How stable haspast climate been? 71(a)(b)Earth24.5 degrees21.6 degreesSun0100200300Thousands of years agoSummer sunshineIce volume400500600
2.2 2.4(c)Figure 4.5Variations in
the Earth’s orbit (a), in its
eccentricity, the orientation
of its spin axis (between
21.6◦and 24.5◦) and the
longitude of perihelion (i.e.
the time of year when the
Earth is closest to the Sun,
see also Figure 5.19), cause
changes in the average
amount of summer
sunshine (in millions of
joules per square metre per
day) near the poles (b).
These changes appear as
cycles in the climate record
in terms of the volume of
ice in the ice caps (c).How stable has past climate been?
The major climate changes considered so far in this chapter have taken
place relatively slowly. The growth and recession of the large polar
ice-sheets between the ice ages and the intervening warmer interglacial
periods have taken on average many thousands of years. However, the ice
core records such as those in Figures 4.4 and 4.6 show evidence of large
and relatively rapid fluctuations. Ice cores from Greenland provide more
detailed evidence of these than those from Antarctica. This is because at
the summit of the Greenland ice cap, the rate of accumulation of snow