The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 281

One violation of this correlation is the sharp rise in temperature that occurred
about 400,000 ybp (Transition 9 in Figure 10.3 ), when solar oscillations were
minimal. Another exception occurred in the sharp rise in temperature at around
800,000 ybp when solar oscillations were moderate. The problem at 400,000 ybp
has been widely discussed in the literature and is referred to as the ''Stage 11
problem'' (based on SPECMAP Stages, see Figure 5.2 ). M&M discussed the
''Stage 11 problem''. They concluded that there is no good solution to this
problem in the astronomical theory. They noted that Raymo (1997) attempted to
account for this problem by developing a complex criterion by which even small
changes in solar input could trigger a termination if enough ice had accumulated,
but that approach has serious problems (see Section 6.4.4 of M&M). Two other
possible approaches to deal with the Stage 11 problem were discussed by M&M.
One approach was to postulate that the same resonant system that drives the
100,000-year cycle also acts as a flywheel to keep the cycle oscillating when the
driving force is small. ''Thus, you don't have to push on a swing every cycle to
keep it high.'' Berger (1999) did this with his resonance memory model. However,
this model seems very artificially contrived to this writer. M&M claim that the
orbital inclination theory ''solves the Stage-11 problem immediately, since no
such minimum in dust accretion occurs at Stage-11 '' However, the detailed

mechanisms involved in this theory are obscure to this writer.
One possible interpretation of Figure 10.3 is that the natural state of the
Earth's climate in the past 800,000 years may have been glacial. During those
periods, of the order of perhaps 50,000 to 60,000 years, when oscillations in
solar input are minimal, solar input never gets high enough to melt summer ice,
climate cools, and ice sheets build up. However, during those periods when oscilla-
tions in solar input are large, solar input gets high enough during the up-lobes of
oscillations to melt summer ice, ice sheets diminish, and climate warms. Even
though there are also steep downward oscillations in solar input, the down-lobes
are insucient to rebuild ice sheets lost in the previous upward oscillation,
probably due to albedo effects and the likelihood that ice sheets build slowly and
disintegrate more rapidly.
Thus, we find that solar input to higher latitudes oscillates relentlessly with a
22,000-year period due to precession of the equinoxes. These oscillations act like a
radio carrier signal. It is the amplitude of the oscillations—not the frequency—
that seems to be of importance. As in AM radio, the signal is amplitude-
modulated due to changes in eccentricity and obliquity. When the amplitude of
oscillations is high, there is a tendency toward reducing global ice and heading
into an interglacial period. When the amplitude is small, ice volume tends to
increase and the ice age deepens. According to this interpretation, the Earth natur-
ally tends toward an ice age (at least over the last few hundreds of thousand
years). Ice sheets build more slowly than they disintegrate. During periods of
small oscillations, solar input does not reach high enough levels to impede this
natural growth of ice sheets. During periods of high amplitude of oscillations,
solar input reaches high enough levels on the up-lobes to reverse ice sheet growth,
and rebuilding of ice sheets in the down-lobes does not occur fast enough to stop

...