Deserts of the world and the Sahel

Detail

What a desert is

• A desert is a large, dry area of land. It gets very little rain, usually less than about 250 mm a year. Plants are few and spread far apart.
• Deserts are not always hot:
• Hot deserts: the Sahara and the Thar.
• Cold deserts: the Gobi. Ladakh is India's cold desert.

• Antarctica is sometimes called a desert. It gets almost no snowfall, so it counts as the largest desert of all. It is a polar desert.

• Köppen climate codes (Class 11):

• BWh is a hot desert, where the average yearly temperature is above 18°C.
• BWk is a cold or mid-latitude desert.
• BS is a steppe or semi-arid climate. It means grassland with a little more rain, for example the Sahel.
• Coastal deserts next to cold currents are often foggy but almost rainless.

Why deserts form: four causes

Cause	How it works	Examples
1. Sinking air under the subtropical highs	Air that rose at the equator flows towards the poles and sinks at about 20°–30° N and S. Sinking air gets warmer as it is squeezed (compressed), so its water vapour does not condense. The result is clear skies and almost no rain.	Sahara, Arabian, Thar, Kalahari, Great Australian deserts, Sonoran
2. Cold ocean currents on west coasts	A cold current cools the air just above the sea. Cool air sits under warmer air above it. This is a temperature inversion, where air gets warmer with height instead of cooler. The air cannot rise to form rain clouds, so there is only fog.	Atacama (Humboldt Current), Namib (Benguela Current), Baja California (California Current)
3. Continental interiors far from the sea	Winds lose their moisture long before they reach the centre of a big continent. This is called continentality.	Gobi, Taklamakan, Karakum, Kyzylkum
4. Rain shadow	Moist wind hits a mountain and is pushed up. The rising air cools and rains on the side facing the wind (the windward side). The air that comes down the far side (the leeward side) is dry and warm.	Patagonia (behind the Andes), Great Basin (behind the Sierra Nevada)

• Pattern to remember: most hot deserts lie on the western sides of continents between about 15° and 30° latitude. Here the subtropical high and the cold west-coast currents work together.
• Many deserts have more than one cause. The Atacama has a cold current and also the Andean rain shadow. The Thar has the subtropical high, and the Aravallis also run parallel to the south-west monsoon winds, so they do not block them and make them rain.

The Sahara: the largest hot desert

• Size and extent: about 9 million sq km. It is the world's largest hot desert. Only Antarctica and the Arctic are bigger as deserts.
• Location: it stretches across North Africa from the Atlantic Ocean in the west to the Red Sea in the east. In the north it borders the Atlas Mountains and the Mediterranean coast. In the south it fades into the Sahel.
• Countries (about a dozen): Morocco (with Western Sahara), Algeria, Tunisia, Libya, Egypt, Mauritania, Mali, Niger, Chad, Sudan and parts of Eritrea.
• Landforms:
• 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.
• Hamada: a bare, rocky plateau. The wind has blown the loose material away.
• Oasis: a green spot where groundwater comes to the surface. Dates are grown and people settle there.
• Mountains:
  • The Ahaggar (Hoggar) is in southern Algeria. Its highest peak is Mount Tahat, about 2,900 m.
  • The Tibesti is in northern Chad. Its peak Emi Koussi, about 3,400 m, is the highest point of the Sahara.

• Qattara Depression in north-west Egypt:

  • It is a large basin whose floor lies about 133 m below sea level.
  • It is one of Africa's lowest points.

• The "Green Sahara":

• About 6,000 years ago the Sahara had lakes, grasslands and animals such as hippos.
• This was the African Humid Period. Small changes in Earth's orbit pushed the monsoon rains further north.
• When the rains moved back south, the land dried into desert.
• Rock art in the Tassili n'Ajjer (Algeria) shows this greener past.

Other hot deserts

Desert	Location	Key facts
Arabian	Arabian Peninsula: Saudi Arabia, Oman, UAE, Yemen	Rub al-Khali ("Empty Quarter") is the largest continuous sand sea in the world
Thar	North-west India (mainly Rajasthan) and Pakistan	Lies between the Aravallis and the Indus plain
Kalahari	Botswana, Namibia, South Africa	Semi-arid: it has grass and scrub, so it is not a true sand desert. It is home to the San people, who are hunter-gatherers. The Okavango Delta in Botswana is an inland delta: the river spreads out and ends in the desert instead of reaching the sea
Australian	Interior and west of Australia	Great Victoria (the largest in Australia), Great Sandy, Gibson, Simpson
Mojave	South-west USA	Death Valley is the lowest and hottest place in North America
Sonoran	USA (Arizona) and Mexico	Known for the saguaro cactus
Chihuahuan	Mexico and USA (Texas, New Mexico)	North America's largest desert

Cold and mid-latitude deserts

Desert	Location	Key facts
Gobi	Southern Mongolia and northern China	Cold winters. It is the source of dust storms that reach Beijing and Korea. It is famous for dinosaur fossils and eggs
Taklamakan	Tarim Basin, Xinjiang (China), between the Tian Shan and Kunlun ranges	A shifting sand desert, ringed by the old Silk Road oasis towns
Karakum	Turkmenistan	Its name means "black sand"
Kyzylkum	Uzbekistan and Kazakhstan, between the Amu Darya and Syr Darya rivers	Its name means "red sand". It lies next to the shrinking Aral Sea
Patagonian	Southern Argentina	A rain-shadow desert east of the Andes. It is the largest desert in the Americas
Great Basin	Nevada and Utah, USA	A rain-shadow desert behind the Sierra Nevada

The Atacama Desert: the driest non-polar place

• Location: a narrow strip about 1,000 km long in northern Chile, between the Pacific Ocean and the Andes.
• Why it is so dry (two causes together):
• Cold Humboldt (Peru) Current:
  • It cools the coastal air and creates a temperature inversion.
  • The cool air cannot rise to form rain clouds.
  • It produces fog (locally called camanchaca) but no rain.
• Andean rain shadow: the Andes block moist air coming from the Amazon basin in the east.

• Result: it is the driest non-polar place on Earth. Some weather stations have recorded almost no rain for years.

• Resources:

• Copper:
  • In 2024 Chile was the world's leading producer of mined copper, with 24% of world production [6].
  • It produced about 5.5 million tonnes in 2024, about 5% more than in 2023 [6].
  • Copper made up about 89% of mining investment in Chile [6].
• Lithium:
  • Chile, Argentina and Bolivia form the "lithium triangle". Lithium here is taken from salt flats (salares) such as the Salar de Atacama.
  • In 2025 Chile was the world's second-largest lithium producer (about 56,000 tonnes, roughly 22% of the world total) [5].
  • World output rose about 31% in 2025 to about 290,000 tonnes [5].
  • India's KABIL (Khanij Bidesh India Ltd) has looked for lithium assets in this region.

• Nitrates (saltpetre):

  • In the War of the Pacific (1879-84), Chile fought Peru and Bolivia for these deposits.
  • Chile won. Bolivia lost its coastline and became landlocked.

• Observatories: the dry air, high altitude and clear skies make the Atacama one of the world's best places to watch the stars. Major telescopes such as ALMA are built here.

The Namib Desert: the oldest desert

• Location: along the Atlantic coast of Namibia, and extending into southern Angola.
• Age: it is considered the oldest desert in the world. It has been dry for tens of millions of years.
• Why it is dry:
• The cold Benguela Current flows north along south-west Africa's Atlantic coast [4].
• The cold current stops rain from forming [4].

• It does create morning fog. The fog collects on the dunes and gives enough water for some thin plant cover [4].

• Life from fog:

• Fog-basking beetles tilt their bodies into the wind. Fog droplets collect on their backs and roll down into their mouths.

• Welwitschia is a strange plant with only two leaves that keep growing. It can live for over 1,000 years and survives on fog.

• Dunes:

• They are among the tallest in the world, up to about 300-350 m (1,000-1,167 ft) in places [7].
• Strong winds from the sea push them up [7].

• The sand comes from thousands of kilometres inland. Rivers, the ocean current and the wind carry it here [3].

• Namib Sand Sea (UNESCO World Heritage Site, 2013):

• It covers over 3 million hectares [3].
• It is the only coastal desert in the world with large dune fields shaped by fog [3].
• It has two dune systems: an old, partly hardened one, with a younger, moving one on top [3].
• Fog is the main source of water here. It supports endemic invertebrates, reptiles and mammals, meaning animals found nowhere else [3].

Comparing the two coastal deserts

Feature	Atacama	Namib
Country	Northern Chile	Namibia (and southern Angola)
Ocean	Pacific	Atlantic
Cold current	Humboldt (Peru)	Benguela
Extra cause	Andean rain shadow	Subtropical high
Known for	Driest non-polar place; copper, lithium, nitrates; observatories	Oldest desert; tallest dunes; fog-based life; World Heritage Site (2013)

The Sahel: the Sahara's southern edge

• Meaning: "Sahel" comes from the Arabic for "shore" or "edge", because it is the "shore" of the Sahara.
• What it is: a semi-arid (partly dry) belt of grass, thorny shrubs and scattered trees. It is a transition zone between the Sahara in the north and the wetter savanna in the south.
• Extent: from Senegal and Mauritania on the Atlantic, through Mali, Burkina Faso, Niger, northern Nigeria and Chad, to Sudan and Eritrea near the Red Sea.
• Rainfall: 200-600 mm a year. It falls in one short summer season, when the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt where the trade winds of the two hemispheres meet) moves north. If the ITCZ does not move far enough north, the rains fail.
• Problems:
• Recurrent drought:
  • The worst was 1967-77. It caused famine and mass deaths of cattle.
  • This drought led the world to set up the UNCCD (UN Convention to Combat Desertification, 1994).
  • Climate change is making rainfall more uneven.
• Desertification: good land turns into desert-like land.
  • Cause: drought, overgrazing, cutting trees for firewood and farming poor soils.
  • Process: the plant cover is lost, the topsoil is blown or washed away, and the soil holds less water.
  • Result: the land can no longer grow crops or grass.
• Food insecurity: poor harvests and rising population mean many people do not have enough food.

• Conflict:

  • Herders and farmers compete for shrinking land and water.
  • Armed groups are active in the region.
  • Coups have taken place.

• Alliance of Sahel States (AES): after military coups, Mali, Burkina Faso and Niger formed the Alliance of Sahel States and left ECOWAS (Economic Community of West African States).

The Great Green Wall

• What it is:
• It is an African Union initiative launched in 2007 [2].

• It began as a response to land degradation and deep poverty caused by repeated severe droughts in the Sahel [2].

• Extent:

• It is a corridor of nearly 8,000 km across the continent, from Senegal to Djibouti [2].

• It covers 11 countries [2]: Senegal, Mauritania, Mali, Burkina Faso, Niger, Nigeria, Chad, Sudan, Eritrea, Ethiopia and Djibouti.

• The idea changed over time: it started as a single line of trees. Now it is a mosaic (a patchwork) of restored forests, farms and grasslands.

• 2030 targets [2]:
• Restore 100 million hectares of degraded land.
• Store over 300 million tonnes of CO₂.

• Create 10 million jobs.

• Progress, from the first status report (September 2020) [2]:

• Close to 18 million hectares restored.
• Over 350,000 jobs created (2007-2018).
• To reach the 2030 goal, the countries would need to restore 8.2 million hectares a year at a cost of about USD 4.3 billion a year.
• So the project is well behind target.

India link: land restoration and desertification

• Land degradation in India: the Desertification and Land Degradation Atlas of India found that 97.85 million hectares (29.77% of India's area) is degraded or turning into desert [9].
• India's pledge:
• India has promised to restore 26 million hectares of degraded land by 2030 [11].
• This will help create an extra carbon sink (land and forests that absorb carbon) of 2.5-3 billion tonnes of CO₂ equivalent [11].

• The National Action Plan to Combat Desertification, 2023 is built around this 26 million ha pledge under the UNCCD [10].

• Progress: 21.76 million hectares have been brought under restoration so far, against the 26 million ha target (counted under the Bonn Challenge, a global goal to restore damaged forest land). This work has created about 1.22 billion person-days of employment [8].

• India's own deserts: the Thar (hot, Rajasthan) and Ladakh (cold). They face the same problems as the Sahel: sand dunes moving onto farmland, wind erosion and overgrazing.

Prelims Hooks

• Largest desert overall: Antarctica (polar). Largest hot desert: Sahara (about 9 million sq km). Largest continuous sand sea: Rub al-Khali (Arabian Desert).
• Desert and cold current pairs: Atacama with the Humboldt (Peru) Current. Namib with the Benguela Current. Baja California with the California Current. Do not mix in warm currents such as the Agulhas or Brazil.
• Rain-shadow deserts: Patagonia (behind the Andes) and the Great Basin (behind the Sierra Nevada). Trap: the Gobi and Taklamakan are mainly caused by continentality, not by subtropical highs.
• Sahara mountains and their countries: Ahaggar is in Algeria. Tibesti is in Chad, and its peak Emi Koussi is the Sahara's highest point. The Qattara Depression (below sea level) is in Egypt.
• Sahara landforms: erg is a sand sea, reg is a stony plain, hamada is a bare rocky plateau.
• Taklamakan lies in the Tarim Basin, Xinjiang. Karakum is in Turkmenistan. Kyzylkum lies between the Amu Darya and Syr Darya.
• Okavango is an inland delta in Botswana, in the Kalahari. The San people live in the Kalahari.
• Namib Sand Sea became a UNESCO World Heritage Site in 2013. It is the only coastal desert with large dune fields shaped by fog [3].
• Chile: the world's top copper producer (24%, 2024) [6] and second-largest lithium producer (2025) [5]. The lithium triangle is Chile, Argentina and Bolivia. The War of the Pacific (1879-84) made Bolivia landlocked.
• Great Green Wall: African Union, 2007. It covers 11 countries from Senegal to Djibouti, about 8,000 km. The target is 100 million ha by 2030 [2]. India's UNCCD pledge is 26 million ha by 2030 [11].

Mains Points

• Pattern of desert distribution (GS-I):
• Most hot deserts lie at about 15°-30° latitude on the western sides of continents. Here the subtropical high and the cold west-coast currents work together.
• Cold deserts lie in continental interiors or in rain shadows.

• So the location of a desert can be explained by how the atmosphere and the oceans circulate.

• The Sahel as a climate-conflict hotspot (GS-I/GS-III):

• Rain is uncertain because of how far north the ITCZ moves. Drought (1967-77) and desertification follow.
• This leads to food insecurity and herders and farmers fighting over land, which then leads to coups and the AES breaking away from ECOWAS.

• Answers can cite the Great Green Wall's shortfall (about 18 million of 100 million ha by 2020) and the need for 8.2 million ha a year [2].

• Desert resources and the energy transition (GS-III):

• The Atacama's copper and lithium make the Andes "lithium triangle" central to battery supply chains [5][6].
• India's KABIL is looking for lithium here, which links resource geography with India's critical-mineral security.

• Mining in fragile, dry ecosystems uses a lot of scarce water.

• India's response to desertification (GS-III):

• About 29.77% of India's land is degraded [9].
• The 26 million ha pledge [11] and the National Action Plan 2023 [10] are India's answer, with 21.76 million ha under restoration so far [8].
• The Thar and Ladakh can learn from the Sahel's "mosaic" approach, which combines restoration with rural jobs.