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 There are two types of desert. One type has formed naturally
and has existed for thousands of years (see preceding page).
There is also a second type of desert, one that is created
over days and months. It is not natural and is formed as a
result of climate changes and human activity.
The process by which dryland is transformed into desert-like
conditions is called desertification. Desertification occurs
when the ecological balance in a dryland area breaks down.
The soil quality becomes progressively worse, and there is
a loss of vegetation covering the land. When the soil becomes
exposed, it is more easily washed away with the rains or blown
away in the dry season. This process is known as erosion.
Deserts around the world are spreading at a rate of 60,000
square kilometres a year. Desertification is a particularly
acute problem in Africa and Asia, which contain almost 70
percent of the world's desertified land. However, the problem
of desertification is not limited to the South. One-third
of North America is classified as dryland, and much of the
agricultural land in Canada and the United States is affected
by some form of erosion.
The main causes of desertification are human activity and
climate change. Human activity causes desertification in the
following ways:
Overburning occurs when vegetation (including trees
and undergrowth) is burned to clear land for farming. This
destroys the protective covering of the land, causing soil
to dry up and turn to powder.
Overcropping happens when people farm the land until
it is no longer fertile, then abandon the land for other areas.
This leaves the soil sapped of nutrients, unable to develop
new plants.
Overgrazing may occur when animals are allowed to graze
freely, restricting plant growth by eating new plant shoots
and leaves. Animals may also destroy trees by eating their
protective bark or trampling the vegetation that protects
soil from erosion.
A major cause of soil erosion is lack of plant cover.
Water and wind both erode soil, and one usually intensifies
the effect of the other.
Deforestation occurs if trees are being cut down more
rapidly than they grow back. Trees are often the only source
of cooking fuel and building materials for people in the South.
The soil is left unprotected and can deteriorate quickly,
turning to sand.
During flood irrigation (a technique which directs
water to dry areas by means of wells or canals) the soil can
become waterlogged if the land is not properly drained. If
this happens, the land may become salt-saturated, which prevents
vegetation from growing.
Continual movement of people is also a threat. Many
groups living at the edge of the desert are nomadic or semi-nomadic.
This means that they often move to new land when the old land
no longer supports them and their livestock. With increasing
population and few empty lands into which people can travel,
there is less land available to support vegetation or animals.
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