type of model predicts changes in sea-surface temperatures and sea ice by treating the ocean as though it were a layer of water of constant depth (typically 50 metres), heat transports within the ocean being specified and remaining constant while climate changes. This kind of model is useful for simulating what the climate would be like for some fixed level of carbon dioxide, but it cannot be used for predicting the rate of change of climate because this is largely determined by processes in the ocean interior.

An OGCM is the ocean counterpart of an AGCM; it is a three-dimensional representation of the ocean and sea ice. OGCMs are useful by themselves for studying ocean circulation, interior processes and variability, but they depend on being supplied with data about surface air temperature and other atmospheric properties. Further information is available about: ocean model development, sea-ice model development, eddy-permitting global ocean model.

The terrestrial carbon cycle is modelled within the land surface scheme of the AGCM, and the marine carbon cycle within the OGCM. The carbon cycle model is needed in order to capture several important climate feedbacks on carbon dioxide concentration, for instance fertilisation of plant growth by carbon dioxide and uptake or outgassing of carbon dioxide by the oceans. Further information is available about ocean carbon cycle modelling.

The Hadley Centre has developed a three-dimensional global atmospheric chemistry model called STOCHEM. The chemical scheme is designed to include the main agents responsible for the production and destruction of ozone and methane in the lower atmosphere.

These are the most complex models in use, consisting of an AGCM coupled to an OGCM. Some recent models include the biosphere, carbon cycle and atmospheric chemistry as well. AOGCMs can be used for the prediction and rate of change of future climate. They are also used to study the variability and physical processes of the coupled climate system. Global climate models typically have a resolution of a few hundred kilometres. Climate projections from the Hadley Centre make use of the HadCM2 AOGCM, developed in 1994, and its successor HadCM3 AOGCM, developed in 1998. Greenhouse-gas experiments with AOGCMs have usually been driven by specifying atmospheric concentrations of the gases, but if a carbon cycle model is included, the AOGCM can predict changes in carbon dioxide concentration, given the emissions of carbon dioxide into the atmosphere. At the Hadley Centre, this was first done in 1999. Similarly, an AOGCM coupled to an atmospheric chemistry model is able to predict the changes in concentration of other atmospheric constituents in response to climate change and to the changing emissions of various gases. Further information is available on: some aspects of ocean simulation in HadCM3 (thermohaline circulation, ventilation, vertical mixing), decadal variability in the ocean of HadCM3.

Recently a global coupled climate model with an eddy-permitting ocean resolution has been developed at the Hadley Centre, in order to better represent important oceanic processes. Further information is available on the HadCEM AOGCM.

Local climate change is influenced greatly by local features such as mountains, which are not well represented in global models because of their coarse resolution. Models of higher resolution cannot practically be used for global simulation of long periods of time. To overcome this, regional climate models, with a higher resolution (typically 50 km) are constructed for limited areas and run for shorter periods (20 years or so). RCMs take their input at their boundaries and for sea-surface conditions from the global AOGCMs. The Hadley Centre has run RCMs for three regions, Europe, the Indian subcontinent and southern Africa and has developed an RCM to run on PCs for any region as part of a regional climate modelling system, PRECIS.