chapter - Atmospheric and Oceanic Science

11.1. Introduction
Global climate models
A large number of climate change experiments using Global Climate Models
(GCMs) have been completed in recent years, both equilibrium and transient experiments,
and both experiments forced with changes in greenhouse gas concentrations
alone and those forced with greenhouse gas and sulphate aerosol changes. A number
of parallel experiments have also been completed using high resolution
Regional Climate Models. Results from a considerable number of these experiments
have been used in climate change impacts and adaptation assessments. It is
not always easy, however, to know which experiment has been used in an impacts
study, nor how the particular modelling results fit into the larger population of
GCMs climate change experiments.
Comprehensive climate models are based on physical laws represented by
mathematical equations that are solved using a three-dimensional grid over the
globe. For climate simulation, the major components of the climate system must be
represented in sub-models (atmosphere, ocean, land, surface, cryosphere and biosphere),
along with the process that go on within and between them. Global climate
models in which the atmosphere and ocean components have been coupled together
are also known as Atmosphere-Ocean General Circulation models (AOGCMs).
Most results to be presented in this report are derived this kind of models.
Climate models have developed over the past few decades as computing
power has increased. During that time, models of the main components, atmosphere,
land, ocean and sea ice have been developed separately and then gradually
integrated. This coupling of the various components is a difficult process. Most
recently, sulphur cycle components have been incorporated to represent the emissions
of sulphur and how they are oxidized to form aerosol particles. Currently in
progress, in a few models, is the coupling of the land carbon cycle and the ocean
carbon cycle (IPCC 2001). The atmospheric chemistry component is currently
modelled outside the main climate model. The ultimate aim is to model as much as
possible of the whole of the Earth's climate system so that all the components can
interact and, thus, the predictions of climate change will continuously take into
account the effect of feedbacks among components. Figure 11.1, obtained from the
Intergovernmental Panel on Climate Change report from 2001 (IPCC 2001), shows
the past, present and possible future evolution of climate models.
In summary, from the description above, one can see that the GCMs can be
very complex, however, can we grasp how do they work? In the next section, some
basic ideas about these models will be addressed.
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