world's largest desert.

The Sahara (in Arabic, desert) encompasses an area of 3.32 million square miles (8.6 million sq km), stretching across eleven countries and Western Sahara, and covering nearly the entire northern region of Africa from the Atlantic Ocean to the Red Sea hills. Parts of the Sahara reach all the way north to the Mediterranean; to the south, it extends nearly 1,500 miles (2,400 km). The two countries with the highest percentage of desert are Libya (99 percent) and Egypt (98 percent). Fifteen percent of the Sahara consists of sand "seas"; the rest is a mixture of hammada (barren rocky plateaus), coarse gravel, two mountain chains in the central regions (with the highest point being 11,204 feet [3,417 m] at the peak of Emi Koussi in Chad), low lands, depressions (the lowest point being 436 feet [133 m] below sea level at the Qattara depression in western Egypt), oases, and transition zones.

The Nile and Niger are its only two permanent rivers. Transition zones receive between 5 and 10 inches (12.7 and 25.4 cm) of rain per year; most of the rest receives fewer than 5 inches (12.7 cm). Large portions of the area receive no rainfall for years at a time. Its climate is among the most inhospitable—the highest evaporation rates, highest temperatures, and lowest humidity (a life-threatening 2.5 percent) have all been registered there. Extreme wind velocities and massive drops in nighttime temperatures, sometimes to subfreezing level, are also a regular feature of the Sahara.

Desertification has slowly encroached upon previous transition zones, such as the sahel belt of vegetation covering fossil sand dunes that separate the Sahara from Equatorial Africa; some also occurs in Arab North African countries. The reasons for the Sahara's continuing expansion range from climatic changes to some direct human influence, such as overgrazing by sheep herds and wood gathering for fuel. The most important minerals found in the Sahara include petroleum and natural gas fields, uranium, phosphates, iron ore, and a long list of other metals.

The four main ethnic groups of the Sahara are all predominantly Berber in origin: the Arabo-Berbers in the north; the Moors (Maures), a mixture of Arab, Berber, and black African groups in the southwestern regions (encompassing parts of present-day Mauritania, Mali, and Western Sahara); the distinctive Twaregs, the most numerous of the four, of the south-central area; and the Tibu of the Tibesti area of Chad, who are also of Berber and black African mixture. Apart from livestock grazing, the old traditional economy included a profitable trade in gold and slaves from West Africa, salt from the desert, and cloth and other products from the Mediterranean coast. The camel, probably introduced in the second century b.c.e., was the backbone of trans-Saharan trade.

Before the prolonged droughts of the 1970s and 1980s, best estimates of the Sahara's population were approximately 2 million persons; about two-thirds were concentrated in oases; the rest engaged in seasonal movements and some were purely nomadic. In Arab North Africa, sedentarization had become almost complete, owing to the erosion of the pastoral economic base. Both "push" and "pull" factors were at work: desertification, which reduced livestock herds; displacement stemming from anti-colonial struggles; the exploitation of oil and gas fields, which provided employment; and the extension of governmental authority, resulting in increased enclosure of land for farming, as well as expanded health and education services.

Historically, the Sahara was a large barrier to aspiring conquerors—Egyptians, Romans, Carthaginians, and Arabs. Islam spread steadily, however, in part from the activities of Muslim traders and scholars. Explorers from Britain and France began to penetrate the Sahara in the early part of the nineteenth century. French conquests began in 1830. Political boundaries in the Sahara were defined only in the late nineteenth and the twentieth centuries. Much was left imprecise by the French, who ruled over most of the region, resulting in a number of border disputes after decolonization, including those between Morocco and Algeria over the Tindouf area, and Libya and Chad with regard to the Aozou strip.

see also aozou strip; berber; deserts; tindouf; twareg; western sahara.

bruce maddy-weitzman