LATITUDINAL DESERT BELTS
CONTINENTALITY AND INLAND DESERTS
COASTAL DESERTS: THE EFFECT OF MARINE UPWELLINGS
ON DESERT DISTRIBUTION
RAIN SHADOWS AND TROPICAL DESERTS
Looking at a satellite image of the whole earth it is easy to spot a series of conspicuous ochre, vegetation-barren areas that run parallel to the equator, in both the northern and southern hemispheres, along two East-West fringes at i5-35° latitude (Figure 1.1). They are the mid-latitude deserts of the world, lying some
2 000-4 000 km away from the equatorial
rainforests. In the northern hemisphere, the succession of mid-latitude subtropical deserts is formed by (1) the Mojave, Sonoran, and Chihuahuan Deserts in North America, (i) the Sahara’s immense swathe in Northern Africa and the Somali-Ethiopian deserts in the Horn of Africa, and (3) the deserts of Asia, including the Arabian, Mesopotamian, Persian, and Thar deserts that stretch from West Asia into Pakistan and India, as well as the Central Asian deserts in Uzbekistan, Turkmenistan, and the Taklimakan and Gobi deserts in China and Mongolia. In the southern hemisphere, the chain is formed by (1) the Atacama, Puna, and Monte Deserts in South America, (i) the Namib and the Karoo in southern Africa, and (3) the vast expanse of the Australian deserts (Allan and others 1993, McGinnies and others 1977, Pipes 1998, Ricciuti 1996).
There are many criteria to define a desert but perhaps the most important one is aridity — the lack of water as the main factor limiting biological processes. One of the most common approaches to measure aridity is through an estimator called the Aridity Index, which is simply the ratio between mean annual precipitation (P) and mean annual potential evapotranspiration (PET, the amount of water that would be lost from water-saturated soil by plant transpiration and direct evaporation from the ground; Thornthwaite 1948). Arid and hyperarid regions have a P/PET ratio of less than 0.i0; that is, rainfall supplies less than i0 per cent of the amount of water needed to support optimum plant growth (UNEP 1997, FAO 2004). Aridity is highest in the Saharan and Chilean-Peruvian deserts, followed by the Arabian, East African, Gobi, Australian, and South African Deserts, and it is generally lower in the Thar and North American deserts. Although the aridity indices vary in the different deserts in the world, all of them fall within the arid and hyperarid categories (Table 1.1).
Thus, the global map of arid and hyperarid regions
can be used as a good approximation to the
boundaries of the Desert Biome (Figure 1.2a).
A bio-ecological criterion can also be used to
map the world's deserts, by lumping together all
the ecoregions of the world that harbour desert
vegetation (identified by the xerophilous life-forms
and the general desert-adapted physiognomy
of the dominant plants). The resulting set of
biologically desert-like ecosystems, modified
from Olson and others ( 2001), provides a second
approximation to the Desert Biome (Figure 1. 2b;
see also Appendices 1 and 2).
A third criterion can be derived from AVHRRsatellite
images of the world. Using a land-cover
index (NDVI, or Normalized Difference Vegetation
Index) the earth has been classified into different
land-cover categories (GLOBIO 2005, USGS
2005). The global map of deserts and semideserts,
defined as large uniform regions with extremely low
vegetation cover, may be used as an alternative
approximation to the Desert Biome (Figure 1. 2c).
Although each approach may have its own sources
of error and the three differ in their definition of what
is a desert, it is surprising how the three alternative
maps coincide (Figure 1.3). The Desert Biome, in
short, is formed by a set of geographic regions
characterized by (a) extremely high aridity, (b) a
large proportion of bare soil, and (c) plants and
animals showing well-defined adaptations to survive
in extremely dry environments. A desert, then, is
a region with very little vegetation cover and large
surfaces of exposed bare soil, where average annual
rainfall is less than 0 percent of the amount needed
to support optimum plant growth, and where plants
and animals show clear adaptations for survival
during long droughts.