ice-age aridity Observation of the current and past extent of aeolian sand dunes (see Fig. 1) indicates that during the past they have expanded considerably. The current distribution of active sand seas is for the most part restricted to geographical areas close to 30° north and south of the Equator (in the vicinity of the descending limbs of Hadley circulation cells), in the inland areas of continents, and in regions influenced by cold ocean currents. These areas are characterized by precipitation levels which do not exceed 150 mm per annum. In the past they were larger, occupying over 50 per cent of the continental land area between 30° N and 30° S. The timing of desert expansion and its relation to global climate cycles has been a subject of great controversy.
Early theories generated late in the nineteenth century by the American geomorphologist G. K. Gilbert correlated cold (glacial) periods with times of pluvial (wet) conditions in arid regions. He studied former lake levels and their relationship to glacial moraines in the Great Salt Lake and Wasatch Mountains area of the south-western United States and concluded that high lake levels correlated with maximum advances of mountain glaciers.
In the mid-1970s this theory was brought into question by three separate lines of evidence. First, radiocarbon dating of some desert and desert-associated sediments suggested that during glacial times the sand dunes had been active. The southern extent of the Sahara, for example, was displaced southwards by some 500 km. Secondly, evidence from lakes in tropical and North Africa indicated that they attained their lowest levels during the Last Glacial Maximum, about 18 000 years ago. Thirdly, dust and aeolian sand input into ocean cores off the coasts adjacent to large deserts were at a maximum during glacial times. Maxima of dust deposition during glacial periods have also subsequently been recognized in polar ice cores. A global correlation between high-latitude glacial expansion and low-mid-latitude aridity was thus established.
It is now realized that cold dry conditions experienced in low-latitude desert regions during glacial times are mainly related to a weakening of the African and Asian monsoonal rainfall regime. The monsoon rains are driven primarily by a strong summer temperature gradient between the warm continental areas and the colder adjacent deserts. Weakening of the land–sea summer temperature differential reduces the intensity of monsoon rains and thus leads to the expansion of deserts. This trend may be further enhanced if the wind-driven upwelling of relatively cold waters is reduced in adjacent oceans. The main influence on summer temperatures is the solar insolation input, which is controlled over glacial– interglacial timescales by the Milankovich or orbital cycles.
Additional evidence of ice-age aridity has been obtained from the analysis of extensive loess sequences in Europe and Asia (Fig. 1). Loess is a fine-grained aeolian sediment, typically liberated by the grinding action of glaciers, which may be transported great distances and indicates both windiness and aridity. Periods of loess deposition are found to correlate with glacial periods when loess profiles are compared to palaeoclimatic evidence from deep-sea cores.
An additional factor which works to enhance ice-age aridity relates to the lowering of sea levels associated with glacial periods. Lower sea levels expose in many coastal areas large expanses of continental shelf (in places over 200 km wide), which result in considerably larger glacial continental areas. The effect of this continentality is that any moisture-laiden air masses travelling inland are likely to have lost much of their moisture by the time they have reached inland continental areas. Pole-to-Equator (meridional) temperature gradients are also at a maximum during ice ages, and this results in the intensitification of trade-wind circulation. Intensified trade winds encourage evaporation.
Over shorter durations, changes in ocean thermohaline circulation and sea-surface temperatures may also have a significant influence, as has been most clearly indicated by a correlation between North Atlantic and Pacific sea-surface temperature anomalies and drought conditions in the Sahel.
Anomalies in the general patterns outlined above do occur. An especially significant example of this is the lake basins examined by G. K. Gilbert in the Great Salt Lakes region. Lakes Bonneville and Lahontan were in this area, reaching a maximum level at about the time of the last glacial maximum. The regional evidence for pluvial conditions at that time is explained by the occurrence of increased rains associated with a south displaced limb of the westerly polar jet, and subsequent increases in the occurrence of winter cyclonic storm activity. The jet was displaced for a period of time when the Laurentide ice sheet reached its maximum late Pleistocene extent. Retreat of the ice sheet during late glacial times has resulted in the return and reconnection of the jet stream limbs in more northern latitudes.
The most recent period of Late Quaternary maximum wetness in arid and semi-arid regions is broadly coincident with the maximum of Northern Hemisphere isolation which occurred approximately 9000 years ago. This was a period of intensive monsoonal rainfall.
Stephen Stokes
Bibliography
Dawson, A. G. (1992) Ice Age Earth: Late Quaternary geology and climate. Routledge, London.
