The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 296

Huybers and Wunsch (2005) concluded: ''the Earth tends to a glacial state
(anthropogenic influences aside) and deglaciates near some, but not all, obliquity
maxima.'' This is closely related to the theory suggested in this topic that during
the past 800,000 years (or more) the Earth has tended to be in a glacial state and
terminations occur when a series of major oscillations in insolation in the NH
occur. Such oscillations occur continually with a period of about 22,000 years due
to precession. The amplitude is controlled by obliquity and eccentricity. Huybers
and Wunsch singled out obliquity. If peak solar intensity, rather than integrated
solar intensity, is the key factor, eccentricity should also be included. In the
astronomical theory, the driving force for terminations is insolation. Obliquity is
involved to the extent that variable obliquity affects insolation, but only in a
secondary way.
Schulz and Zeebe (2006) noticed that when peak solar inputs to latitudes 65 N
and 65 S are compared over the past several hundred thousand years there occur
occasional periods of up to about 3,000 years in which irradiance increases with
time at both latitudes. This occurs despite the fact that solar inputs to these
latitudes are overall mostly out of phase, with one increasing while the other
decreases. This can be discerned in Figure 10.14 (see p. 308) at around 22,000 and
137,000 ybp . What appears to happen is that there is a brief period when the 65 N
curve starts to turn upward after bottoming out while the 65 S curve has not quite
reached its apex. Schulz and Zeebe (2006) also noted that periods where NH and
SH irradiances have been simultaneously increasing for at least 1,000 years always
seem to occur near times when ice age terminations occur. Therefore, they
''hypothesized that the glacial termination trigger is the synchronous, prolonged
( 1,000 yr) increase in SH and NH insolation, the insolation canon.'' Further,
they showed that prior to about 800,000 years ago such overlapping periods
where NH and SH irradiances have been simultaneously increasing did not occur
as frequently, and they suggested that this may be tied to the change from the
''41K world'' prior to 800 kybp to the ''100K world'' over the past 800,000 years.
While Schulz and Zeebe (2006) seem to have found a correlation between
simultaneous increase in NH and SH insolation, Figure 10.14 provides no evi-
dence that there was a unique cause-effect relationship between these occurrences
and terminations of ice ages. There is no physical reason to believe that the
''insolation canon'' has any validity at all. It is more instructive to note that both
of these periods occur at the end of a prolonged rise in insolation in the SH. The
fact that insolation in the NH has turned upward seems irrelevant considering that
insolation in the NH was very low during both periods. The insolation canon
seems to be merely a statistical quirk of no significance.
Clark et al. (2009) carried out a rather thorough chronology of the last 50,000
years with particular emphasis on the Last Glacial Maximum around 20,000
years ago. They concluded that the onset of deglaciation is induced by some com-
bination of peak northern insolation in summer, tropical Pacific sea surface
temperatures, and atmospheric CO 2 . However, there is no reason to believe that
tropical Pacific sea surface temperatures and atmospheric CO 2 should change of
their own volition; they seem more likely to be effects than causes. In the end, it