How stable haspast climate been? 73for other regions. Compare this with a temperature rise during the twen-
tieth century of about 0.6◦C and the rates of change of a few degrees
centigrade per century projected to occur during the twenty-first century
because of human activities (see Chapter 6).
The ice core data (Figures 4.6) demonstrate that a series of rapid
warm and cold oscillations called Dansgaard–Oeschger events punctu-
ated the last glaciation. Comparison between the results from ice cores
drilled at different locations within the Greenland ice cap confirm the
details up to about 100 000 years ago. Comparison with data from Antarc-
tica suggests that the fluctuations of temperature over Greenland (perhaps
up to 16◦C) have been larger than those over Antarctica. Similar large
and relatively rapid variations are evident from North Atlantic deep sea
sediment cores.
Another particularly interesting period of climatic history, more re-
cently, is the Younger Dryas event (so called because it was marked by
the spread of an arctic flower,Dryas octopetala), which occurred over a
period of about 1500 years between about 12 000 and 10 700 years ago.
For 6000 years before the start of this event the Earth had been warming
up after the endof the last ice age. But then during the Younger Dryas
period, as demonstrated from many different sources of paleoclimatic
data, the climate swung back again into much colder conditions similar
to those at the end of the last ice age (Figure 4.7).The ice core record
shows that at the end of the event, 10 700 years ago, the warming in
the Arctic of about 7◦C occurred over only about fifty years and was
associated with decreased storminess (shown by a dramatic fall in the
amount of dust in the ice core) and an increase of precipitation of about
fifty per cent.
Two main reasons for these rapid variations in the past have been
suggested. One reason particularly applicable to ice age conditions is
that, as the ice-sheets over Greenland and eastern Canada have built
up, major break-upshave occurred from time to time, releasing massive
numbers of icebergs into the North Atlantic in what are called Heinrich
events. The second possibility is that the ocean circulation in the North
Atlantic region has been strongly affected by injections of fresh water
from the melting of ice. At present the ocean circulation in this region
is strongly influenced by cold salty water sinking to deep ocean levels
because its saltiness makes it dense; this sinking process is part of the
‘conveyor belt’ which is the major feature of the circulation of deep ocean
water around the world (see Figure 5.18). Large quantities of fresh water
from the melting of ice would make the water less salty, preventing it
from sinking and thereby altering the whole Atlantic circulation.
This link between the melting of ice and the ocean circulation
is a key feature of the explanation put forward by Professor Wallace