Deserts of the World - Design with the Desert: Conservation and Sustainable Development - page 26

in the mid-Holocene or the intense pluvial or lacustral phase of tropical Africa in the early
Holocene), through to major Pleistocene events related to the glacial and interglacials
of higher latitudes (which may have had a duration of the order of 100,000 years), and
through to the long-term “geological changes” of the order of millions of years associated
with major shifts in the positions of the continents, major tectonic and orogenic events, and
the configurations of the ocean basins and their associated circulation systems.
The short-term fluctuations are related to changes in the general atmospheric
circulation, with such changes as the water temperatures of the Pacific Ocean associated
with the El Niño effect or the zonality or meridionality of the Rossby waves playing a
major rôle. 122 Medium-term fluctuations, such as the early Holocene pluvial of the tropics,
may be related to changes in Earth geometry, and, at around 9000 year BP, theoretical
insolation receipt during the Northern Hemisphere summer may have been larger than
now (by about 7%), thereby leading to an intensification of the monsoonal circulation
and associated precipitation. 69 At a longer timescale, related to the interglacial and glacial
fluctuations of the Pleistocene, a variety of factors may have been involved. For example,
during a major glacial phase, tropical aridity may have been heightened by an increased
continentality of climate resulting from the withdrawal of the sea from the continental
shelves as a consequence of glacial eustasy. Such a drop in sea level, together with an
extension of sea ice, would result in less evaporation from the ocean surface, leading to less
rain. Moreover, the worldwide cooling of the oceans would lead to less evaporation and
convection and the generation of fewer tropical cyclones.
Furthermore, thermal variations provoked by the growth and decay of the great ice sheets
decisively influenced the patterns of the general atmospheric circulation. For example,
in theory, an increased temperature gradient resulting from the presence of the great
Scandinavian and Laurentide ice sheets would result in stronger westerlies, an equatorial
displacement of major circulation features, and an intensification and shrinking of the
Hadley cell zone and the zone of extratropical Rossby wave circulation. The result would
have been a greater degree of westerly flow and midlatitude cyclogenesis over an area
such as North Africa. 123 A corresponding displacement of the subtropical high-pressure
zone would have displaced the aridity maximum into West Africa. The decreased thermal
contrast between the two hemispheres compared to the situation today would have had
an impact. At present, the Southern Hemisphere, in comparison with the Northern, is
much cooler and its temperature gradient greater. This is because of the varying amounts
and distribution of ocean and land in the two hemispheres. In the Southern Hemisphere,
the stronger temperature gradient produces a more intense atmospheric circulation and
is probably largely responsible for the asymmetry that exists, whereby the meteorological
equator lies in the Northern Hemisphere. During a glacial phase, intense continental
glaciation should have led to a displacement of this meteorological equator to a position
more coincident with the geographical equator (i.e., southward). This would disrupt the
annual march of the monsoon and thereby reduce precipitation amounts in such areas as
the Thar and southeast Arabia.
The trigger for the glacial/interglacial fluctuations themselves is now thought to lie in
the Milankovitch mechanism of Earth orbital fluctuations, with amplitudes of the order of
about 100,000, 43,000, and 19,000-24,000 years. 124 Spectral analysis has shown such wave-
lengths in ocean-floor sediments and in Chinese loess profiles.125 125
The longest scales of change, which include the establishment of the world's arid areas,
are related to variations in the configurations of the continents and oceans, occasioned
by global tectonics. This mechanism operated in a variety of ways. First, the drift of the
continents changed their latitudinal positions and thus their position with respect to major