Simple climate models
In Chapter 5 a detailed description was given of
general circulation models (GCMs) of the atmo-
sphere and the ocean and of the way in which they
are coupled together (in AOGCMs) to provide sim-
ulations of the current climate and of climate per-
turbed by anthropogenic emissions of greenhouse
gases. These models provide the basis of our pro-
jections of the detail of future climate. However,
because they are so elaborate, they take a great deal
of computer time so that only a few simulations can
be run with these large coupled models.
To carry out more simulations under differ-
ent future emission profiles of greenhouse gases
or of aerosols or to explore the sensitivity of fu-
ture change to different parameters (for instance,
parameters describing the feedbacks in the atmo-
sphere which largely define the climate sensitiv-
ity), extensive use has been made of simple climate
models.^9 These simpler models are ‘tuned’ so as to
agree closely with the results of the more complex
AOGCMs in cases where they can be compared.
The most radical simplification in the simpler mod-
els is to remove one or more of the dimensions so
thatthe quantities of interest are averaged overlat-
itude circles (in two-dimensional models) or over
the whole globe (in one-dimensional models). Such
models can, of course, only simulate latitudinal or
global averages – they can provide no regional in-
formation.
Figure 6.3 illustrates the components of such a
model in which the atmosphere is contained within
a ‘box’ with appropriate radiative inputs and out-
puts. Exchange of heat occurs at the land surface
(another ‘box’) and the ocean surface. Within the
ocean allowance ismade for vertical diffusionand
vertical circulation. Such a model is appropriate
for simulating changes in global average surface
temperature with increasing greenhouse gases or
aerosols. When exchanges of carbon dioxide across
the interfaces between the atmosphere, the land and
the ocean are also included, the model can be em-
ployed to simulate the carbon cycle.Figure 6.3The components of a simple ‘upwelling–diffusion’ climate model.