Ice core data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 145

data that presumably represent worldwide conditions over several million years.
Of particular interest is a comparison of Greenland and Antarctic ice core data
over the past 150,000 years, which encompasses the last ice age and the previous
interglacial period. A comparison of NH and SH climate synchrony over the past
50,000 years was reported by Blunier et al. (1998). Of critical importance in such
studies is the absolute accuracy of time scales, particularly when they are based on
comparing the composition of entrapped gases. That is because of the relatively
large time lag of the entrapped air relative to the ice in which it is stored in
Antarctic cores. Despite the great diculties involved, Blunier et al. (1998)
claimed that they established absolute chronological scales that allow comparison
of the two sites with an accuracy of about 100 years, depending on the age:
''Because of the rapid mixing time of the atmosphere ( 1 year between
hemispheres), large-scale changes in the concentration of long-lived atmospheric
gases are essentially globally synchronous. This synchronicity provides a tool for
correlating ice core chronologies and thereby comparing the timing of climate
and other environmental change, recorded by various proxies in the ice, between
the hemispheres. The correlation is complicated by the fact that air is trapped in
bubbles 50 to 100m below the surface, creating an age offset between the trapped
air and the surrounding ice. This age offset ( D -age) must be corrected for when
comparing the timing of climate events recorded in the ice by stable isotopes or
other proxies.''
The value of D age for Greenland sites was estimated to be around 800 years
whereas for Antarctica it ranged from 6,100 years in recent times to 6,300 years at
earlier times. Their result is shown in Figure 4.15 . This work was extended to a
90,000-year period in a later publication as shown in Figure 4.16 . The CH 4 data
in Figure 4.16 are very similar for Antarctica and Greenland. This indicates that
rapid mixing of atmospheric gases does indeed occur. However, the d 18 O data for
Greenland and Antarctica show significant differences.
The data in Figure 4.16 indicate that over the past 90,000 years the Earth has
been predominantly in an ice age, with the current interglacial period starting
about 15,000 years ago. However, the Ice Age in Greenland was interspersed with
about two dozen so-called Dansgaard-Oeschger (D-O) events characterized by
sudden warming followed by slow cooling with an overall duration of a few
thousand years and a Greenland temperature amplitude of up to 15 C. Despite
the qualitatively different temperature patterns between Greenland and Antarctica,
there appears to be a correlation between major discontinuities in Greenland and
slope changes in Antarctica. The seven vertical lines shown in Figure 4.16 indicate
where major sudden temperature increases occurred in Greenland. There seems to
be a causal relationship between these occurrences and temperature patterns in
Antarctica. The data in Figure 4.16 suggest that each sudden increase in tempera-
ture in Greenland was preceded by a rather slow moderate temperature rise in
Antarctica for a few thousand years. Each sudden rise in Greenland occurs near
the end of a more protracted rise in Antarctica.