precipitation during Quaternary ice ages

precipitation during Quaternary ice ages During the Quaternary ice ages, changes in the pattern of global atmospheric circulation, as well as changes in the hydrological budget resulted in complex changes in regional patterns of precipitation. In high latitudes the greatest changes arose as a result of the growth of the major ice sheets of the northern hemisphere and also because of the development of sea ice over large areas of ocean. In the southern hemisphere, the Antarctic ice sheet increased in surface area and was also associated with a major expansion of sea ice. In both hemispheres, where ice sheets existed and where there was also a large increase in sea-ice cover there was an associated expansion of the areas affected by permanent high atmospheric pressure. This, in turn, promoted aridity and the displacement of mid-latitude cyclones to lower latitudes. Indeed, certain high-latitude areas were sufficiently arid to be incapable of supporting the growth of glacier ice. In these regions (for example northern Alaska, north-eastern Siberia, and northern Greenland) ice-age aridity was associated with the growth of permafrost.

The expansion of ice sheets in both hemispheres, together with decreased air temperatures, led to complex changes in atmospheric circulation in low latitudes. In many low-latitude areas during ice ages the position of the Intertropical Convergence Zone (ITCZ) was changed, and this led to changes in the distribution of monsoon precipitation. Thus, for example, in Africa during the last ice age, the position of the ITCZ was displaced several hundred kilometres southwards (Fig. 1). This, in turn, led to the failure of the south-west monsoon over the Gulf of Guinea and the displacement of the south-west monsoon over eastern Africa caused by strengthened north-easterly trade winds. By contrast, most areas of Africa during ice-age winters appear to have been dominated by dry northerly and north-easterly winds. These changes in atmospheric circulation led to increased ice-age aridity over most areas of Africa and this is reflected by an ice-age River Nile that scarcely functioned.

Similar climatic changes may have occurred over northern areas of South America during ice ages (Fig. 2). Here, displacement of the ITCZ over the area presently occupied by the equatorial rainforest may have caused a more restricted winter north-east monsoon and pronounced aridity. These changes in tropical South America and in Africa led to a greatly decreased area of equatorial rainforest during the last ice age. This widespread tropical aridity is also illustrated by the occurrence of windblown sediments in areas of present-day equatorial rainforest and by the presence of wind-deposited sediments offshore in sea-bed sediments.

In the southern hemisphere, the northern extension of sea-ice cover to latitude 50 °S resulted in the northward displacement of the Antarctic polar atmospheric front. This change resulted, in turn, in the northward displacement of midlatitude cyclones over Argentina, southern Africa, southern Australia, and Tasmania. A strengthened cell of Antarctic high pressure also accounted for the northward displacement of mid-latitude cyclones in the southern hemisphere that tracked farther northwards across the southern Pacific, Indian, and Atlantic oceans.

Not all areas were affected by decreased precipitation during ice ages. For example, in western North America the mid-latitude cyclones that were steered along the southern margin of the Laurentide ice sheet resulted, through the combined influence of increased precipitation and decreased evaporation, in the formation of a number of large lakes (for example, Lake Bonneville). Similarly, increased precipitation in areas south of the ice sheet caused the formation of a shallow lake in Death Valley.

Alastair G. Dawson

Bibliography

Dawson, A. G. (1992) Ice Age Earth: late Quaternary geology and climate. Routledge, London.
Goudie, A. S. (1983) Environmental change. Clarendon Press, Oxford.
Hamilton, A. C. (1982) Environmental history of East Africa: a study of the Quaternary. Academic Press, London.