The world’s great deserts were formed by natural processes interacting
over long intervals of time. During most of these times, deserts have
grown and shrunk independent of human activities. Paleodeserts, large
sand seas now inactive because they are stabilized by vegetation, extend
well beyond the present margins of core deserts, such as the Sahara. In
some regions, deserts are separated sharply from surrounding, less arid
areas by mountains and other contrasting landforms that reflect basic
structural differences in the regional geology. In other areas, desert
fringes form a gradual transition from a dry to a more humid
environment, making it more difficult to define the desert border.
[Illustration: Overgrazing has made the Rio Puerco Basin of central New
Mexico one of the most eroded river basins of the American West and has
increased the high sediment content of the river (photograph by Terrence
Moore).]
[Illustration: Linear dunes of the Sahara Desert encroach on Nouakchott,
the capital of Mauritania. The dunes border a mosque at left (photograph
by Georg Gerster).]
These transition zones have very fragile, delicately balanced
ecosystems. Desert fringes often are a mosaic of microclimates. Small
hollows support vegetation that picks up heat from the hot winds and
protects the land from the prevailing winds. After rainfall the
vegetated areas are distinctly cooler than the surroundings. In these
marginal areas, human activity may stress the ecosystem beyond its
tolerance limit, resulting in degradation of the land. By pounding the
soil with their hooves, livestock compact the substrate, increase the
proportion of fine material, and reduce the percolation rate of the
soil, thus encouraging erosion by wind and water. Grazing and the
collection of firewood reduces or eliminates plants that help to bind
the soil.
This degradation of formerly productive land—_desertification_—is a
complex process. It involves multiple causes, and it proceeds at varying
rates in different climates. Desertification may intensify a general
climatic trend toward greater aridity, or it may initiate a change in
local climate.
Desertification does not occur in linear, easily mappable patterns.
Deserts advance erratically, forming patches on their borders. Areas far
from natural deserts can degrade quickly to barren soil, rock, or sand
through poor land management. The presence of a nearby desert has no
direct relationship to desertification. Unfortunately, an area
undergoing desertification is brought to public attention only after the
process is well underway. Often little or no data are available to
indicate the previous state of the ecosystem or the rate of degradation.
Scientists still question whether desertification, as a process of
global change, is permanent or how and when it can be halted or
reversed.
[Illustration: Camels and other animals trample the soil in the semiarid
Sahel of Africa as they move to water holes such as this one in Chad
(photograph courtesy of the U.S. Agency for International Development).]