72 Climates ofthe past
Figure 4.6Variations in Arctic temperature over the past 100 000 years as
deduced from oxygen isotope measurements (in terms ofδ^18 O) from the
‘Summit’ ice core in Greenland. The quantityδ^18 O plotted in Figures 4.6 and 4.7
is the difference (in parts per thousand) between the^18 O/^16 O ratio in the sample
and the same ratio in a laboratory standard. The overall shape of the record is
similar to that from the Vostok ice core shown in Figure 4.4 but much more
detail is apparent in the ‘Summit’ record’s stable period over the last 8000 years.
A change of five parts per thousand inδ^18 O in the ice core corresponds to about
a7◦C change in temperature.has been higher than that at the Antarctica drilling locations. For a given
period in the past, the relevant part of the Greenland ice core is longer
and more detail of variations over relatively short periods is therefore
available.
The data show that the last 8000 years have been unusually stable
compared with earlier epochs. In fact, as judged from the Vostok (Fig-
ure 4.4) and the Greenland records (Figure 4.6) this long stable period in
the Holocene is a unique feature of climate during the past 420 000 years.
It has been suggested that this had profound implications for the devel-
opment of civilisations.^10 Model simulations (see Chapter 5) indicate
that the detail of long-term changes during the Holocene is consistent
with the influence of orbital forcing (Figure 4.5).
It is also interesting to inspect the rate of temperature change during
the recovery period from the last glacial maximum about 20 000 years
ago and compare it with recent temperature changes. The data indicate
an average warming rate of about 0.2◦C per century between 20 000
and 10 000 years before present (BP) over Greenland, with lower rates