100 Modellingthe climate
of evaporation) and albedo (reflectivity to solar radiation). The models
keep track of the changes in soil moisture through evaporation and pre-
cipitation. The albedo depends on soil type, vegetation, snow cover and
surface wetness.Validation of the model
In discussing various aspects of modelling we have already indicated
how some validation of the components of climate models may be carried
out. The successful predictions of weather-forecasting models provide
validation of important aspects of the atmospheric component, as do
thesimulations mentioned earlier in the chapter of the connections be-
tween sea surface temperature anomalies and precipitation patterns in
some parts of the world. Various tests have also been carried out of the
ocean component of climate models; forinstance, through comparisons
between the simulation and observation of the movement of chemical
tracers (see box below).
Once a comprehensive climate model hasbeen formulated it can be
tested in three main ways. Firstly, it can be run for a number of years
of simulated time and the climate generated by the model compared in
detail to the current climate. For the model to be seen as a valid one, the
average distribution and the seasonal variations of appropriate parame-
ters such as surface pressure, temperature and rainfall have to compare
well with observation. In the same way, the variability of the model’s
climate must be similar to the observed variability. Climate models that
are currently employed for climate prediction stand up well to such
comparisons.^14
Secondly, models can be compared against simulations of past cli-
mates when the distribution of key variables was substantially different
than at present; for example, the period around 9000 years ago when
the configuration of the Earth’s orbit around the Sun was different (see
Figure 5.19). The perihelion (minimum Earth–Sun distance) was in July
rather than in January as it is now; also the tilt of the Earth’s axis was
slightly different from its current value (24◦rather than 23.5◦). Result-
ing from these orbital differences (see Chapter 4), there were significant
differences in the distribution of solar radiation throughout the year.
The incoming solar energy when averaged over the northern hemisphere
was about seven per cent greater in July and correspondingly less in
January.
When these altered parameters are incorporated into a model, a dif-
ferent climate results. For instance, northern continents are warmer in
summer and colder in winter. In summer a significantly expanded low