Variability of the Earth’s orbit: The astronomical theory - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 271

''In summary, both ice-volume and deep-water temperature contributions to
the [ d 18 O] signal likely changed across the MPT, with an increase in ice volume
accompanied by a decrease in deep-water temperatures. The details of the
trajectory that each component followed, however, remain unclear and await
development of a high-resolution record of deep-water temperature during the
MPT.''
Since the cyclic variability of solar input to high latitudes has not changed
over the past 2.7 million years, it seems evident that the origin of the MPT must
lie in the internal climate system. Clark et al. (2006) provided a lengthy and
detailed discussion of various models that have been proposed to account for the
existence of the MPT, but none of these are convincing to this writer.
9.8 NORTH OR SOUTH?
If solar intensity variability at high latitudes due to orbital changes is the key
factor in producing ice ages and interglacials, should we seek variability in the
north, the south, or both? Figure 9.12 shows that solar intensity in the north and
south are out of phase by about 11,000 years. Therefore, any model that predicts
the accumulation of ice in the NH due to changes in solar intensity in the NH
with a time lag of T years is indistinguishable from a model that predicts the
accumulation of ice in the NH due to changes in solar intensity in the SH with a
time lag of (T þ 11,000) years.
Most studies of glacial-interglacial cycles have assumed a priori that variations
in solar intensity in the NH (rather than the SH) are relevant, since ice sheets form
preferentially in the NH. However, since insolation in the NH and the SH are out
of phase by merely 11,000 years, considerable uncertainty creeps into the proce-
dure. Henderson and Slowey (2000) analyzed sediment cores taken in the
Bahamas and used an improved dating technique based on 234 U and 130 Th. They
concluded that the penultimate deglaciation (end of the previous ice age) took
place over a period of about 5,000 years centered on 135.2 3.5 kybp . Henderson
and Slowey (2000) pointed out that ''this date is 8 kyr before the peak in
northern hemisphere insolation'' and suggested: ''deglaciation is initiated by a
mechanism in the southern hemisphere or tropics.'' Figure 9.12 shows that solar
intensity in the north was near a minimum at 135 kybp , whereas that in the south
was near a maximum. However, if ocean sediments can be used to measure global
ice volume, then time lags must be involved. Nevertheless, the Imbrie model
( Figure 9.14 ) does not show a significant reduction in ice volume until 120 kybp
and there is no sign of a termination as early as 135 kybp in their model. A con-
siderable amount of work has been done on terminations of ice ages since
Henderson and Slowey's study in 2000, and this is documented in Section 10.2.3.
There is evidence that the SH may play a significant role in terminations.
However, it should be noted that the Earth is moving fastest through its perihelion
when the Earth is close to the Sun producing high peak solar intensity, as men-