Location and General Description

From the Atlantic Ocean in the west, the greater Sahara stretches across Africa to the Red Sea and down to the highlands of Ethiopia, encompassing an area 9,100,000 square kilometers (km²). This ecoregion covers the central Sahara Desert, between 18° and 30° N, and has an area of 4,619,260 km². The northern and southern margins of the Sahara, which receive more rainfall and have greater vegetation cover, are described separately.

The surface of the desert ranges from large areas of sand dunes (erg Chech, Raoui), to stone plateaus (hamadas), gravel plains (reg), dry valleys (wadis), and salt flats. Several deeply dissected mountain massifs (Ahaggar, Tassili N'Ajjer, Tibesti, and Aïr) rise from the desert areas, and are delineated as separate ecoregions. Vast underground aquifers that underlie much of the region sometimes penetrate the surface, resulting in oases.
The Sahara is located in a climatic divide. The Intercontinental Convergence Zone moves up from the south, but stops before the center of the Sahara, and consequently hardly carries any rain. Similarly, the winter rainfall of North Africa does not reach far south enough to regularly bring rain to the central Sahara. Consequently, the rainfall, albeit extremely rare, can fall in any season. The annual rainfall is below 25 millimeters (mm), and in the eastern part of the desert it is less than 5 mm per annum. The scarcity of rainfall in this ecoregion is aggravated by its irregularity, as no rain may fall for many years in some areas, followed by a single intense thunderstorm.
The Sahara is one of the hottest regions in the world, with mean annual temperatures exceeding 30°C. In the hottest months, temperatures can rise over 50°C, and temperatures can fall below freezing in the winter. A single daily variation of -0.5°C to 37.5°C has been recorded. The Sahara is also extremely windy. Hot, dust-filled winds create dust devils which can make the temperatures seem even hotter.
The extreme aridity of this area is a relatively recent feature. Much larger areas of the Sahara had adequate water only 5000 to 6000 years ago. It is not clear how much of this ecoregion was covered with vegetation, but in other parts of the Sahara the vegetation was closer to the savanna woodlands of eastern and southern Africa. Currently the ecoregion is in a "hyper arid" phase, with high summer temperatures, lower winter temperatures and rainfall between 0 and 25 mm per annum.
Precambrian rocks are exposed in few places across the Sahara. During the Mesozoic much of North Africa was under water and marine deposits were deposited. The area was uplifted in the Middle Tertiary and has been eroding ever since. Shifting sands and bare rocks cover only about one-fifth of the greater Sahara. More than half of the area comprises soils known as yermosols, with shallow profiles over gravel or pebble beds. These soils have been developing over the past 50 million years.
In terms of the phytogeographical classification of White, the ecoregion is classified as the Sahara regional transition zone. Throughout most of the ecoregion there is very little perennial vegetation. Where it occurs, it is confined to areas where ground water reaches the surface or to areas with runoff. The plants that are present tend to be much more diversified in western Sahara than eastern Sahara, due to the lack of rain to the east. They have strictly Sahara-Arabian affinities, with exceptional adaptations to aridity. Large expanses of ergs and regs will be devoid of any visible plant life for years, but following rainfall, vegetation cover may reach more than 50 percent on sand dunes and 20 percent on the gravel plains.