The
Sahara (
Arabic:
الصحراء الكبرى,
aṣ-Ṣaḥrāʾ al-Kubrā , ‘the Greatest Desert’) is the world’s hottest
desert, the third largest desert after
Antarctica and the
Arctic.
[1] At over 9,400,000 square kilometres (3,600,000 sq mi), it covers most of
North Africa, making it almost as large as
China or the
United States. The Sahara stretches from the
Red Sea, including parts of the
Mediterranean coasts, to the outskirts of the
Atlantic Ocean. To the south, it is delimited by the
Sahel, a belt of
semi-arid tropical savanna that composes the northern region of central and western
Sub-Saharan Africa.
Most of the rivers and streams in the Sahara are seasonal or intermittent, the chief exception being the
Nile River, which crosses the desert from its origins in central Africa to empty into the Mediterranean. Underground
aquifers sometimes reach the surface, forming oases, including the
Bahariya,
Ghardaïa,
Timimoun,
Kufra, and
Siwa.
The central part of the Sahara is
hyper-arid, with little vegetation. The northern and southern reaches of the desert, along with the highlands, have areas of sparse grassland and
desert shrub, with trees and taller shrubs in
wadis where moisture collects.
To the south, the Sahara is bounded by the Sahel, a belt of dry
tropical savanna with a summer rainy season that extends across Africa from east to west. The southern limit of the Sahara is indicated botanically by the southern limit of
Cornulaca monacantha (a drought-tolerant member of the
Chenopodiaceae), or northern limit of
Cenchrus biflorus, a
grass typical of the Sahel.
[12][13] According to climatic criteria, the southern limit of the Sahara corresponds to the 150 mm (5.9 in) isohyet of annual precipitation (this is a long-term average, since
precipitation varies annually).
[14]
Climate
The climate of the Sahara has undergone enormous variations between wet and dry over the last few hundred thousand years.
[15] During the last
glacial period, the Sahara was even bigger than it is today, extending south beyond its current boundaries.
[16] The end of the glacial period brought more rain to the Sahara, from about 8000 BC to 6000 BC, perhaps because of
low pressure areas over the collapsing
ice sheets to the north.
[17]Once the ice sheets were gone, the northern Sahara dried out. In the southern Sahara though, the drying trend was soon counteracted by the
monsoon, which brought rain further north than it does today. The monsoon season is caused by heating of air over the land during summer. The hot air rises and pulls in cool, wet air from the ocean, which causes rain. Thus, though it seems counterintuitive, the Sahara was wetter when it received more
insolationin the summer. This was caused by a stronger tilt in Earth’s
axis of orbit than today, and
perihelion occurred at the end of July around 7000 BC.
[18]By around 4200 BC, the monsoon retreated south to approximately where it is today,
[9] leading to the gradual
desertification of the Sahara.
[19] The Sahara is now as dry as it was about 13,000 years ago.
[15] These conditions are responsible for what has been called the
Sahara pump theory.
The Sahara has one of the harshest climates in the world. The prevailing north-easterly wind often causes
sand storms and
dust devils.
[20] When this wind reaches the Mediterranean, it is known as
sirocco and often reaches hurricane speeds in North Africa and southern Europe. Half of the Sahara receives less than 20 mm (0.79 in) of rain per year, and the rest receives up to 100 mm (3.9 in) per year.
[21] The rainfall happens very rarely, but when it does it is usually torrential when it occurs after long dry periods.
The southern boundary of the Sahara, as measured by rainfall, was observed to both advance and retreat between 1980 and 1990. As a result of
drought in the Sahel, the southern boundary moved south 130 kilometers (81 mi) overall during that period.
[22]Recent signals indicate that the Sahara and surrounding regions are greening because of increased rainfall. Satellite imaging shows extensive regreening of the Sahel between 1982 and 2002, and in both Eastern and Western Sahara a more than 20-year-long trend of increased grazing areas and flourishing trees and shrubs has been observed by climate scientist Stefan Kröpelin.
[23]
Political Affairs 