Geoscience Reference
In-Depth Information
the Namib Desert of southern Africa; Fig. 5.2).
Gobabeb research station (Namibia) data records
show that coastal fog is a significant contributor
of moisture, even though the research station
is some 60 km inland and 408 m a.s.l. Here fog
moisture may well exceed rainfall in some years.
Another significant source of moisture may be
dew. Fewer records on dew are available but
data from the Negev, Israel, suggest that dew
occurs on an average of 195 nights per year at
the ground surface, and provides 33 mm yr
−1
of
moisture, although only 0.35 mm for any one
dew night (Evanari et al. 1982).
thus coastal climates. It has been postulated that
the closure of one such gateway reinforced the
hyper-aridity of the Atacama Desert of South
America (Hartley 2003).
5.1.2.3 Anthropogenic agents
Humans have been credited with forming and
expanding many modern deserts (Ehrlich &
Erlich 1970). The expansion of desert areas
through human intervention is known as
'desertification' (
sensu
Mabutt 1978, 1985).
This process, and its definition, however, has
become increasingly disputed (e.g. Williams
1994). There is no doubt that mismanagement
of land through overgrazing and soil saliniza-
tion has occurred since the Holocene increase
in human population. The human population
is estimated to have increased from 10
7
at the
start of the Holocene Epoch (10 ka) to 10
8
by
5 ka and 10
9
by 2 ka (May 1978). Typically,
however, the increase in desert areas is tem-
porary and associated with intervals of drought,
which in most cases return to their natural non-
desert habit once the drought has ended and
recovery set in. In 1992 the Rio Earth Summit
thus defined desertification as 'land degrada-
tion in arid, semi-arid and dry subhumid areas
['susceptible drylands'] resulting from various
factors, including climatic variations and human
actions' (UNEP 1994).
5.1.2.2 Tectonics
Arid environments can be found in a range of
tectonic settings. These include currently stable
intraplate settings such as cratons (e.g. the
Australian Desert) and passive margins (e.g.
the Namib). Arid regions can also be found in
extensional tectonic settings such as the Mojave
and Chihuahua deserts, and compressional con-
tinental margins such as the Atacama and Thar
deserts. Some deserts, such as the Sahara, are so
large that they may include several tectonic set-
tings (the Sahara spans all of the above settings).
Despite the apparent lack of correlation between
tectonic settings and arid region occurrence,
tectonics are a major control on the location and
persistence of arid environments. Tectonics con-
trol the regional and global topography of the
continents, which in turn has significant effects on
the local climate. The main impact of tectonics
as a control on the development of arid areas is
through the two processes listed below.
1
Orogeny.
Tectonics can generate mountains,
which create local rain shadows. On a larger
scale, large orogenic belts such as the Himalayas
can divert major air circulation, leading to
aridity. The associated uplift will also lead to an
increase in the continentality of an area.
2
Continental drift.
Tectonics can affect latitude
over many millennia through continental drift
(consider for example Britain which was much
closer to the Equator 250 Ma in the Permo-
Triassic, and subject to desert conditions). Plate
tectonics can also lead to the closure of oceanic
gateways, which can affect oceanic currents and
5.1.3 Variability of arid environments
through time
The geological record of aeolian dunes and eva-
porite sediments indicates that arid areas have
been present since the Precambrian (Glennie
1987). The Atacama is probably the oldest
extant desert in the world (Hartley et al. 2005),
dating back to the Jurassic (some 150 Ma), but
most deserts have retained their position over the
last 2 Ma. Although the locations have remained
relatively fixed over these geological time-scales,
the actual surface area has changed. For example,
some of the drier areas of Africa appear to have
been expanding as a result of persistent droughts.
Typically, however, these fluctuations are tem-
porary. Quaternary records from tropical and
