Geoscience Reference
In-Depth Information
centennial to decadal fluctuations of the last 10,000 years. In these five chapters, we
offer a synthesis of past environmental and climatic changes in the deserts of Africa
(including the Arabian Peninsula), Asia, North and South America, and Australia.
Topics covered separately in the earlier specialist chapters are brought together for
the convenience of readers interested in those particular continents. Because the scope
and limitations of the type of evidence used to reconstruct past changes have already
been discussed in the topical chapters, the evidence is simply presented without further
elaboration.
The pressing issues of historic droughts and floods discussed in
Chapter 23
lead
us to consider the causes of, consequences of, and possible solutions to the thorny
problem of contemporary desertification (
Chapter 24
). Indeed, the very word 'desert'
comes from the Latin verb
deserere
(past participle
desertum
), meaning to abandon.
Implicit in the term 'desert' is the notion that these now dry areas were once able
to support more abundant life but have since become 'deserted'. There is growing
concern today that human actions are contributing to the spread of desert-like condi-
tions in previously fertile and well-vegetated land, a complex set of processes known
as 'desertification' (Mabbutt,
1978
; Mabbutt,
1979
; UNEP,
1992a
; UNEP,
1992b
;
Mainguet,
1994
; Williams and Balling,
1996
; Williams,
2000
; Williams
2002b
; Wil-
liams,
2004
). The causes of desertification are complex and often controversial; they
include droughts, human mismanagement and the aftermath of war, and they are
reviewed in the last three chapters of this volume. Two thousand years ago, the Roman
historian Tacitus (ca. 56-ca. 120 AD) wrote scornfully of the scorched-earth policies
favoured by some of the Roman emperors and their generals:
Ubi solitudinem faciunt
pacem appellant
('They create a desert and call it peace').
The Intergovernmental Panel on Climate Change (IPCC) volumes and cognate
research papers have shown the nexus between ecosystem responses to possible future
changes that could be triggered by global warming and the impact on societies living
in the arid, semi-arid and dry subhumid regions of the world.
Chapter 25
reviews these
issues, and the concluding
Chapter 26
provides a succinct but robust set of guiding
principles for achieving ecologically sustainable use of deserts. Because nearly half
of the land area of the globe is considered dry or seasonally dry (see
Tabl e 1 . 1
)and
provides a home for about a fifth of the world's present human population, these are
not trivial matters.
Whether reconstructions of past climates can provide us with a useful template for
assessing likely future changes is hard to gauge, but given that global climate models
are of necessity limited by the assumptions upon which they are based (which may
or may not be valid), it would be unwise to ignore what has already occurred. Indeed,
one could argue that knowledge of past events is often our only reasonably sure guide
to what might happen in the future.
