Geoscience Reference
In-Depth Information
1.5 Aims and structure of this volume
As noted at the beginning of this chapter, this topic has three main aims, all of which
are implicit in its title. The first aim requires us to consider very carefully the type
of evidence used to reconstruct past climatic changes in deserts and desert margins.
Until this is done, it is not possible to embark upon the second aim, which is to assess
how deserts have responded to past climatic changes. The third aim is to consider how
they might respond to future climatic changes. At this point it is pertinent to enquire
just what the term
climatic change
denotes. The answer has to be a qualified one,
given that it depends entirely on the scale (in time and space) at which the deserts are
being studied. As we shall see, different climate proxies provide different levels of
temporal and spatial detail, including the means of unravelling seasonal variations in
precipitation.
Although deserts share a number of common attributes or diagnostic characteristics,
each desert is unique and reflects the subtle interplay between local biophysical
influences, including rock type, tectonic history, climate and biota. So as to avoid the
pitfalls of over-facile generalisation, each of the world's larger deserts will provide
examples to illustrate the discussion of past, present and future change.
In order to set the scene for what follows,
Chapter 2
deals with the causes of
aridity,
Chapter 3
with the tectonic setting and geological evolution of the major
deserts,
Chapter 4
with adaptations to life in deserts and with the influence of fire
on the biota and
Chapter 5
with the history of climatic research in deserts. Because
chronology is essential to any history of past climatic change in deserts,
Chapter 6
explains the need for careful dating and describes some of the more commonly used
methods. Many attempts to reconstruct past environmental fluctuations on land and in
the ocean rely heavily on isotopic analyses, so
Chapter 7
explains some of the isotopic
and geochemical techniques most widely used in reconstructing past change.
Thenextninechapters(
8
through
16
) consider the degree to which dunes, dust,
rivers, lakes, glacial landforms, speleothems and tufas, soils, duricrusts and plant
and animal fossils can provide precise and accurate information about past climatic
change in deserts and desert margins. Desert margins are emphasised because deserts
have been much more extensive at intervals in the relatively recent geological past,
leaving behind a legacy of now fixed and vegetated sand dunes and other relics of
their presence, such as mantles of desert dust or loess on which soils have since
developed. One of these chapters (
Chapter 12
) summarises the reasons behind the
persistent debate over the nature and significance of
pluvial
episodes in deserts. The
use of archaeological evidence in interpreting past change and the role of prehistoric
humans in modifying deserts environments are covered in
Chapter 17
. The history
of past climatic fluctuations in each of the major deserts is reviewed in Chapters
18
through
22
, starting with the impact of Cenozoic cooling and desiccation before
proceeding to the millennial scale fluctuations of the past 2 million years and the
