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DESERTIFICATION OF ARID LANDS
By
H. E. Dregne
________________________________________
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.
________________________________________
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.
By
H. E. Dregne
________________________________________
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.
________________________________________
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.