Geoscience Reference
In-Depth Information
the plant cover along the southern borders of the Sahara, such an averaging has very
little meaning. The 1950s were wetter-than-average years in northern Sudan, and the
Lamprey survey came after a decade of below-average rain.
Other workers have made similar poorly supported claims, generally in the wake
of several years of drought. See, for example, the publications of the forester E.P.
Stebbing (
1935
;
1937a
;
1937b
;
1938
), with their apocalyptic titles about 'the encroach-
ing Sahara' and 'the man-made desert in Africa'. Unfortunately, this type of strident
oversimplification becomes widely circulated and quoted out of context. For instance,
in their popular account
Population, Resources, Ecology: Issues in Human Ecology
,
Paul and Anne Ehrlich write expansively that 'the vast Sahara desert itself is largely
man-made, the result of overgrazing, faulty irrigation, deforestation, perhaps com-
bined with a shift in the course of a jet stream. Today the Sahara is advancing
southward on a broad front at a rate of several miles per year' (Ehrlich and Ehrlich,
1970
, p. 166). Such a claim (echoing Lamprey,
1975
) is patent nonsense. We have
already seen in Chapters
1
to
3
that the deserts predate the advent of humans by many
millions of years and that they are where they are for sound geographical reasons that
have nothing to do with humans. What we can say with some confidence is that during
colder, drier and windier intervals throughout the late Quaternary, such as during Last
Glacial Maximum 20,000 years ago, many of the tropical deserts of Africa and Asia
were indeed more extensive than they are today, meaning that the presently vegetated
and stable dunes along their now semi-arid margins were active at those times, as was
discussed in
Chapter 8
.
24.4 Causes of anthropogenic desertification
As an exercise in perception, the author once asked senior government natural resource
managers from many parts of Asia and some parts of Africa and South America to
list what they considered to be the primary causes of desertification. The resulting
list of causes tallies almost exactly with those identified in the chapter on deserti-
fication in
Agenda 21
, which had been published by that time and widely circulated
among relevant government departments and agencies. The causes identified were
deforestation, overgrazing, population pressure, inappropriate agricultural practices
(shifting cultivation with too short a fallow; cultivating on very steep slopes; overuse
of fertilizers), over-irrigation, poor drainage, fire, drought, floods and climate change.
Conversations that the author has held at intervals over the last fifty years with cul-
tivators and pastoralists in northern Africa (Sudan, Ethiopia, Niger, Kenya, Somalia,
Tunisia, Algeria), arid northern China (Xinjiang Province and Inner Mongolia) and
the drier parts of India (Rajasthan, Andhra Pradesh and Madhya Pradesh) suggest that
other causes may be of equal importance, at least at a local or regional level. These
include low prices for cash crops and high prices for subsistence items such as grain,
invasions of weeds and plants unpalatable for livestock, an increase in the frequency
