Why is the sahara a desert?

Question

For my homework Lol

Answer 1

Desert landscapes are mostly moulded by the wind but water has also played its part. many deserts including the Sahara once had climates that were far more moist than they have today. deserts are often completely dry for several years, but many centimetres of rain may fall in a few hours during one freak storm.

the chief agent of erosion, transportation and deposition in deserts is the wind. wind-blown sand polishes rocks and cuts low caves in cliffs. in many deserts wind-blown sand carves out deep hollows.

Answer 2

Around 5,000 BC, there was a sudden shift in the weather pattern of the Sahara.  The area's Monsoonal climate disappeared to the south and never returned.  The shift was caused by El Nino in the Atlantic Ocean changing course and leaving the Sahara the vast desert it is today.

Answer 3

Ignore kt's and dylan's answers. One is way off and the other is just a jumbled and hard-to-read copy and paste off of Wikipedia. Just post the links people, sheesh!

Lets start off with what is a desert. The thing all deserts have in common is that they are dry. The exact figure is that they all receive less than ten inches of rain a year on average. Most of the Sahara gets only a few inches of rain per year, so it fits nicely.

Answer 4

because before it used to be an ocean

Answer 5

The history and reason the sahara is a desert.

The Sahara (Arabic: الصَحراء الكُبرى‎, aṣ-ṣaḥrā´ al-koubra, "The Great Desert", (صحراء (help·info))) is the world's largest hot desert and the world's second largest desert after Antarctica [1]. At over 9,000,000 square kilometres (3,500,000 sq mi), it covers most parts of northern Africa; an area stretching from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean. It is almost as large as the continental United States, and is larger than Australia. The Sahara has an intermittent history that may go back as much as 3 million years.[2] Some of the sand dunes can reach 180 meters (600 feet) in height.[3] Its name is the Arabic word meaning "desert": (صَحراء), "ṣaḥrā´" .[4][5]

The boundaries of the Sahara are the Atlantic Ocean on the west, the Atlas Mountains and the Mediterranean Sea on the north, the Red Sea and Egypt on the east, and the Sudan and the valley of the Niger River on the south. The Sahara is divided into western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains (a region of desert mountains and high plateaus), Tenere desert and the Libyan desert (the most arid region). The highest peak in the Sahara is Emi Koussi (3415 m) in the Tibesti Mountains in northern Chad.

The Sahara divides the continent of Africa into North and Sub-Saharan Africa. The southern border of the Sahara is marked by a band of semiarid savanna called the Sahel; south of the Sahel lies the lusher Sudan and the Congo River Basin. Most of the Sahara consists of rocky hamada; ergs (large sand dunes) form only a minor part.

People lived on the edge of the desert thousands of years ago[6] since, immediately after the last ice age, the Sahara was a much wetter place than it is today. Over 30,000 petroglyphs of river animals such as crocodiles (which still exist in parts of the desert)[7] survive, with half found in the Tassili n'Ajjer in southeast Algeria. Fossils of dinosaurs, including Afrovenator, Jobaria and Ouranosaurus, have also been found here. The modern Sahara, though, is not as lush in vegetation, except in the Nile Valley, at a few oases, and in the northern highlands, where Mediterranean plants such as the olive tree are found to grow. The region has been this way since about 3000 BC. Some 2.5 million people live in the Sahara, most of these in Egypt, Mauritania, Morocco and Algeria. Dominant ethnicities in the Sahara are various Berber groups including Tuareg tribes, various Arabised Berber groups such as the Hassaniya-speaking Maure (Moors, also known as Sahrawis), and various "black African" ethnicities including Tubu, Nubians, Zaghawa, Kanuri, Peul (Fulani), Hausa and Songhai. Important cities located in the Sahara include Nouakchott, the capital of Mauritania; Tamanrasset, Ouargla, Bechar, Hassi Messaoud, Ghardaia, El Oued, Algeria; Timbuktu, Mali; Agadez, Niger; Ghat, Libya; and Faya-Largeau, Chad.

It has been reported that the Sahara is expanding south by as much as 30 miles per year, overwhelming degraded grasslands,[8] [9] taking over the Sahel, the dry tropical savanna that has defined the Sahara's southern limit. Global warming and poor farming methods have been given as possible causes.[10] One report states that all of Africa could eventually become a massive desert. This spreading of deserts is known as "desertification," and the phenomenon is occurring in other desert areas worldwide.

[edit] Geography



A geographical map of Africa, showing the ecological break that defines the Saharan areaThe Sahara covers huge parts of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Morocco, Niger, Western Sahara, Sudan and Tunisia. It is one of three distinct physiographic provinces of the African massive physiographic division.

The desert landforms of the Sahara are shaped by wind (eolian) or by occasional rains, and include sand dunes and dune fields or sand seas (erg), stone plateaus (hamada), gravel plains (reg), dry valleys (wadi), and salt flats (shatt or chott).[11] Unusual landforms include the Richat Structure in Mauritania.

Several deeply dissected mountains and mountain ranges, many volcanic, rise from the desert, including the Aïr Mountains, Ahaggar Mountains, Saharan Atlas, Tibesti Mountains, Adrar des Iforas, and Red Sea Hills. The highest peak in the Sahara is Emi Koussi, a shield volcano in the Tibesti range of northern Chad.

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, Kufrah, and Siwah.

The center 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 north, the Sahara reaches to the Mediterranean Sea in Egypt and portions of Libya, but in Cyrenaica and the Magreb, the Sahara borders Mediterranean forest, woodland, and shrub ecoregions of northern Africa, which have a Mediterranean climate characterized by a winter rainy season. According to the botanical criteria of Frank White[12] and geographer Robert Capot-Rey,[13][14] the northern limit of the Sahara corresponds to the northern limit of Date Palm cultivation (Phoenix dactylifera), and the southern limit of Esparto (Stipa tenacissima), a grass typical of the Mediterranean climate portion of the Maghreb and Iberia. The northern limit also corresponds to the 100 mm isohyet of annual precipitation.[15]

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 Chenopodiaceae), or northern limit of the Cenchrus biflorus, a grass typical of the Sahel.[13][14] According to climatic criteria, the southern limit of the Sahara corresponds to the 150 mm isohyet of annual precipitation (keeping in mind that precipitation varies strongly from one year to another).[15]

[edit] Climate history



An oasis in the Ahaggar MountainsThe climate of the Sahara has undergone enormous variation between wet and dry over the last few hundred thousand years.[16] During the last ice age, the Sahara was bigger than it is today, extending south beyond its current boundaries.[17] The end of the ice age brought better times to the Sahara, from about 8000 BC to 6000 BC, perhaps due to low pressure areas over the collapsing ice sheets to the north.[18] Once the ice sheets were gone, the northern part of the Sahara dried out. However, not long after the end of the ice sheets, the monsoon, which currently brings rain only as far as the Sahel, came further north and counteracted the drying trend in the southern Sahara. The monsoon in Africa (and elsewhere) is due to heating during the summer. Air over land becomes warmer and rises, pulling in cool wet air from the ocean, which causes rain. Paradoxically, the Sahara was wetter when it received more solar insolation in the summer. Changes in solar insolation are caused by changes in the Earth's orbital parameters (9,000 years ago the Earth's axis had a stronger tilt than it does presently, and perihelion occurred at the end of July).[19]

By around 3400 BC, the monsoon retreated south to approximately where it is today,[20] leading to the gradual rather than abrupt desertification of the Sahara.[21] The Sahara is currently as dry as it was about 13,000 years ago.[16] 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 the sand to form sand storms and dust devils.[22] Precipitation, while rare, is not unknown. Half of the Sahara receives less than 2 cm of rain a year, with the rest receiving up to 10 cm a year.[23] The rainfall happens very rarely, but when it does it is usually torrential when it occurs after long dry periods, which can last for years.

[edit] Ecoregions

The Sahara comprises several distinct ecoregions, whose variations in temperature, rainfall, elevation, and soils harbor distinct communities of plants and animals. According to the Worldwide Fund for Nature (WWF), the ecoregions of the Sahara include:

Atlantic coastal desert: The coastal desert occupies a narrow strip along the Atlantic coast, where fog generated offshore by the cool Canary Current provides sufficient moisture to sustain a variety of lichens, succulents, and shrubs. It covers 39,900 square kilometers (15,400 square miles) in Western Sahara and Mauritania.[24]

North Saharan steppe and woodlands: This ecoregion lies along the northern edge of the desert, next to the Mediterranean forests, woodlands, and shrub ecoregions of the northern Maghreb and Cyrenaica. Winter rains sustain shrublands and dry woodlands that form a transition between the Mediterranean climate regions to the north and the hyper-arid Sahara proper to the south. It covers 1,675,300 square kilometers (646,800 square miles) in Algeria, Egypt, Libya, Mauritania, Morocco, Tunisia, and Western Sahara.[25]

Sahara desert: This ecoregion covers the hyper-arid central portion of the Sahara where rainfall is minimal and sporadic. Vegetation is rare, and this ecoregion consists mostly of sand dunes (erg, chech, raoui), stone plateaus (hamadas), gravel plains (reg), dry valleys (wadis), and salt flats. It covers 4,639,900 square kilometers (1,791,500 square miles) of Algeria, Chad, Egypt, Libya, Mali, Maurit