Variability of the Earth’s orbit: The astronomical theory - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 254

Figure 9.11. Relative peak solar intensity in summer at 65 N over 800,000 years.
varying obliquity and eccentricity. In a sense, this is like an amplitude-modulated
radiowave where a high-power carrier wave is modulated in amplitude by the
signal of interest. As we shall see in Chapter 10, there is some evidence that the
amplitude of peak solar input in Figure 9.10 may be important in contributing to
changes in the Earth's climate whereas the fact that solar input oscillates with a
22,000-year period may not be so important.
Solar input to high northern latitudes is maximized when the Earth is closest
to the Sun in northern summer. However, the seasons of the Earth are reversed in
the two hemispheres. If the Earth is closest to the Sun during northern summer,
this implies that the Earth will be farthest from the Sun in southern summer.
Hence, peak solar intensity in summer at high latitudes in the two hemispheres
will be anti-phased (as shown in Figure 9.12 ).
9.6 CONNECTION BETWEEN SOLAR VARIABILITY AND
GLACIATION-DEGLACIATION CYCLES ACCORDING TO THE
ASTRONOMICAL THEORY
The astronomical theory of ice ages postulates that the distribution variability of
peak solar input in summer to the Earth caused by quasi-periodic variations in the
Earth's orbit about the Sun is a major contributing factor to the sequences of
glaciation and deglaciation that have occurred over the past 3 million years.
However, the mechanism by which this variability affects the climate is obscure. A
prevalent belief is that the magnitude of solar input to higher northern latitudes
during local summer determines the amount of ice that can survive the summer.
According to this concept, ice will spread at high northern latitudes during years
with low summer solar input and retract when summer solar input is high. The
question arises as to which measure to use for solar intensity. As Figure 9.8