Environmental Engineering Reference
In-Depth Information
in the 1970s and early 1980s (Callaghan
et al.
1985). Le Houerou (1977) has estimated that,
in the areas along the desert margins in Africa
and the Middle East, a family of five will
consume, every year, all of the fuel available on
one hectare of woody steppe. With a population
close to 100 million dependent upon this form
of fuel in the area concerned, as much as 20
million hectares per year are being destroyed, and
all of that area is potentially open to
desertification.
No change in the land-use is required to
initiate such a progression, in some regions. The
introduction of too many animals into an area
may lead to overgrazing and cause such
environmental deterioration that after only a few
years the land may no longer be able to support
the new activity. Forage species are gradually
replaced by weeds of little use to the animals,
and the soil becomes barren and unable to recover
even when grazing ceases. In all of these cases,
the land has been laid waste with little active
contribution from climate. Human activities have
disturbed the environmental balance to such an
extent that they have effectively created a desert.
Although human activities have been widely
accepted as causing desertification, and the
processes involved have been observed, there is
increasing concern that the human contribution
has been overestimated. Current academic and
popular attitudes to desertification owe a lot to
the findings of a United Nations Conference on
Desertification (UNCOD) held in Nairobi,
Kenya in 1977. At that conference, the role of
human activities in land degradation was
considered to be firmly established, and the
contribution of drought was seen as secondary
at best. Since human action had caused the
problem, it seemed to follow that human action
could solve it. In keeping with this philosophy,
UNEP was given the responsibility for taking
global initiatives to introduce preventive
measures which would alleviate the problem of
desertification (Grove 1986). Fifteen years and
$6 billion later, few effective counter measures
have been taken, and the plan of action is widely
seen as a failure (Pearce 1992a).
The data upon which the UNEP responses
were based are now considered by many
researchers to be unrepresentative of the real
situation. Nelson (1990), for example, has
suggested that the extent of irreversible
desertification has been over-estimated,
although he does not deny that it remains a
serious concern in many parts of the world. The
main problems with the data arose from the
timing and method of collection, and were
aggravated by the UNEP premise that human
activity was the main cause of the land
degradation that produced desertification. The
basic data, apparently indicating the rapid
creation of desert-like conditions, were collected
in the early 1970s at a time of severe drought in
sub-Saharan Africa. They therefore failed to
give sufficient weight to the marked rainfall
variability characteristic of the area, and in
consequence over-represented the effects of the
drought. A great deal of the information was
obtained using remote sensing. The changing
location of vegetation boundaries were
identified from satellite photography, for
example. This seemed to confirm the steady
encroachment of the desert in areas such as the
Sudan, and the results were incorporated in the
UNCOD report of 1977. Since then, however,
they have been widely disputed, and subsequent
studies have found no evidence that the large
scale desertification described in the 1970s
continued into the 1980s. (For summaries of
current thinking on desertification see Nelson
(1990), Pearce (1992f) and Hulme and Kelly
(1993).)
Failure to appreciate the the extent of annual
fluctuations in vegetation boundaries—
differences of as much as 200 km were reported
on the Sudan/Chad border between 1984 and
1985—combined with inadequate ground
control may have contributed to the problem
(Nelson 1990). A general consensus seems to be
forming among those investigating the issue that
the approach to defining desertification in terms
of vegetation needs to be re-examined. In parts
of East Africa, for example, drought and possibly
overgrazing have combined to allow the normal
