Validation of themodel 101Figure 5.19Changes in the Earth’s elliptical orbit from the present
configuration to 9000 years ago and (right hand side) changes in the average
solar radiation during the year over the northern hemisphere.
pressure region develops over north Africa and south Asia because of
the increased land–ocean temperature contrast. The summer monsoons
in these regions are strengthened and there is increased rainfall. These
simulated changes are in qualitative agreement with paleoclimate data;
for example, these data provide evidence for that period (around 9000
years ago) of lakes and vegetation in the southern Sahara about 1000 km
north of the present limits of vegetation.
The accuracy and the coverage of data available for these past pe-
riods are limited. However, the model simulations for 9000 years ago,
described above, and those for other periods in the past have demon-
strated the value of such studies in the validation of climate models.
A third way in which models can be validated is to use them to predict
the effect of large perturbations on the climate. Good progress is being
achieved with the prediction of El Ni ̃no events and the associated climate
anomalies up to a year ahead (see earlier in the chapter). Other short-term
perturbations are due to volcanic eruptions, the effects of which were
mentioned in Chapter 1. Several climate models have been run in which
the amount of incoming solar radiation has been modified to allow for the
effect of the volcanic dust from Mount Pinatubo, which erupted in 1991
(Figure 5.21). Successful simulation of some of the regional anomalies
of climate which followed that eruption, for instance the unusually cold
winters in the Middle East and the mild winters in western Europe, has
also been achieved by the models.^15