Geoscience Reference
In-Depth Information
2
Present-day desert environments
El sharia el howa
The way of the wind
Arabic expression for desert tracks known only to smugglers and locals
2.1 Introduction
Before considering why deserts are arid and when this aridity set in, it is useful to
consider where the world's major deserts are presently situated. The distribution of
the world's major deserts (
Chapter 1
,
Figure 1.1
) is closely linked to latitude and
to distance from the sea. The Saharan and Arabian deserts, which extend eastwards
across the deserts of Iran, Afghanistan and Pakistan into the Thar Desert of India,
lie on or close to the Tropic of Cancer. The deserts of Australia, the Kalahari and
the Atacama are traversed by the Tropic of Capricorn. On the other hand, the deserts
of central Asia, including the Taklamakan and Gobi deserts of China and Mongolia,
are situated in the interior of mid-latitude continental regions. A number of deserts
are also located in the rain shadow of high mountain ranges, such as the Andes, the
Rockies, the Himalayas and the Altai, Tian Shan and Kunlun ranges in central Asia.
Why is this so?
Two sets of factors are responsible for this very particular distribution pattern. One
involves the tectonic events that culminated in the global cooling and desiccation of
the Cenozoic, a topic discussed in the next chapter (
Chapter 3
); the other is bound up
with the present global atmospheric circulation system, itself a product of Cenozoic
and earlier tectonic history. If we accept Lyell's dictum that in matters geological the
present is the key to the past, it is logical to begin with the causes of present-day
aridity before turning to the evidence for past aridity.
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