Sahara

Topic: World Physical Features and Natural Regions · NCERT: Beyond NCERT

Meaning

The Sahara is the world's largest hot desert. It covers about 9 million sq km across North Africa, from the Atlantic Ocean in the west to the Red Sea in the east. It matters because it shows clearly how the subtropical high-pressure belt creates deserts. Its southern edge, the Sahel, is a hotspot for drought, desertification and conflict.

Explanation

Why the Sahara is so dry

• Main cause: sinking air under the subtropical high.
• Air rises at the equator and flows towards the poles. It sinks at about 20°–30° N and S.
• Sinking air is squeezed (compressed), so it gets warmer. Its water vapour does not condense.

• Result: clear skies and almost no rain, usually less than about 250 mm a year.

• Western-side pattern: most hot deserts lie on the western sides of continents between about 15° and 30° latitude. The Sahara follows this pattern.

• Köppen class: the Sahara is a BWh climate, a hot desert where the average yearly temperature is above 18°C.

Landforms of the Sahara

• Erg: a large "sand sea" of dunes.
• Reg: a stony or gravel plain.
• This is the most common Sahara surface.

• Sand covers only a small part of the desert, so the common picture of the Sahara as endless dunes is wrong.

• Hamada: a bare, rocky plateau. The wind has blown the loose material away.

• Oasis: a green spot where groundwater comes to the surface. People settle there and grow dates.
• Mountains:
• Ahaggar (Hoggar): in southern Algeria. Its highest peak is Mount Tahat, about 2,900 m.

• Tibesti: in northern Chad. Its peak Emi Koussi, about 3,400 m, is the highest point of the Sahara.

• Qattara Depression: a large basin in north-west Egypt. Its floor is about 133 m below sea level, which makes it one of Africa's lowest points.

The "Green Sahara": a desert that was once wet

• Cause: small changes in Earth's orbit pushed the monsoon rains further north.
• Result: about 6,000 years ago, the Sahara had lakes, grasslands and animals such as hippos. This time is called the African Humid Period.
• Then: the rains moved back south, and the land dried into desert.
• Evidence: rock art in the Tassili n'Ajjer (Algeria) shows this greener past.

Its edges

• North: the Sahara borders the Atlas Mountains and the Mediterranean coast.
• South: it slowly turns into the Sahel, a semi-arid (partly dry) belt of grass and thorny shrubs. The Sahel's name comes from the Arabic for "shore" or "edge".

Where it occurs

• Continent: North Africa, from the Atlantic Ocean to the Red Sea.
• Countries (about a dozen): Morocco (with Western Sahara), Algeria, Tunisia, Libya, Egypt, Mauritania, Mali, Niger, Chad, Sudan and parts of Eritrea.
• Key places and their countries:
• Ahaggar Mountains and Tassili n'Ajjer: Algeria
• Tibesti Mountains and Emi Koussi: Chad

• Qattara Depression: Egypt

• Southern fringe, the Sahel: from Senegal and Mauritania through Mali, Burkina Faso, Niger, northern Nigeria and Chad to Sudan and Eritrea.

• India link: the Thar in Rajasthan is also a hot desert formed under the subtropical high. Like the Sahel, it faces moving sand dunes, wind erosion and overgrazing.

Don't confuse with

• Antarctica: this is the largest desert of all, but it is a polar desert. The Sahara is the largest hot desert. Only Antarctica and the Arctic are bigger as deserts.
• Rub al-Khali (Arabian Desert): this is the largest continuous sand sea. The Sahara is the largest hot desert, but most of it is reg (stony plain), not sand.
• Sahel: this is a semi-arid (BS, steppe) belt with 200-600 mm of rain a year. It is not part of the true desert. It is the transition zone between the Sahara and the savanna.
• Kalahari: this is also in Africa, but in the south (Botswana, Namibia, South Africa). It is semi-arid, with grass and scrub, so it is not a true sand desert.

Prelims Hooks

• Largest hot desert: Sahara (about 9 million sq km). Largest desert overall: Antarctica. Largest continuous sand sea: Rub al-Khali.
• Mountain and country pairs: Ahaggar (Hoggar) is in Algeria, and Tibesti is in Chad. Trap: the Sahara's highest point is Emi Koussi (Tibesti), not Mount Tahat (Ahaggar).
• Qattara Depression (about 133 m below sea level) is in Egypt, not Libya.
• Landform matching: erg is a sand sea, reg is a stony plain (the most common surface), and hamada is a bare rocky plateau.
• Cause of the Sahara: it is mainly due to the subtropical high (sinking air). Trap: continentality explains the Gobi and Taklamakan, not the Sahara.
• Green Sahara: about 6,000 years ago, during the African Humid Period. The rock art at Tassili n'Ajjer (Algeria) is evidence of it.

Mains Points

• Pattern of desert location (GS-I):
• The Sahara lies under the subtropical high, where sinking, warming air blocks rain.
• This explains why most hot deserts lie at about 15°–30° on the western sides of continents.

• The "Green Sahara" shows that climate can change naturally: small shifts in Earth's orbit moved the monsoon belt and turned grassland into desert.

• The Sahara's edge as a climate-conflict hotspot (GS-I/GS-III):

• Sahel rain depends on how far north the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt where the trade winds of the two hemispheres meet) moves. If it does not move far enough, the rains fail.
• Drought (worst in 1967-77), overgrazing and tree-cutting lead to desertification. This drought led the world to set up the UNCCD (1994).

• These problems lead to food insecurity, fights between herders and farmers, and coups. Mali, Burkina Faso and Niger formed the Alliance of Sahel States and left ECOWAS.

• Policy response (GS-III):

• The African Union's Great Green Wall (launched 2007) runs nearly 8,000 km from Senegal to Djibouti across 11 countries. It aims to restore 100 million ha by 2030, but only close to 18 million ha had been restored by 2020 [1].
• India faces a similar problem: 29.77% of its land is degraded [2]. It has pledged to restore 26 million ha by 2030 [3], and it can learn from the Sahel's "mosaic" approach for the Thar.

Related concepts

• Atacama Desert
• Namib Desert
• Sahel
• Great Green Wall

Read more

• Deserts of the world and the Sahel