The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 278

presumably have to encompass short-term variations. It would seem likely that
solar-driven processes are inadequate.
The termination of the last ice age
The last period involving a major climate change was the termination of the last
ice age that began roughly 20 kybp . Figures 4.22 and 9.12 show the behavior of
northern and southern insolation. As it turns out, southern insolation rose from a
minimum around 31 kybp to a maximum at 20 kybp and then proceeded down-
ward to the present low value. Is it possible that the termination of the last ice
age began in the far SH as insolation rose to a peak around 20 kybp ? Stott et al.
(2007) used radiocarbon ( 14 C) dating of ocean sediments to establish the timing of
deep-sea and tropical surface ocean temperature changes during the last glacial
termination and compared this history with the timing of CO 2 changes and
deglacial warming in southern high latitudes during the last glacial termination.
They concluded that the onset of deglacial warming throughout the Southern
Hemisphere occurred long before deglacial warming began in the tropical surface
ocean. Both the rise in CO 2 and the increase in tropical sea surface temperatures
did not begin to change until approximately 1,000 years after Southern Ocean
warming began. In a second paper, Timmermann et al. (2009) carried out model-
ing to show the likely cause of initiation of deglaciation after 20 kybp was the
increase in insolation during austral spring when the southern ice pack was at a
maximum, coupled with the sea ice-albedo feedback as the sea ice went into
retreat. As the CO 2 concentration rose, this added another warming feedback.
This explanation seems to fit the last several deglaciations. However, there were
many increases in SH insolation in the past few hundred years that did not lead
to a deglaciation so this appears to be a necessary condition but not a sucient
one. Furthermore, their models for solar insolation do not agree with Figures
4.16b and 9.12 .
The last few ice ages
Figure 10.2 shows the variation of solar input to 65 N over the past 800,000 years.
This figure shows that solar input to higher latitudes rapidly oscillates due to pre-
cession of the seasons as time progresses. The variability of eccentricity imposes an
envelope on these oscillations with a period of about 100,000 years. Obliquity
during periods of higher eccentricity affects the amplitude of the envelope. Some
periods have higher amplitudes of oscillation. These are earmarked with þ , þþ ,
or þ þþ signs in the figure, depending on the relative amplitudes.
Figure 10.3 compares temperature measured at Antarctica with yearly solar
input at 65 N over the past 800,000 years. In this figure vertical dashed lines are
drawn at temperature peaks in the Antarctic ice core data. Arrows depict trends
between cycles. The shaded areas correspond to the periods of high solar
amplitude from Figure 10.2 . In most cases, periods with high-amplitude solar
oscillations correspond to interglacial periods, whereas periods with low-amplitude
solar oscillations correspond to ice ages.