Geoscience Reference
In-Depth Information
comparing vegetation and insect evidence. It takes years
for trees to spread from their refuge areas, but animals and
insects can move more quickly in response to a change in
climate as long as their food supply moves too. By about
12,200
BP
, it is believed, the climate of Britain was similar
to that at present (
Figure 9.4
).
This warmth did not last. A
Heinrich event
took place
in what is termed the Younger Dryas period (see box,
p. 179). Ice melting from the Canadian ice sheet and a
change in direction of the meltwaters from the Mississippi
valley into the St Lawrence led to a massive outpouring of
fresh water into the north Atlantic. Coarse sediment was
deposited there and a general cooling of surface waters took
place. As a consequence, a significant ice advance appears
to have occurred in north-west Europe and the former
Soviet Union, with possible signs of cooling in some other
parts of the world. In Britain
cirque glaciers
became re-
established in many parts of the uplands, and in western
Scotland ice advanced towards the lowlands near Glasgow.
Mean July temperatures fell below 10
C and trees tem-
porarily disappeared from Britain. This brief period of
about 1,200 years has recently been redated from Greenland
ice cores to extend from about 12,800
BP
to 11,600
BP
and
is called the
Loch Lomond stadial
. It was too short for exten-
sive glacier growth but the piles of gravelly debris in many
mountain cirques show the deposition which took place
Following the final retreat of the continental ice sheets
from Europe and North America between 10,000
BP
and
7000
BP
the climate rapidly ameliorated in middle and
Sudden climate changes
NEW DEVELOPMENTS
As dating methods have improved it has become possible to determine the rates of temperature change in the past
as indicated by ice cores and deep-sea sediments. Where rates of accumulation or sedimentation are adequate and
continuous it is possible to record indicated temperature changes to within a decade. This is a very important feature
of previous climates, as the present increase in temperature needs to be related to what has happened in the past.
Could such a rate of change be produced by natural processes or are the present rates of increase greater than
anything that has been observed before?
As recently as the 1980s scientists became aware of the rapidity with which climate might change within the geological
record. Evidence from ice cores taken from Greenland and deep-sea sediment from the North Atlantic clearly indicated
sudden changes in temperature conditions
(
Figures 9.13
and
9.14
)
. In the last glacial period rapid warming of about
5-7
C in a few decades was followed by periods of slower cooling and then a more rapid return to glacial conditions.
Twenty-three such oscillations have been found during the last glacial period and are called Dansgaard-Oeschger
events after the Danish and Swiss scientists working on the cores. These patterns are similar to those found in the
North Atlantic sediments. The most dramatic of these oscillations may be linked with the sedimentary Heinrich layers
found in the deep-sea cores. Six Heinrich events have been identified during the last glacial period. Their main
characteristic is a sudden increase in ice-rafted debris - debris that could not have been transported by liquid water.
They have been interpreted as the result of massive iceberg discharge from north-east Canada and the Hudson Strait,
leading to a decrease of oceanic salinity in the North Atlantic. They are believed to have occurred at the end of the
cooling cycle and were followed by a sudden change to warmer sea surface temperatures. One of the most important
features of these changes is their suddenness: they must indicate that rapid changes in atmospheric circulation took
place within a few years. They are a feature of both during the last glacial period and at its termination. Unless it can
be shown that they occur only during and towards the end of glacial phases, they may be of relevance to the present-
day sudden warming.
These sudden changes are still not fully understood and may result from a variety of causes. Evidence from other
parts of the world suggests that the changes were at least hemispheric in their extent and the longer oscillations in
the Greenland ice core have weaker and smoother counterparts in Antarctica. Hence we can be confident that they
are not the result of local factors. One major probable factor is the sudden changing of surface and subsurface ocean
currents as a result of temperature or salinity changes. In Britain we can appreciate the importance of the North
Atlantic Drift in maintaining our present climate. Changes in its surface position for whatever reason would result in
a sudden decrease in temperatures and different precipitation distributions.
