potential. There are no worldwide assessments but case studies have been conducted
in several areas. How much land has undergone desertification? There are several
varying estimates depending on particular definitions of desertification; however,
it is apparent that the total exceeds a billion hectares (about two-and-a-half billion
acres).
Clearly, naturalclimatechange can lead to desertification. The Sahara of
northern Africa is a significant case in point. There is good proxy evidence from
pollen and lake levels that the Sahara once harbored a much greener environment.
All evidence points to two drying periods: 6,700 to 5,500 years ago and a more
abrupt drying from 4,000 to 3,600 years ago. The drying climate eradicated wide-
spread low bushes and annual grasses and led to the present-day stark covering of
bare rock and sand. This result was devastating to nascent agriculture and led to its
disappearance from the area, destroying some civilizations. There is abundant evi-
dence that the spreading sands encroached on the monumental works of the Old
Kingdom Egyptians. Another natural climate change can be sensed in the 13th
century Mongol outpourings from central Asia. Progressive desiccation of already
marginal lands placed pressure on the remaining grasslands. Some authors have
attributed the ferocity of the empire-building of Genghis Khan as borne of chang-
ing climate. Although this single cause is not sufficient to explain Mongolian
expansion, it certainly must have been a contributing factor. These two cases are
cautionary and hint at deep societal problems when we ponder the possibility of
global warming.
The Sahel, the transition between the Sahara and theforestsof central Africa at,
roughly, 15°to 18°N across the entire longitudinal extent of Africa has been
pointed to as a pernicious sort of desertification affected by humans, first notice-
able in the droughty late 1960s and early 1970s. There are several features of
Sahelian societies that deepened the practical severity of the droughts and have
lessened the carrying capacity of the Sahel to the point it has not yet recovered.
There has been a long-standing shift from nomadic to sedentary societies since
the beginning of European colonization. This shift forcedpopulationsto stay
put and intensively use thelandscaperesources around them. Significant popula-
tion growth has exacerbated the demand on lands; instead of “resting” in between
nomadic visits, the land is now continuously exploited. In many areas the exploi-
tation is nonsustainable. Expanded domestic herding promoted overgrazing and
subsequent killing of grasses, especially around water sources. Many trees and
shrubs were removedfor use as fuel and wood. Others were cleared for agricul-
ture. The result was the exposure of dry soil to theatmospherewhich is readily
eroded by wind and water. Poorly managed irrigated areas underwent salinization,
killing crops and precluding natural revegetation. Soil fertility is invariably low-
ered by the removal of most all of the biomass. This created barren landscapes
Desertification 99