Desertification

Much of the world’s land surface is arid or semiarid (see Deserts). First used by the French scientist Aubreville in 1949, the concept of desertification has maintained its perceived importance in an increasingly populated world. Desertification is land degradation in arid and semiarid areas and has a negative connotation for sustaining human populations. It is a transformation that includes the lessening of water resources, decreased fertility of soil, and disappearance of most of the biomass. This is the result of interrelated processes that are difficult to sort. Some desertification is natural and some a complicated interplay of natural and human causes. Desertified land has a larger yearly moisture deficit (precipitation minus evapotranspiration), more modest humidity, and higher temperatures than land that has not undergone desertification.

Desertification can be identified using land resources satellites. The assessment of biomass status is now common and has monitoring 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, natural climate change 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 widespread 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 evidence 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 changing 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.