Quaternary ice sheets and climate

Quaternary ice sheets and climate It is now generally agreed that during the Quaternary temperate conditions similar to those of the present existed for only relatively short intervals of time. For the majority of the time, glacial conditions prevailed, and for long time-intervals ice sheets covered large areas of North America, Europe, and Russia. During periods of glaciation the ice sheets in Antarctica and Greenland expanded in size, while in many upland areas a significant lowering of the snow-line (frequently by as much as 700 metres) resulted in the widespread development of alpine glaciers and ice caps (Fig. 1).

During ice ages the various ice sheets played an important part in altering the Earth's climate. The most direct influences were in the northern hemisphere, where large ice sheets in North America and Eurasia resulted in the development of topographic barriers that greatly altered patterns of air flow in the troposphere. Much of what we know of the changing patterns of climate associated with the development of Quaternary ice sheets has been established as a result of the development of numerical models of the Earth's climate during ice ages. This has been supplemented by empirical studies of proxy indicators of climate change (for example biostratigraphical and lithostratigraphical investigations, that have been used to check the accuracy of the mathematical models).

From a climatological point of view, the most important influence of ice sheets on climate arises from the cooling of the overlying air by the surface of the ice sheets. As a result, the air that overlies any ice sheet is stable and results in the formation of permanent high pressure that, in turn, causes cyclonic depressions to be steered round individual ice sheets. Thus, in the case of the Antarctic ice sheet during the last glaciation, cyclones that formed in the Antarctic Circumpolar Ocean were displaced further northwards and this, in turn, led to increased precipitation in lower latitudes (e.g. across Southern Australia).

In the northern hemisphere, cyclones were similarly steered round the permanent cells of high pressure located above the ice sheets in North America and Eurasia. This process appears to have been associated with a splitting of the mid-latitude jet stream around both the major ice sheets (Fig. 2). Thus, the North American ice sheet was associated with a northern jet stream limb that tracked across northern Alaska and thereafter descended to the south-east to merge with the southern jet stream limb over the North Atlantic Ocean. This limb of the jet stream tracked across the continental United States, where it induced the generation of rain-bearing depressions. Accordingly, during periods of time when an ice sheet existed over North America, much of the continental area of the United States was influenced by increased rainfall and decreased evaporation, the latter a result of lower temperatures.

In Europe there appears to have been a similar split in the mid-latitude jet stream so that a northern jet stream limb tracked across Arctic Russia while a southern section of the jet stream was deflected southwards across the Mediterranean region (Fig. 2). In a similar manner to North America, the southward displacement of the jet stream across Europe resulted in increased precipitation over the Mediterranean.

Further east the behaviour of the jet stream is less well known: it depends largely upon the ice age conditions in the Tibetan plateau. A popular view is that during the last glaciation a large ice sheet formed over the Tibetan plateau and that this ice sheet caused a similar split in jet-stream flow. A contrasting view is that there were relatively small accumulations of ice in Tibet during the last glaciation, since the area was greatly affected by ice-age aridity. This interpretation is consistent with the formation of a single mid-latitude jet stream. This particular controversy is very important from a climatological point of view, since it is of importance for the patterns of ice-age tropospheric air flow over China during ice ages.

The patterns of jet-stream displacement described above were also partly a function of the great thickness that some of the ice sheets attained. In North America, Europe, and Russia the ice sheets were in the region of 3000–4000 m in thickness and, as a result, they presented topographic barriers to air flow within the upper troposphere. The presence of ice sheets (particularly in the northern hemisphere), together with the development of sea ice in the polar oceans, also resulted in an increase in meridional (north–south) temperature gradients. This process in turn led indirectly to increased rates of heat transfer between low and high latitudes, and this resulted in increased rates of air flow. Furthermore, the presence of the ice sheets together with decreased ocean temperatures led to increased aridity at lower latitudes.

An additional factor affecting the patterns of atmospheric circulation round Quaternary ice sheets arises from the fact that in many areas large lakes were created next to the ice-sheet margins. In the case of the Russian ice sheet a series of large lakes were formed along its southern margin. The largest of these, the Pur and Mensi lake complex occupied an area of 1.5 million square kilometres. Numerous large lakes were similarly formed along the southern margin of the Laurentide ice sheet in North America. In addition, numerous large lakes were formed further south of the ice sheet margin, principally as a result of decreased evaporation rates and increased rainfall (for example, Lake Bonneville: see pluvial lakes). These lakes are important from a climatological point of view since they provided a supply of moisture for mid-latitude cyclones that tracked across these continental land masses. At present, the significance of these moisture sources for the growth of ice sheets is not fully understood.

The recognition that ice-age conditions were associated with a profound redistribution of global weather systems implies that, over relatively short geological time periods, the dynamics of the troposphere responded to climate change in an unstable manner. The rates at which parts of the atmosphere may be susceptible to such change are at present unknown. However, recognizing that major changes have taken place in the relatively recent past provides a more secure basis from which to predict future changes.

Alastair G. Dawson

Bibliography

Lamb, H. H. (1982) Climate history and the modern world. Methuen, London.
Broecker, W. and and Denton, G. (1990) What drives glacial cycles? Scientific American, January 1990, 43–50.
Dawson, A. G. (1992) Ice Age Earth: Late Quaternary geology and climate. Routledge, London.