| Copyright ©2003, part of The YKTA Corporation, and its licensor's. All rights reserved. |
| Home I Mind I Body I Life I Tools I New I Music Instrument I Links I Contact Us I |
| Defining Desertification . |
Defining Desertification
"Desertification" is defined in this web site, and in this research
program as the reduction or spatial reorganization of net primary
production in arid and semi-arid lands. As early as 1949, the
scientist Aubreville noticed land degradation that seemed to be
extending north into semi-arid and sub-humid regions of North
Africa from the more arid zones of the Sahara (Aubreville, 1949).
The term Aubreville coined for this process was "desertification".
The problem had been observed in the Mediterranean World
nearly two millennia before by the Roman senator Cicero who
spoke of the destruction of the north African forests and their
replacement by barren, desert-like areas. The devastating Sahel
droughts of 1968-73 and the apparent accelerated southward
advance of the Sahara Desert led to extensive international
discussion of the problem and the formation of the United Nations
Conference on Desertification (UNCOD).
At a meeting convened during 1977 in Nairobi, Kenya, UNCOD
defined desertification as follows:
"Desertification is the diminution or destruction of the biological
potential of land, and can lead ultimately to desert-like conditions. It
is an aspect of the widespread deterioration of ecosystems, and
has diminished or destroyed the biological potential, i.e. plant and
animal production, for multiple use purposes at a time when
increased productivity is needed to support growing populations in
quest of development." (UNEP, 1978)
During the following years various agencies, scientific institutions
and individual scientists found the above definition to be
inadequate (UNEP, 1992). In response various groups developed
their own definition, which, not unexpectedly, led to a significant
amount of confusion. Additionally, cyclic oscillations of vegetation
productivity related to climate fluctuations had been observed in
satellite data, and there was a need to differentiate between
desertification and these cyclic climatic oscillations (UNEP, 1992).
In 1992 UNCOD settled upon the following definition of
desertification:
"Desertification is land degradation in arid, semi-arid, and dry sub-
humid areas resulting mainly from adverse human impact." (UNEP,
1992)
Perhaps the most significant aspect of this definition of
desertification is its focus on "human impact." This differentiates
the issue of desertification from simple climatic fluctuations such
as drought, but it should be noted that drought can cause an
exacerbation of damage derived from human activities. The
definition of desertification offered by Mainguet (1994) states that
desertification is "revealed by drought," but "caused by human
activities." Nearly one-quarter of the vegetated land area of the
Earth has been highly disturbed by human activity and an
additional 28 percent has been moderately disturbed (World
Resources Institute [WRI], 1993). This massive level of human
disturbance suggests that there are large areas of the Earth's
surface that have either been degraded through human activity or
have a significant potential of being degraded through human
activity.
Extent of the Problem
"In fact, so devastating seems the occupation of man that, with a
few striking exceptions, a desert or near-desert condition is often
associated with his long habitation of a region. Two major factors
are believed to account for the growth of man-made deserts. In the
first place, semi-arid to semi-humid regions proved the most
favorable sites for the early development of human culture. Such
areas, however, stand in a condition of delicate ecological balance
between humid and true desert climates. In the second place,
processes of soil erosion are accelerated by the exposure of soil
surfaces hitherto protected by complete mantles of vegetation,
whether grass or forest, by heavy grazing and cultivation. It is only
within the past decade that experimental studies of these
processes have been made. So enormous have been the
differences in soil wastage and superficial runoff of rain waters
from bared sloping lands, as compared with similar surfaces
protected by a complete coverage of vegetation, that new light is
thrown on the problem of the decadence of former civilizations."
(Lowdermilk, 1935)
The World Resources Institute (1992) reports that "over the past 45
years, about 11 percent of the Earth’s vegetated soils became
degraded to the point that their original biotic functions are
damaged, and reclamation may be costly or in some cases
impossible." UNCOD stated that as of 1992 desertification affects
70 percent of the world's drylands (3.6 billion hectares) or nearly
one-fourth of the total land area of the planet (UNEP, 1992).
However, UNCOD estimates have been questioned by workers
such as Warren and Agnew (1988) who pointed out that about half
of the arid area being used for the estimates is too arid for any
form of agriculture. In fact, the UNEP (1992) definition of
"rangeland" includes "non-agricultural, largely unoccupied drylands
that are unused or used only occasionally by nomadic pastoralists,"
which suggests that the estimated area of degraded rangeland
may be inflated, but, since the estimate of total rangeland would
also be inflated, this may not change the fraction of usable arid
lands that has been degraded. Even if the area of usable arid
lands is only half of the value quoted by UNCOD, the region at risk
represents one-sixth of the Earth's surface, and the reduced area
of arid lands still included are those most used by humans.
The second important aspect of the extent of land degradation is
the question of the potential for reversing the degradation. Nelson
(1988) expressed the view that "the extent of desertification as an
irreversible state has probably been exaggerated, although it is
correct to classify it as a serious problem." Topsoil lost to erosion
can usually be rebuilt given time, and the well-known process of
biological succession shows that a forest ecosystem can replace
an abandoned field in a few centuries. However, on the scale of
people's need to use the land for sustenance, a few centuries is
forever. Human effort can accelerate the regeneration of topsoil or
ecosystems, shortening the time need for regeneration, but this
may require considerable economic expense. In extreme cases of
degradation where the clay fraction of the soil is nearly completely
lost or invading flora becomes too firmly entrenched, natural
recovery to the original ecosystem will not occur. Warren and
Agnew (1988) point out lack of the knowledge needed to assess
the resilience or recoverability of soils and plant communities.
Although the effects of land degradation have been less severe in
the United States than they have been in areas such as the Sahel
or Western Rajasthan (India), the United States itself has a great
potential for the degradation of drylands. The historian Walter
Prescott Webb once said of the Western United States:
"Draw a line anywhere from the region’s eastern boundary to the
Pacific, stand on its mid-point and you will find yourself either in the
desert or near it. If we do not understand the West it is because we
perversely refuse to recognize this fact.. When the desert pokes a
hot finger into the border regions, the people speak of a drought;
when it pulls the finger back, they say 'the country is getting more
seasonable.' At the heart of the desert there is no drought, there is
only an occasional mitigation of dryness." (Webb, 1957)
The Western U.S. has been one of the most rapidly growing
regions of the country over the last 100 years (Sheridan, 1981).
Agricultural output from this region comprised more than 18
percent of the total agricultural output of the United States in 1977
(Sheridan, 1981).
That year the arid lands produced 66 percent of the nation's cotton,
39 percent of US barley, and 21 percent of the total wheat
production (Sheridan, 1981). The annual rainfall in much of the
West averages less than 51 cm (20 inches). This has long been
considered to be below the threshold of successful agriculture
without irrigation, and it was predicted that "these lands will
maintain but a scanty population" (Powell, 1879). In order to
support the large population and agricultural output of the West
groundwater has been pumped out of the ground faster than it can
be recharged, most of the region's rivers have been dammed to
make water available on demand, and water has been transported
over long distances from where it is more plentiful to where it is
scarce (Sheridan, 1981). In 1893, John Wesley Powell told the
International Irrigation Congress in Los Angeles, (Stegner, 1954)
"you are piling up a heritage of conflict and litigation over water
rights for there is not sufficient water to supply the land." This
prophecy has been borne out as witnessed by the conflicts over
water that have increased in intensity over the last few decades.
Given the large potential area of the United States in which
degradation of drylands can occur and the unprecedented stresses
due to its large population and vast agricultural output, it should not
be a surprise to find that significant desertification has occurred in
North America. Dregne (1983) reported that nearly 1.3 million
square kilometers of North American drylands have been "severel"
or "very severely" degraded. Over the last 100 years the Sonoran
and Chihuahuan deserts have become perceptibly more barren,
even though these areas have been deserts for a very long time
(Sheridan, 1981). This problem is likely to get worse, not better:
"The introduction of sprinkler irrigation systems, especially the
center pivot systems, has enabled previously unsuitable rolling
sandy lands to be cropped successfully. If and when those soils are
abandoned, for reasons of economy or shortage of water, the
United States will face an even greater wind erosion threat than it
has had to cope with in the past" (Dregne 1983).
We can see the double-edged nature of the water problem here.
The rate at which the water is being used in these arid regions
around the world for irrigated agriculture is not sustainable, and for
that reason the environmentally prudent thing would seem to be to
severely curtail irrigation in these areas. When the irrigation is
stopped, the area that had been under cultivation may become a
source of blowing sand which further damages the environment of
the drylands.
The Processes of Desertification
Another significant item in the current UNCOD definition of
desertification listed above is the idea of "land degradation."
UNCOD defined land degradation as follows:
"Degradation implies reduction of resource potential by one or a
combination of processes acting on the land. These processes
include water erosion, wind erosion and sedimentation by those
agents, long-term reduction in the amount or diversity of natural
vegetation, where relevant, and salinization and sodication."
(UNEP, 1992)
Sheridan (1981) characterized the affliction of land degradation as
having the following symptoms: declining groundwater tables,
salinization of topsoil and water, reduction of surface waters,
unnaturally high soil erosion, and the desolation of native
vegetation. Dregne (1983) indicates the major processes of
desertification in arid regions as: water erosion, wind erosion,
salinization, waterlogging, and soil compaction. Additionally, the
mechanical destruction of desert pavements and calcrete (caliche)
makes the soil more vulnerable to erosion. Through these different
processes, the land is made more barren, and it may become
effectively like a desert, even without climate changes. Before
continuing further to consider how to assess and monitor land
degradation, we should look briefly at some of these processes.
The Challenge of Monitoring Desertification
"The image created has too often been of inexorably advancing
sands, as opposed to more subtle, more complex, pulsating
deteriorations, sometimes with reversals, but at least, with
substantial periodic remissions, radiating out from centers of
excessive population pressure." Nelson (1988)
Desertification is not, as sometimes envisioned, an invasion of non-
desert areas from a desert core. Sheridan (1981) compares
desertification to guerrilla warfare with no real "front line". Dregne
(1983) describes desertification as a patch of land degraded
through human abuse that then spreads outward if the abuse
continues. A United Nations report (UNEP, 1978) describes the
spread of desertification as follows: "These degraded patches, like
a skin disease, link up to carry the process over extended areas."
Mainguet (1994) clearly states, "The theory of the encroaching
desert, which has now been scientifically rejected, is still a fixed
idea in the minds of governments, donors, and journalists; this must
change."
The extent of this “subtle and insidious process” (UNEP, 1978) of
land degradation can be difficult to assess from observations on
the ground. Ground observations may quickly show that soil is
blowing from a farmer's field or the salinization is beginning in the
soil; however, this single observation does not indicate the state of
degradation a few miles away. Each site requires manpower-
intensive biomass surveys and soil sampling. Furthermore, as
indicated by the quotation from Nelson (1988) above, the
desertification processes fluctuate over time, so repeated and
ongoing observations are required to determine if progressive
degradation is actually occurring and to track its progress. Warren
and Agnew (1988) stated that one of the main problems in our
understanding of land degradation was "the ignorance in
establishing trends and speeds, because trends and speeds
require sequential surveys." In addition, sequential surveys are of
limited use if baselines for comparison are not established
(Nelson, 1988; Rodenberg, 1991; Murray et al., 1994).
Remote sensing from satellites offers a potential means to survey
all arid regions of the globe. However, remote sensing can not
entirely supplant the use of ground observations as was widely
believed in the 1970's and early 1980's (Mainguet, 1994). Still,
satellite remote sensing can provide a powerful adjunct to ground
observations by extending in time and space field observations
made at a single point in a given region. Satellite remote sensing
thus can monitor much larger areas than ground surveys and aid in
targeting key ground observations. In this way, ground observation
anchors satellite observations, with the satellite remote sensing
effectively multiplying the value of the data acquired on the ground
and identifying the specific localities where additional and repeat
ground surveys are most needed.
Mainguet (1994) cites the importance of "remote sensing
supported by verifications on the ground for a better perception of
the extent and processes of degradation." Dregne and Tucker
(1988) indicate that 30 to 40 years of observation by satellites and
ground studies will be required to confidently determine any
permanent changes in the boundaries of the Sahel.
Desertification
Concept definition: 1) The development of desert conditions as a
result of human activity or climatic changes. 2) The process of land
damage which allows the soil to spread like a desert in arid and
semi-arid regions. There is a loss of vegetative cover and the soil
deteriorates in texture, nutrient content and fertility. Desertification
affects the lives of three-quarters of the world's population, 70% of all
drylands and one quarter of the total land area of the planet. There are
many reasons for desertification, but the majority are caused by
human activities, overgrazing, deforestation, poor land management
and over-exploitation. Agenda 21 states that the priority in combating
desertification should be establishing preventive measures for lands
that are not yet, or are only slightly, degraded. (Source: LBC / WRIGHT)
