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DESERTIFICATION OF ARID LANDS
By
H. E. Dregne
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ABSTRACT
Desertification is a land degradation problem of major importance in the arid regions of the world. Deterioration in soil and plant
cover have adversely affected nearly 50 percent of the land areas as the result of human mismanagement of cultivated and range
lands. North America and Spain have the largest percentage of their arid lands affected. Overgrazing and woodcutting are
responsible for most of the desertification of rangelands, cultivation practices inducing accelerated water and wind erosion are
most responsible in the rain-fed croplands, and improper water management leading to salinization is the cause of the
deterioration of irrigated lands. In addition to vegetation deterioration, erosion, and salinization, desertification effects can be seen
in loss of soil fertility, soil compaction, and soil crusting. Urbanization, mining, and recreation are having adverse effects on the
land of the same kind as is seen on range, dry farming, and irrigated lands. Combating desertification can be done successfully
using techniques already known if financial resources are available and the political will to act is present.
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INTRODUCTION
Desertification of the arid lands of the world has been proceeding--sometimes rapidly, sometimes slowly--for more than a
thousand years. It has caused untold misery among those most directly affected, yet environmental destruction continues. Until
recently, few if any lessons seemed to have been learned from the past, in part because the problem was an insidious one that
went unrecognized in its early stages or was seen as a local one affecting only a small population, and in part because new land
was always available to start over again. As long as remedial action could be deferred by moving on to new frontiers, land
conservation had little appeal. It was not until the 20th century--when easy land expansion came to an end--that governments and
people finally realized that continued careless degradation of natural resources threatened their future.
INTERNATIONAL DIRECTIONS
The decade of the 1950's witnessed the first worldwide effort to call attention to the problems and potentials of arid regions. It
started when the United Nations Educational, Scientific, and Cultural Organization (UNESCO) launched its Major Project on
Scientific Research on Arid Lands in 1951. That project led to publication of a newsletter, the provision for funds for establishing
and strengthening arid land research institutes, organization of conferences and symposia, and publication of a series of
research reviews and special reports on a wide range of topics. The Major Project was terminated in 1962 and the arid land
program was merged with the broader UNESCO natural resource program.
The impetus generated by the UNESCO project led to expanded interest in, and support of, arid lands studies throughout the
world. By 1970, knowledgeable scientists were well aware of the magnitude of the land destruction that had taken place in the
past, and that was becoming even more serious as population pressures increased.
At about the same time, one event served to focus world attention on desertification: the 1969 to 1973 drought in the African
Sahel. Recognition of the severity of the drought affecting six countries on the southern border of the Sahara (Mauritania, Senegal,
Mali, Upper Volta, Niger, and Chad) was slow to develop. Droughts, after all, were not unusual in the Sahelian countries; an
equally bad or even worse one had struck the same region during the years from 1911 to 1914, and several other droughts had
occurred before and after that time. It was only when the situation had become catastrophic that relief measures were undertaken
on a large scale. Among the aftereffects of the human toll and the millions of livestock that died due to the drought, was the call by
the United Nations General Assembly for the convening of an International Conference on Desertification. The conference was
held in Nairobi, Kenya, in August and September of 1977, attended by representatives of nearly 100 nations and many
international organizations; governmental and nongovernmental. In addition to providing a forum by which the world's attention
was drawn to the land destruction that was occurring, the conference addressed the problem of combating desertification of arid
lands. A plan of action was drawn to the land destruction that was developed and approved by the delegates (United Nations,
1978). Responsibility for following up and coordinating the implementation of the Plan of Action to Combat Desertification was
entrusted to the United Nations Environment Program.
DEFINITION
Desertification is a term that has been in use since at least 1949 when Aubreville, a perceptive and well-informed botanist and
ecologist, published a book on "Climate, Forets, et Desertification de l'Afrique Tropicale" (Aubreville, 1949). Aubreville thought of
desertification as the changing of productive land into a desert as the result of ruination of land by man-induced soil erosion. He
associated it with the humid and subhumid tropics where he worked. The causes of land destruction were tree cutting,
indiscriminate use of fire, and cultivation, which exposed the soil to water and wind erosion. Desertification was not the result of
the Sahara spreading outward but of localized activity that could begin anywhere. Aubreville was quite clear in his conclusion that
desertification in tropical Africa was due to man's activity, and that there had been no significant climatic change during the past
thousand or more years. Most of that destructive activity has occurred within recent historic time by the action of agricultural
populations.
Despite the fact that a world conference has been held on the subject, there is no generally accepted definition of desertification.
For most people, the word probably evokes a mental picture of a barren forbidding landscape resembling Death Valley or the
Sahara. Fortunately, that grim picture does not apply to most of the land that has undergone desertification.
A definition of desertification should recognize that it is a land degradation process that involves a continuum of change, from
slight to very severe degradation of the plant and soil resource, and is due to man's activities. The definition used here is the
following:
Desertification is the impoverishment of terrestrial ecosystems under the impact of man. It is the process of deterioration in these
ecosystems that can be measured by reduced productivity of desirable plants, undesirable alterations in the biomass and the
diversity of the micro and macro fauna and flora, accelerated soil deterioration, and increased hazards for human occupancy.
DESERT SPREAD AND DROUGHTS
A common misapprehension about desertification is that it spreads from a desert core, like a ripple on a pond. The truth is that
land degradation can and does occur far from any climatic desert; the presence or absence of a nearby desert has no direct
relation to desertification. Desertification usually begins as a spot on the land-scape where land abuse has become excessive.
From that spot, which might be around a watering point or in a cultivated field, land degradation spreads outward if the abuse
continues. Ultimately the spots may merge into a homogeneous area, but that is unusual on a large scale.
A second misconception is that droughts are responsible for desertification. Droughts do increase the likelihood that the rate of
degradation will increase on non-irrigated land if the carrying capacity is exceeded. However, well-managed land will recover from
droughts with minimal adverse effects when the rains return. The deadly combination is land abuse during good periods and its
continuation during periods of deficient rainfall. The famous satellite photograph taken during the 1970's drought of a green
rectangle (the Ekrafane Ranch) surrounded by brown denuded land in eastern Niger demonstrated dramatically the value of good
range management during favorable and unfavorable years.
HISTORICAL DEVELOPMENT
Land degradation is by no means a new problem, despite the attention focused upon it in recent years. In some quarters, there is
a tendency to blame desertification upon land pressures generated by the population explosion of the middle 20th century. While
the rapidly expanding population has greatly exacerbated the situation, desertification is not a new phenomenon.
Historical evidence shows that serious and extensive land deterioration occurring several centuries ago in the arid regions had
three epicenters: the Mediterranean Sea, The Mesopotamian Valley, and the loessial plateau of China. There were other places
where destructive changes in soil and plant cover had occurred but they were small in extent or are not well known.
Last One Hundred Years
Desertification in Africa and elsewhere began long before the 1969-1973 drought struck the Sahel. Stebbing (1937a, 1937b) was
the most persistent of those sounding the alarm over the rapid deforestation of the Sahelian and Sudanian vegetative zones. He
looked upon desertification in West Africa as forest degradation that leads to erosion and, as a last stage, to barren sand or rock.
That process is hastened, he contended, by blown sand from the Sahara being deposited on the deforested land. It was this
latter activity that led him to refer to the "encroaching Sahara," a term he later regretted using (Stebbing, 1938). He disclaimed any
attempt to give the idea that the Sahara was a vast sand field advancing in great waves like the incoming tide of a sea. The latter
concept, however, has proved so attractive to numerous writers on desertification that it now represents a common view on the
subject (Cloudsley-Thompson, 1974). There apparently is something fascinating about the idea of an expanding desert
threatening mankind. Encroachment of moving sand dunes on desert oases and transportation routes is an aspect of
desertification that is of small areal extent but is locally important and highly visible.
Warnings similar to those of Stebbing for the Sahel were made by other scientists in southern Africa, North America, South
America, Asia, and Australia during the 1920's and 1930's. Research has been undertaken in many countries to develop
techniques of grazing management and soil and water conservation that would halt and reverse desertification. As a result, there
is now a good understanding of the basic principles of land conservation. Field application of those principles has been slow,
unfortunately, and land degradation continues to undermine efforts to improve human well-being.
OCCURRENCE OF DESERTIFICATION
Desertification affects nearly all of the arid regions, to varying degrees, except for the extremely arid climatic deserts such as the
Sahara, Atacama, and Taklimakan. An indication of the extent and intensity of the desertification that has occurred in the past .
The classification system used in the preparation of continental desertification maps is based on four classes of desertification:
slight, moderate, severe, and very severe. The criteria for each class are as follows:
Slight: Little or no degradation of the soil and plant cover has occurred.
Moderate: (1) 26 to 50 percent of plant community consists of climax species, or (2) 25 to 75 percent of original topsoil lost, or (3)
soil salinity has reduced crop yields 10 to 50 percent.
Severe: (1) 10 to 25 percent of plan community consists of climax species, or (2) erosion has removed all or practically all of the
topsoil, or (3) salinity controllable by drainage and leaching has reduced crop yield by more than 50 percent.
Very Severe: (1) Less than 10 percent of plant community consists of climax species, or (2) land has many sand dunes or deep
gullies, or (3) salt crusts have developed on very slow permeable irrigated soils.
The "very severe" category represents the extreme condition that many people associate with desertification. It is land so badly
degraded that its utility by man or animals is virtually zero and the degradation is economically irreversible, for most purposes.
While there are many small areas of land that fit into this category, there are few areas large enough to be shown on the
continental maps. Practically all of the world's desertification can, at this point, be reversed.
Delineations on the maps, because of the small scale employed, usually are combinations of different desertification classes, for
example, and area shown as moderately desertified may have inclusions of slightly desertified and severely desertified land.
Most of the land used for agriculture in the arid regions is at least moderately desertified. The 52.1-percent figure for land in the
category of slight desertification consists mainly of the naturally barren climatic deserts (hyperarid regions) where man's
presence has been minimal.
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