Geoscience Reference
In-Depth Information
26
Towards sustainable use of deserts
He prayeth best, who loveth best
All things both great and small
Samuel Taylor Coleridge (1772-1834)
The Rime of the Ancient Mariner
(1798)
26.1 Introduction
In the opening chapters of this volume, we saw that the distribution of our deserts is
governed by long-established tectonic, topographic and climatic factors and that the
onset of climatic desiccation in all of the world's deserts began many millions of years
ago and had nothing to do with any recent human influence. Evidence from pollen
analysis, molecular genetics and sediment geochemistry, discussed in later chapters
of the volume, also confirms the antiquity of the deserts. However, within and beyond
the confines of the present-day deserts, there is abundant and well-dated evidence
that past climates in what are now arid and semi-arid environments have frequently
been wetter than they are today. This evidence is derived from defunct river systems
and fossil lakes within the deserts, with their rich stores of aquatic fossils, as well as
from cave deposits and fossil soils. We also saw that the response of rivers, lakes and
glaciers to climatic change is often quite variable, so we should avoid relying on only
one source of information.
The fossil remains of the first bipedal hominids date back more than 5 million
years in what is today the arid Afar Rift of Ethiopia and the desert region of northern
Chad. The rich vertebrate fauna from these two areas indicates that the climate at that
time was somewhat less arid than it is today. These wetter intervals alternated with
more arid phases, during which vast deserts like the Sahara were even more extensive
than at present and desert dunes were active well beyond their existing limits. At
these times, great plumes of desert dust were blown out to sea, often as far as the
Amazon Basin, Greenland and Antarctica. Some of this dust remained on land to form
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