Environmental Engineering Reference
In-Depth Information
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.
These sudden changes are still not fully understood and may result from a variety of
causes. Evidence from other parts of the world suggest 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.
occurred in north-west Europe and the former Soviet Union, with possible signs of
cooling in 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 temporarily
disappeared from Britain. This brief period of about 1200 years has recently been redated
from Greenland ice cores to extend from about 12,900 to 11,700 BP and is called the
Loch Lomond stadial
. It was too short for extensive glacier growth but the piles of
gravelly debris in many mountain cirques show the deposition which took place when the
cirque glaciers melted (Plate 9.4).
Following the final retreat of the continental ice sheets from Europe and North
America between 10,000 and 7000 BP the climate rapidly ameliorated in middle and high
latitudes. A thermal maximum was reached about 5000 years ago, when temperatures are
believed to have been 1°-2° C higher than those of today. Lake levels in tropical areas
indicate moist conditions in the early part of this postglacial period, with a general
decline in levels subsequently. Evaporation and earth movements also affect lake levels,
as well as precipitation, so climatic interpretation is not always easy.
The thermal maximum was followed by a period of slowly declining temperatures and
fluctuations in precipitation. Within the general cooling there were cooler periods such as
around 2000 years ago and warmer ones, for example about AD 1000 to 1200. At that
time there were few severe winter storms in the Atlantic. The Vikings took advantage of
this quieter period to colonize Iceland and Greenland and probably visited North
America. By AD 1200 cooling began to set in, with increased storminess. In at least four
major sea floods of the Dutch and German coasts in the thirteenth century the death toll
was estimated at more than 100,000. At the same time, drought was starting to affect
Native American settlements in Iowa and South Dakota, but in parts of China moister
conditions prevailed. An increase in the strength of the westerly circulation in the
northern hemisphere has the effect of decreasing precipitation to the lee of the Western
Cordillera of the United States but can lead to an increase where jet streams converge
after splitting around the Tibetan plateau, so the differences are not contradictory.
