The Sahelian drought that began in 1968 was responsible for the deaths of between 100,000 and 250,000 people, the disruption of millions of lives, and the collapse of the agricultural bases of five countries (photograph by Daniel Stiles, UNEP).
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.
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).
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.
Linear dunes of the Sahara Desert encroach on Nouakchott, the capital of Mauritania. The dunes border a mosque at left (photograph by Georg Gerster).
Increased population and livestock pressure on marginal lands has accelerated desertification. In some areas, nomads moving to less arid areas disrupt the local ecosystem and increase the rate of erosion of the land. Nomads are trying to escape the desert, but because of their land-use practices, they are bringing the desert with them.
It is a misconception that droughts cause desertification. Droughts are common in
arid and semiarid lands. Well-managed lands can recover from drought when the
rains return. Continued land abuse during droughts, however, increases land
degradation. By 1973, the drought that began in 1968 in the Sahel of West Africa
and the land-use practices there had caused the deaths of more than 100,000
people and 12 million cattle, as well as the disruption of social organizations
from villages to the national level.
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).
While desertification has received tremendous publicity by the political and news media, there are still many things that we don't know about the degradation of productive lands and the expansion of deserts. In 1988 Ridley Nelson pointed out in an important scientific paper that the desertification problem and processes are not clearly defined. There is no consensus among researchers as to the specific causes, extent, or degree of desertification. Contrary to many popular reports, desertification is actually a subtle and complex process of deterioration that may often be reversible.
LEFT: Off-road vehicles significantly increase soil loss in the delicate desert
environment of the western United States. In a few seconds, soils that took
hundreds of years to develop can be destroyed (photograph by Terrence Moore).
RIGHT:Goat seeks food in the sparsely vegetated Sahel of Africa (photograph courtesy of the U S. Agency for International Development).
Oases and farmlands in windy regions can be protected by planting tree fences or grass belts. Sand that manages to pass through the grass belts can be caught in strips of trees planted as wind breaks 50 to 100 meters apart adjacent to the belts. Small plots of trees may also be scattered inside oases to stabilize the area. On a much larger scale, a "Green Wall," which will eventually stretch more than 5,700 kilometers in length, much longer than the famous Great Wall, is being planted in northeastern China to protect "sandy lands"--deserts believed to have been created by human activity.
More efficient use of existing water resources and control of salinization are other effective tools for improving arid lands. New ways are being sought to use surface-water resources such as rain water harvesting or irrigating with seasonal runoff from adjacent highlands. New ways also being sought to find and tap groundwater resources and to develop more effective ways of irrigating arid and semiarid lands. Research on the reclamation of deserts also is focusing on discovering proper crop rotation to protect the fragile soil, on understanding how sand-fixing plants can be adapted to local environments, and on how grazing lands and water resources can be developed effectively without being overused.
If we are to stop and reverse the degradation of arid and semiarid lands, we must
understand how and why the rates of climate change, population growth, and food
production adversely affect these environments. The most effective intervention
can come only from the wise use of the best earth-science information available.
Straw grids (one of which is shown at left) and vegetation irrigated by water from the Yellow River stabilize dunes in this part of China's Tengger Desert (shown at right) and protect a nearby railroad from windblown sand
From wasteland to vineyard. Ground water and underground channels help this vineyard flourish on land reclaimed from desert pavement in China's Turpan Depression.