70 Climates ofthe past
The third variation is of the time of year when the Earth is closest to
the Sun (the Earth’s perihelion). The time of perihelion moves through
the months of the year with a period of about 23 000 years (see also
Figure 5.19); in the present configuration, the Earth is closest to the Sun in
January.
As the Earth’s orbit changes its relationship to the Sun, although the
total quantity of solar radiation reaching the Earth varies very little, the
distribution of that radiation with latitude and season over the Earth’s
surface changes considerably. The changes are especially large in polar
regions where the variations in summer sunshine, for instance, reach
about ten per cent (Figure 4.5(b)). James Croll, a British scientist, first
pointed out in 1867 that the major ice ages of the past might be linked
with these regular variations in the seasonal distribution of solar radi-
ation reaching the Earth. His ideas were developed in 1920 by Milutin
Milankovitch, a climatologist from Yugoslavia, whose name is usually
linked with the theory. Inspection by eye of the relationship between the
variations of polar summer sunshine and global ice volume shown in
Figure 4.5 suggestsa significant connection. Careful study of the corre-
lation between the two curves confirms this and demonstrates that sixty
per cent of the variance in the climatic record of global ice volume falls
close to the three frequencies of regular variationsin the Earth’s orbit,
thus providing support for the Milankovitch theory.
More careful study of the relationship between the ice ages and the
Earth’s orbital variations shows that the size of the climate changes is
larger than might be expected from forcing by the radiation changes
alone. Other processes that enhance the effect of the radiation changes
(in other words, positive feedback processes) have to be introduced to ex-
plain the climate variations. One such feedback arises from the changes in
carbon dioxide influencing atmospheric temperature through the green-
house effect, illustrated by the strong correlation observed in the climatic
record between average atmospheric temperature and carbon dioxide
concentration (Figure 4.4). Such a correlation does not, of course, prove
the existenceof the greenhouse feedback; in fact part of the correlation
arises because the atmospheric carbon dioxide concentration is itself in-
fluenced, through biological feedbacks (see Chapter 3), by factors related
to the average global temperature. However, as we shall see in Chapter 5,
climates of the past cannot be modelled successfully without taking the
greenhouse feedback into account.^8
An obvious question to ask is when, on the Milankovitch theory, is
the next ice age due? It so happens that we are currently in a period of
relatively small solar radiation variations and the best projections for the
long term are of a longer than normal interglacial period leading to the
beginning of a new ice age perhaps in 50 000 years’ time.^9