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Hyper-arid
Arid
Semi-arid
Dry-subhumid
Figure 1.2
The global distribution of drylands.
oceans due to the effects of tectonic plate movements and
orogeny, and changes in global climate, have, however,
caused the positions and extent of arid zones to change
through geologic time.
Sedimentary evidence suggests that the Namib is prob-
ably the oldest and most persistent of current arid zones
on Earth, dating back 80 million years to the Cretaceous
(Ward, Seely and Lancaster, 1983), though such a great
antiquity by no means meets with full agreement (see
Tankard and Rogers, 1978; Vogel, Rogers and Seely,,
1981). Notwithstanding the effects of subsequent climatic
perturbations on the extent and intensity of aridity, many
other deserts would seem to date from the Tertiary. Deep
sea-core evidence indicates that aeolian material existed
off West Africa from 38 million years ago in the Oligocene
(Sarnthein, 1978), while other deserts such as those of
Australia (Bowler, 1976) and the Atacama (Clark et al. ,
1967) date from the Miocene.
1.5
The distribution of arid zones
Table 1.2 shows the distribution of arid zones today ac-
cording to continent. While Africa and Asia each contain
almost a third of the global arid zone, inspection of Figure
1.2 clearly shows that Australia is the most arid continent,
with approximately 75 % of the land area being arid or
semi-arid.
Table 1.1
The extent of the global drylands (expressed as a percentage of the global land area).
Classification
Dry-subhumid
Semi-arid
Arid
Hyper-arid
Total
Koppen (1931)
14.3
12.0
26.3
Thornthwaite (1948)
15.3
15.3
30.6
Meigs (1953)
15.8
16.2
4.3
36.3
Shantz (1956)
5.2
24.8
4.7
34.7
UN (1977) a
13.3
13.7
5.8
32.8
UNEP (1992)
9.9
17.7
12.1
7.5
47.2
 
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