DESERTS ARE AN environmental extreme, characterized by low rainfall that is highly variable intra-annu-ally and interannually, with a coverage of about 27.7 Mkm.2, equivalent to 35 percent of the earth’s surface and, thus, representing one of the largest terrestrial biomes. They occur mainly within latitudes between 5-35 degrees north of the equator and are characterized by very high aridity, very little vegetation cover and large surfaces of exposed bare soil, and high adaptations of plants and animals for survival during long droughts. According to bio-ecological definitions, the world’s deserts represent all ecoregions of the world that harbor desert vegetation, identified by the xerophilous life-forms and the general desert-adapted physiognomy of the dominant plants.
Desert climate is characterized by precipitation of less than 9.84 in. (250 mm.) with very high variability, high diurnal variations of temperature, and strong solar radiation. Desert air is very dry; incoming solar and outgoing terrestrial radiation are intense, with large daily temperature fluctuations; and potential evaporation is high. Extreme desert systems already experience wide fluctuations in rainfall and are adapted to coping with sequences of extreme conditions.
Deserts can be hot or cold. Among the hot deserts are ones with two rain seasons (Sonora desert, Karoo); with one rainy season (Northern Sahara, Mohave Desert, Middle-Asian deserts); deserts with summer rain (Southern Sahara, inner Namib, Atacama); deserts with few rains at any season (Central Australia); coastal deserts without rains, but with fog (North Chilean coastal deserts, outer Namib); and deserts without any rain or vegetation (Central Sahara). The Sahara in north Africa and the Namib desert in southwest Africa are classified as the hottest deserts in the world, with average monthly temperatures above 86 degrees F (30 degrees C) during the warmest months and extremes above 122 degrees F (50 degrees C). The diurnal temperature range often is large; winter nights in the Namib Desert sometimes are as cold as minus 14 degrees F (10 degrees C) or lower.
Aridity is the most prominent indicator, commonly measured by the Aridity Index, an estimator for the ratio between mean annual precipitation and mean annual potential evapotranspiration, which is less than -40 for arid deserts and -20 for semi-deserts. Aridity is highest in the Saharan and Chilean-Peruvian deserts, followed by the Arabian, East African, Gobi, Australian, and South African deserts, and lower in the Thar and North American deserts. This high aridity, as well as typical pulse-type variations in desert environments, are caused by global atmospheric and oceanic phenomena, such as the position of the jet streams, the movement of polar-front boundaries, the intensity of the summer monsoon, El Nino Southern Oscillation events, and even longer-term ocean cycles, such as the Pacific Decadal Oscillation.
Climate change
Driven by these large-scale forces, the intensity and frequency of rain pulses at a local scale may vary substantially with time, and future projections indicate an increasing likelihood of even more episodic climate events and inter-annual variability in deserts. Continental deserts could experience more severe, persistent droughts. In the Americas, temperate deserts are projected to expand substantially under doubled CO2 climate scenarios. About one third of the Sahel was projected to aridify with warming of 2.7-3.6 degrees F (1.5- 2 degrees C) by about 2050, with a general equa-torward shift of vegetation zones. Alternative climate scenarios show less pronounced changes, even shifts to wetter climates in the Sahel zone and movement of vegetation zones, accordingly. The potentially positive impact of rising atmospheric CO2 remains a significant uncertainty, which also could offset current projections, especially because it is likely to increase plant productivity, leading to contrasting scenarios.
Deserts also play an important role in the regulation of climate, air quality, and atmosphere composition of remote areas through wind-blown desert dust; also the desert albedo influences rainfall and bio-geochemistry of other terrestrial and marine ecosystems, in this way increasing vulnerability of remote regions to climate change. The 4th Intergovernmental Panel on Climate Change (IPCC) assessment projects increased dust flux rates, which may increase aridity and suppress rainfall outside deserts, with opposite effects under wetting scenarios, associated with a decline of atmospheric burden by soil aerosols of 20-60 percent. Changes of rainfall in deserts are projected to be in the range of 0.2 mm. day-1.
Erosion
Typical desert soils are aridisols, characterized by little weathering of the maternal rocks and low organic matter in the surface layer, formed under the typical influences of desert conditions by strong winds, scattered but torrential rains, and high temperatures. The materials in these soils are often cemented together forming water-impervious hardpans, sometimes containing salts or gypsum. The low soil cover exposes deserts to more wind erosion than any other environment, and to water erosion, as well, if slopes are steep and rain does fall. Desert landscapes come in two categories: shield deserts and mountain-and-basin deserts.
Very low biomass cover is very likely to make some desert dunes susceptible to aeolian erosion, and, with regional warming of between 4.5-6.3 degrees F, (2.53.5 degrees C) most dune fields could be reactivated by 2100. About 10-20 percent of deserts are ecologically degraded by an imbalance between demand and supply for ecosystem goods and services. Because of the extremely slow rate of biological activity in deserts, these ecosystems take decades, if not centuries, to recover from even slight damage. Moreover, because traditional livelihoods in deserts require large areas, they are particularly vulnerable to political and environmental changes. Irreversible damages have been caused in previously good agricultural grounds in deserts by large-scale modern developments, such as dam construction for water and energy supplies.
Biodiversity
Previous, present, and future climate regimes structure desert ecosystems in a way that requires a physical and behavioral adaptation to the patch dynamics of primary production, water, and nutrient cycling in space and time. During pulses of bounty, the fragile seedlings of desert plants can germinate, establish, and prepare for long droughts, burying their roots deep into the desert soils and, to a large extent, it is this heterogeneity of pulses that drives the high biodiversity of desert ecosystems. Many organisms in the deserts already are near their tolerance limits. Desert biodiversity is likely to be vulnerable to climate change, especially in biodiversity "hotspots," such as in the succulent Karoo biome of South Africa, where 2,800 plant species face potential extinction as bioclimatically suitable habitat is reduced by 80 percent with a global warming of 2.7-4.9 degrees F (1.5-2.7 degrees C).
Deserts support about 10 people per sq. km. Humans in deserts undergo considerable dehydration, and therefore, have to cope with the dry environment for their survival with a panoply of behavioral, cultural, and technological adaptations. Traditionally, desert-dwellers were of three types: hunter-gatherers, pasto-ralists, and farmers, whose livelihoods are adapted to the spatial and temporal patchiness of their environment. The movements of pastoralists, for instance, mimic the variability and unpredictability of the landscape and range reserves. Desert agriculture occurs mostly around oases and desert rivers, which often provide silt and nutrients through flooding cycles.
Finally, the specific aesthetic features and atmospheres of deserts, their silence, wideness, beauty, bareness, and emptiness has always created an intimate, spiritual relationship between humans and the desert landscape, leading to the creation of all three monotheistic religions in desert regions. They still remain places of spiritual inspiration and meditation today.
The Namib Desert can range from 14 to above 122 degrees F; such desert variations can be connected to global phenomena.
