Deserts of the World - Design with the Desert: Conservation and Sustainable Development - page 4

In late Cenozoic times, aridity became a prominent feature of the Saharan environment, 12
probably because of the occurrence of several independent but roughly synchronous
geological events 13 :
• As the African plate moved northward, there was a migration of northern Africa
from wet equatorial latitudes (where the Sahara had been at the end of the Jurassic)
into drier subtropical latitudes.
• During the late Tertiary and Quaternary, uplift of the Tibetan plateau had a dra-
matic effect on world climates, helping to create the easterly jet stream, which now
brings dry subsiding air to the Ethiopian and Somali deserts.
• The progressive build-up of polar ice caps during the Cenozoic climatic decline
created a steeper temperature gradient between the Equator and the Poles, and
this in turn led to an increase in Trade Wind velocities and their ability to mobilize
sand into dunes.
• Cooling of the ocean surface may have reduced the amount of evaporation and
convection in low latitudes, thus reducing the amount of tropical and subtropical
precipitation.
Thus, although the analysis of deep-sea cores in the Atlantic offshore from the Sahara
indicates that some aeolian activity dates back to the early Cretaceous, 14 it was probably
around 2-3 million year BP that a high level of aridity became established. From about 2.5
million year BP, the great tropical inland lakes of the Sahara began to dry out, and this is
more or less contemporaneous with the onset of mid-latitude glaciation. Aeolian sands
become evident in the Chad basin at this time, and such palynological work indicates
substantial changes in vegetation characteristics. 15
In the Pleistocene, a clearer pattern of climatic oscillations became apparent, with
alternations of aridity and greater humidity, although dates are a matter of controversy,
especially in North Africa. 16 Each cycle may have been of the order of 100,000 years in
duration, with nine-tenth of the cycle involving a gradual buildup toward peak aridity,
coldness, and aeolian activity, followed by a rapid but short-lived return to milder and
wetter conditions. 13 During dry phases, ocean cores demonstrate that large quantities of
dust were generated by the Sahara, 17 and there was an equator-ward spread of dune fields
in the Sahelian-Soudano zone. 18 The situation becomes especially clear at the time of the
maximum of the last glaciation (the “Ogolian” of French workers), when aeolian turbidites *
were deposited on the Atlantic continental shelf. There was a substantial increase in dust
output, and fluvial inputs from rivers such as the Senegal were minimal. 19 Desiccation of
lakes took place on the southern side of the Sahara between 23,000 and 16,000 year BP, and
active dunes extended up to 280 miles southward into the Sahel, blocking the courses of
the Senegal and Niger rivers and probably also crossing the Nile. 20
Toward the end of the last glacial, at around 12,500 year BP, there was a major change
in environmental conditions, characterized by a redevelopment of lakes in the Chad basin
and eastern Africa. Extensive lakes and swamps also formed along the Blue and White
Niles in the Gezira area. A peak of humidity may have occurred around 9000-8000 year
B P, 21 and, if one assumes that temperatures were broadly the same as today, the rainfall
increase may have been as much as 65%. There were other lake oscillations during the
course of the Holocene, but from about 5000 year BP conditions began to deteriorate
irregularly toward the present situation. The general trend toward aridity is reflected in
* Aeolian turbidites result from undersea mass movement of sediments deposited by wind.