Geoscience Reference
In-Depth Information
1
Climatic change in deserts: An introduction
The desert shall rejoice, and blossom as the rose.
Isaiah 35.1
1.1 Introduction
In Topic Four of
The Histories
, Herodotus (ca. 485-425 BC) repeats the tale of a
group of people from the small town of Sirte in northern Libya who, goaded into an
irrational fury by the south wind that had dried out their water storage tanks, declared
war on the wind and marched into the desert, where 'the wind blew and buried them
in sand'. It was not always thus. A few thousand years earlier, numerous bands of
cattle herders roamed what were then the vast grassy plains of the Sahara, and before
then herds of African herbivores including antelopes, giraffes and even elephants
had ventured into what was at that time a well-watered savanna landscape strewn
with perennial rivers and freshwater lakes. The evidence of these past changes is still
obvious to the observant traveller. Scattered across the 5,000 km width of the Sahara
fromMauritania to the Red Sea is an abundance of beautifully executed rock paintings
of Neolithic cattle, sheep and goats, as well as rock engravings of the wild herbivores,
all of which were forced out by a progressively drier climate.
As the Saharan example shows, deserts are superb repositories of past climatic
events. The very aridity to which they owe their existence has facilitated the preserva-
tion of landforms, sediments and soils developed under very different environmental
conditions, as well as evidence of the former presence of plants, animals and pre-
historic humans in areas now too arid to support much life. Contrary to the popular
view of deserts as regions almost entirely covered by sand dunes - only a fifth of the
Sahara is so covered - deserts are more likely to consist of rugged mountain ranges
and dissected plateaux interspersed with vast gravel plains, intermittently active rivers
and sporadically flooded lakes (
Figures 1.1
to
1.6
). Indeed, many of the landforms that
are considered so characteristic of deserts are in fact inconsistent with present-day
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