Global Warming

The past millionyears 69

Figure 4.4Variations over the last 160 000 years of polar temperature and
atmospheric carbon dioxide concentrations derived from the Vostok ice core
from Antarctica. It is estimated that the variation of global average temperature
is about half that in the polar regions. Also shown is the current carbon dioxide
concentration of about 370 ppm and the likely rise during the twenty-first
century under various projections of its growth.

The most obvious place to look for the cause of regular cycles in
climate is outside the Earth, in the Sun’s radiation. Has this varied in the
past in a cyclic way? So far as is known the output of the Sun itself has not
changed to any large extent over the last million years or so. But because
of variations in the Earth’s orbit, the distribution of solar radiationhas
varied in a more or less regular way during the last millennium.
Three regular variations occur in the orbit of the Earth around the
Sun (Figure 4.5(a)). The Earth’s orbit, although nearly circular, is actually
an ellipse. The eccentricity of the ellipse (which is related to the ratio
between the greatest and the least diameters) varies with a period of
about 100 000 years; that is the slowest of the three variations. The Earth
also spins on its own axis, the axis of spin being tilted with respect
to the axis of the Earth’s orbit, the angle of tilt varying between 21.6◦
and 24.5◦(currently it is 23.5◦) with a period of about 41 000 years.