Environmental Engineering Reference
In-Depth Information
years, with the warm part of the cycle lasting only a relatively short time before another
gradual development of ice and then a rapid melt period (Figure 9.3).
During the cold phases, ice advanced across much of north-west Europe from
Scandinavia and across much of North America from centres over northern Canada. It
appears that ice occupied these areas for only a short period of time. For the remainder of
the cold phase the climate on the tropical side of the ice sheets was cold and dry, and
provided a source area for much of the wind-blown sediments. In the tropics, lake levels
indicate that the glacial periods were generally arid, as is indicated by the wider spread of
active sand dunes. At the same time, sea level fell by up to 150 m because so much water
was locked up in the form of ice in ice caps, ice sheets and glaciers. Oceanic currents
almost certainly changed in intensity and even direction in response to the new conditions
and coastlines.
The last glacial period reached its maximum intensity about 18,000 to 16,000 years
Before the Present (BP). This term is used whenever dating is based on radiometric
methods involving isotopic decay such as carbon-14 (radiocarbon) or potassium-argon
(K-Ar) (see box). After this time, much more information is available about changes of
climate.
The advances and retreats of glaciers can be used to interpret changes of temperature
and moisture regimes. Former lake shorelines may indicate changes in former moisture
conditions (Plate 9.2). Ice cores have been taken through the Antarctic and the Greenland
ice sheets, where estimates of seasonal climatic conditions may be interpreted from the
layers of ice. Sudden oscillations giving rise to warm events or 'interstadials' have been
found in the Greenland ice cores, with up to twenty-four such events between 115,000
and 14,000 BP. Even the nature of atmospheric composition may be determined from the
content of air inclusions within the ice. Cores from deep oceanic sediment can also
provide information about what was happening at the surface in environmental terms.
Information from proxy data indicates that at about 18,000 BP large parts of the
northern hemisphere, especially Europe and North America, were ice-covered (Figure
9.4). Antarctic ice advanced farther equatorwards but, because the southern hemisphere
continents cover only small areas in temperate latitudes, ice developed only in highland
areas, increasing glacier size and frequency. Sea surface temperatures as estimated from
foraminiferal remains and oxygen isotope analysis indicate major decreases in some
areas, such as the north-east Atlantic, where the warm oceanic current changed its
position and temperature reductions of up to 10° C occur. In other areas, like the South
China Sea, the difference was less marked. Temperatures of about 27° C predicted for
northern hemisphere summer off Hong Kong (Figure 9.4) compare with about 28.5° C
today. Recent work on tropical sea surface temperatures at 18,000 BP in other areas, such
as Brazil, suggests that temperatures may have been considerably lower, by 3-5°.
