(from the Arabic sahra, “desert”), a desert in North Africa and the largest tropical desert in the world, occupying roughly one-quarter of the African continent.
The desert occupies southern Morocco and Tunisia, most of Algeria, Mauritania, Libya, and Egypt, and the northern parts of Mali, Niger, Chad, and the Sudan. It stretches for a maximum of 5,700 km from east to west and as much as 2,000 km from north to south in the middle. Various methods are used to determine the boundaries of the Sahara, including the isohyets for 100 mm and 200 mm, the boundary of fruit-bearing date palms in the north and other botanical characteristics in the south, and indexes of aridity. Some geographers consider the Nile Valley to be the eastern boundary of the Sahara. In view of these differences, estimates of the area of the Sahara vary from 6 to 8 million sq km.
Topography. The relief is dominated by plains with elevations of less than 500 m; along the Atlantic and Mediterranean coasts the plains lie below 200 m. Major uplifts are found only in the central Sahara, where the Ahaggar (Tahat, 3,003 m) and Tibesti (Emi Koussi, 3,415 m) highlands bear traces of active Neogene and Anthropogene volcanism (lava fields and geyser deposits in the Tibesti massif). The highlands are dissected by deep canyons and wadis, the dry beds of ancient and present-day streams. South of the Ahaggar and Tibesti highlands are the Iforas (to 728 m), Air (to 1,900 m), and Ennedi (to 1,310 m) plateaus. There are many nondraining basins, some of them lying below sea level. Among the largest are the Chott Melghir in the Algerian Sahara (26 m below sea level) and the al-Fayyum, Siwa, and Qattara (133 m below sea level) in the Libyan Desert.
The desert landscape is highly diversified. Some 70 percent of the Sahara is occupied by rocky deserts (hammadas), by gravel deserts, called regs in the Algerian Sahara and serirs in the Libyan Desert, by sandy deserts (ergs), and by flat saline plains known as sebkhas. The remainder of the area consists of mountain landscapes with rock and gravel deserts. Thick accumulations of sand covering some 2.2 million sq km are found in the depressions and major wadis. The largest are the Erg Iguidi, Erg Chech, the Great Western Erg, and the Great Eastern Erg. The sands of the ergs, rising in ridges, are stabilized to some extent by xerophytic vegetation. There are ridges with elevations of up to 200–300 m, as well as circular and star-shaped dunes. Shifting sands are found in the southern parts of the northern and northeastern Sahara, in the Idehan Ubari, Idehan Marzuq, Ténéré, and Libyan Desert.
Geological structure and minerals. Geologically, the Sahara constitutes the northwestern part of the African-Arabian Platform, whose Precambrian basement emerges in the Rigaibat, Ahaggar, Tibesti, and al-Uwaynat massifs and in the western projection of the Nubian-Arabian Shield. Together, the outcrops form the central Sahara uplift zone. To the north and south of this zone stretch the north Sahara and south Sahara subsidence zones, filled by Phanerozoic deposits.
Minerals include petroleum and gas, iron ore (Kediet Ijill), and copper ore (Akjoujt). The Precambrian basement contains deposits of gold, and ores of rare metals, tungsten, and uranium are associated with younger intrusions.
Climate. Excluding the north, which has a subtropical climate, the Sahara has a marked tropical desert climate, dry and hot. The climate is strongly influenced by the northeasterly trade winds that prevail over most of the Sahara throughout the year. The present arid climate has lasted roughly 10,000 years. During the Ice Age the climate was more humid, and the prehistoric population engaged not only in livestock raising and farming but also in hunting and even fishing, as shown by cliff drawings in various parts of the Sahara.
Air dryness (a relative humidity of 30–50 percent), an enormous moisture deficit, and a high rate of evaporation (potentially 2,500–6,000 mm) are characteristic everywhere but in the narrow coastal zones. The north Sahara receives most of the precipitation in winter and the south Sahara, in summer. The annual precipitation, 100–200 mm in the border regions, decreases to less than 50 mm over most of the Sahara plains and is normally less than 100 mm in the mountains. In the interior there may be no rain for several years in a row, although occasional downpours cause flash floods in the wadis and mudflows in the mountains. Abundant morning dew (condensation caused by low nighttime temperatures) is typical for most of the Sahara and promotes the formation of a surface dust crust. Snow falls for a short time in the high parts of the Ahaggar and Tibesti highlands almost every year.
High daytime temperatures and great daily and annual fluctuations in temperature are characteristic. Temperatures may reach 56°-58°C, almost the highest on earth, and the surface of the ground may heat up to 70°-80°C. The mean July temperature reaches 37.2°C at Adrar. Ground frosts occur almost everywhere during winter nights, and in the central mountains nighttime temperatures as low as - 18°C have been recorded. Long windy periods and dust (or sand) storms lasting many days are frequent, with the wind velocity increasing from 3–5 m per sec to 50 m per sec or more. The winds are locally called sirocco, khamsin, or simoom.
Rivers and lakes. With the exception of the Nile, there are no permanent streams flowing through the Sahara. After heavy rainfall the dry watercourses of the north Sahara are filled with water, as are the channels of the ancient river network spreading out from the Ahaggar and Tibesti highlands (Igharghar, Tafasaset, and Tamanrasset wadis). Small relict lakes have survived in the marginal regions of the Sahara and in the central mountains. They are somewhat swampy and often strongly mineralized.
The Sahara has large basins of groundwater, including artesian water, most of it occurring in continental Lower Cretaceous sandstones. The groundwater supports life in the oases, which are usually located in low-lying areas where there are springs or where it is easier to establish wells. The north Sahara is richest in groundwater. Here, the underground flow is more abundant, and there are more artesian wells with significant reserves. In the south Sahara groundwater is less abundant and lies deeper, and there is virtually no artesian water. Active drilling for water is under way in the Sahara, and the new basins of underground water that have been discovered are being used for water supply in the petroleum- and gas-producing regions. In the northern Sahara the newly discovered groundwater reserves are used for irrigation as well.
Soils and vegetation. The typical soils of most of the Sahara are the primitive soils of tropical deserts and semideserts (gravel, pebble, and sand), frequently saline. Limestone and gypsum crusts ranging in thickness from a few centimeters to 1–2 m are widespread. The vegetation belongs to the Holarctic floristic region and includes many Mediterranean species. There are roughly 1,200 species of higher plants, although an average of only 150 species are encountered over 1,000 sq km, about one-tenth the number found in Europe and one twenty-fifth the number found in the humid tropics.
Differences in the vegetative cover are governed by the Sahara’s position in two climatic zones and by characteristics of the relief and surface lithology (whether the area is a rocky, sandy, or other type of desert). Most of the vegetation consists of perennial drought-resistant grasses and shrubs with extensive and deep (up to 15–20 m) root systems, as well as ephemerals that appear after rainfall. The vegetation is extremely sparse everywhere, and the regs, hammadas, and parts of the sand accumulations are completely devoid of vegetation. The mountain regions, where relicts of Neogene flora have survived and where there are many endemic plants, are richer in vegetation. The most common trees and shrubs in the south Sahara are various species of acacia, tamarisk, Ephedra and Genista.
Almost everywhere the vegetation has been strongly affected by human activity—livestock grazing, the gathering of useful plants, and fuel extraction. As a result of the large-scale destruction of vegetation and the cultivation of light sandy soils, the shifting sands are advancing the oases. In 1974, Algeria launched the Green Wall Project, under which stone pines, eucalyptuses, and other trees were planted to block the advance of the sands over a 1,500-km stretch. In addition to dates, the principal crop, figs, olives, citrus and other fruit, and various vegetables are raised in the oases. Doum palms are grown in the southern oases.
Wildlife. The fauna belongs to the Holarctic and Ethiopian zoogeographic regions and includes some 4,000 species, most of them invertebrates. Roughly 40 percent of all the species are African endemics, although not more than 10–12 percent are Sahara species. The highlands of the central Sahara are isolated habitats for typical Sahara animals. Excluding birds, more than 50 percent of which are migratory, there are very few species in the drier regions, where the biomass of animals drops to 2 kilograms per hectare or less.
There are only about 60 species of mammals in the Sahara, among them the addax, the ungulate best adapted to long periods without water and now in danger of extinction, several species of gazelles, the mouflon (Ahaggar and Tibesti highlands), and one species of monkey (Air Plateau and Tibesti Highlands). Predators include fennecs, and, in the border regions, jackals, cheetahs, and hyenas. Common rodents are the gerbils and jerboas; lagomorphs are represented by the Sahara hare. Lizards (skinks, monitors) and snakes (horned vipers, cobras) are found everywhere in large numbers. Small crocodiles inhabit the relict lakes in the Ahaggar Highlands. Most Sahara animals are nocturnal.
Population. Excluding the densely populated regions of the Nile Valley and Delta, the Sahara has a population of roughly 3 million, most of whom live in the northern part of the Algerian Sahara, in the western and southern border regions of the Sahara, and in groups of oases (about two-thirds of the population), where the density reaches 1,000 persons per sq km or more. The present ethnic makeup of the Sahara population is diverse, with Berber-Arab peoples predominating. There are roughly 30,000 Tuareg, nomadic herders related to the Berbers, living in the Ahaggar Highlands and on the Air Plateau. The Negroid Tibbu inhabit the Tibesti Highlands. The chief cities are Touggourt, Ouargla, Laghouat, Touat, and Tidikelt in Algeria, al-Kufrah, Sabhah, Tazerbo, Jaghbub, and Marzuq in Libya, and Siwa, Dakhilah, al-Farafirah, and al-Bahriyah in Egypt.
Economy. The traditional occupations of the Sahara population are nomadic livestock raising, the gathering of wild plants and fruits, and farming in the oases. Dates are the principal crop, but grains and vegetables are also raised. The nomads and seminomads raise camels, sheep, and goats. Each spring they set out from the oases with their herds in search of summer pastures.
During the 1950’s and 1960’s the discovery and exploitation of major petroleum and natural gas deposits in the Algerian Sahara and in Libya fundamentally changed the economic significance of the Sahara. It became one of the world’s major petroleum-producing regions. Rock salt is extracted at Taoudenni in Mali, hard coal at Kenadsa in Algeria, iron ore at Kediet Ijill in Mauritania (9.3 million tons in 1972), copper ore at Akjoujt in Mauritania, and phosphorites at Bu Craa in the Western Sahara.
In addition to the pipelines that connect the petroleum and gas deposits with the Mediterranean ports in Algeria, Tunisia, and Libya, motor-vehicle and air transport are important. The Trans-Sahara Highway from Algeria to Niger (Agades) has an asphalt surface as far as Aïn Salah. A railroad connects Abadía with northern Algeria and Morocco. Touggourt is linked by rail with the Mediterranean coast, and ore is carried by rail from Ijil to the Atlantic coast.
M. B. GORNUNG