Sahara

• 1. Introduction

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  The Sahara spans 9,200,000 square kilometers across North Africa, making it the largest hot desert on Earth. Only two deserts exceed it in size: Antarctica and the northern Arctic. Its name comes from an Arabic word that simply means desert. The desert is so large and bright that, in theory, it could be detected from other stars as a surface feature of Earth, using near-current technology.

  Yet this place was not always the parched expanse it is now. For several hundred thousand years, the Sahara has swung between desert and savanna grassland, following a 20,000-year cycle tied to the slow wobble of Earth's axis. What forces drive that swing, and what did the green Sahara hold before the sand returned? Who lived here when lakes still glittered across the interior, and how did human hands and ancient empires leave their mark on a landscape that became one of the harshest on the planet? The answers stretch from a triple burial laid on a bed of flowers to a vanished empire that drank fossil water from tunnels bored into mountains.

• 2. A Landscape Of Extremes

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  Emi Koussi, a shield volcano in the Tibesti range of northern Chad, is the highest peak in the Sahara. Around it the desert reveals itself as far more than sand. The Sahara is a patchwork of hamada rocky plateaus, ergs of dune-covered sand seas, gravel plains called reg, dry valleys known as wadi, dry lakes, and salt flats. Many of its dunes rise over 180 meters. Several deeply dissected mountains rise from the floor, among them the Aïr Mountains, the Ahaggar Mountains, the Saharan Atlas, the Adrar des Iforas, and the Red Sea Hills.

  The Richat Structure in Mauritania stands out as one of the desert's strangest landforms, a feature unlike the surrounding terrain. The central Sahara is hyperarid, with sparse vegetation, and it splinters into named subdivisions such as the Tanezrouft, the Ténéré, the Libyan Desert, the Eastern Desert, and the Nubian Desert. These extremely arid areas often receive no rain for years at a time.

  The desert's edges are drawn not by surveyors but by plants. The botanist Frank White and the geographer Robert Capot-Rey marked the northern limit by the northern boundary of date palm cultivation and the southern boundary of esparto grass. To the south, the Sahel marks the change, signaled botanically by the southern limit of the drought-tolerant Cornulaca monacantha. Across this expanse sit cities such as Nouakchott, the capital of Mauritania, alongside Timbuktu in Mali, Agadez in Niger, and Faya-Largeau in Chad.

• 3. The Engine Of Aridity

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  The Sahara sits in the horse latitudes, beneath the subtropical ridge, a belt of warm high pressure where air from the upper troposphere sinks, warming and drying as it falls. That descending air prevents clouds from forming and stops the convective overturning that would otherwise bring rain. The result is a sky that stays clear and weather that stays sunny, dry, and stable.

  The descending airflow is strongest over the eastern part of the desert, in the Libyan Desert, described as the sunniest, driest, and most nearly rainless place on the planet, rivaling the Atacama Desert in Chile and Peru. The Sahara High, which produces this effect, is the eastern continental extension of the Azores High centered over the North Atlantic. Its subsidence nearly reaches the ground during the coolest part of the year.

  The Atlas Mountains of Algeria, Morocco, and Tunisia deepen the dryness. They act as a barrier, casting a strong rain shadow that strips humidity from disturbances arriving along the polar front. What little rain does fall comes mainly from the Intertropical Convergence Zone, a belt of low-pressure systems near the equator that delivers a brief and irregular rainy season to the Sahel and the southern Sahara. The annual rates of potential evaporation across this desert are the highest recorded anywhere on Earth.

• 4. Heat And The Cold Myth

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  A sand temperature of 83.5 degrees Celsius has been recorded in Port Sudan, a reading that captures how punishing the Saharan surface becomes. Ground temperatures of 72 degrees were measured in the Adrar of Mauritania, and 75 degrees in Borkou in northern Chad. During daytime the sand can easily reach 80 degrees or more.

  Most of the desert enjoys more than 3,600 hours of bright sunshine each year, over 82 percent of daylight hours. In a wide eastern area the figure climbs above 4,000 hours. The highest values come close to the theoretical maximum: 4,300 hours, about 98 percent of all daylight hours, has been recorded in Upper Egypt at Aswan and Luxor, and in the Nubian Desert at Wadi Halfa. The annual average direct solar irradiation reaches around 2,800 kilowatt-hours per square meter per year, giving the desert a huge potential for solar energy.

  The world's highest mean monthly maximum temperature, 47 degrees Celsius, was recorded in Bou Bernous, a remote town in the Algerian Desert at 378 meters above sea level, rivaled only by Death Valley in California. In Salah, known across Algeria for its extreme heat, average highs run 43.8 degrees in June, 46.4 in July, 45.5 in August, and 41.9 in September.

  It is a myth that nights turn especially cold after the hottest days. On average, nighttime temperatures simply run cooler than daytime ones. The smallest day-to-night swings appear along humid coasts, sometimes less than a 10-degree difference, while the largest occur inland in the dry southern Sahara. Winter nights can still drop to freezing or below, and the frequency of such frosts is shaped by the North Atlantic Oscillation, with more frost when that oscillation runs positive.

• 5. The Green Sahara

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  At its largest extent, sometime before 5000 BCE, Lake Mega-Chad covered an estimated 350,000 square kilometers, the largest of four Saharan paleolakes. Lake Chad today is the shrunken remnant of that former inland sea. This vanished water belonged to a wetter world, one the climate cycle has produced again and again.

  The cycle turns on a 41,000-year swing in which the tilt of the Earth changes between 22 and 24.5 degrees, altering the path of the North African Monsoon. The idea that orbital changes in solar heating drive global monsoon strength was first suggested by Rudolf Spitaler in the late nineteenth century. The meteorologist John Kutzbach formally proposed and tested it in 1981, and the framework, named the Orbital Monsoon Hypothesis by Ruddiman in 2001, is now widely accepted.

  The end of the last glacial period brought more rain from about 8000 BCE to 6000 BCE. Around 4200 BCE the monsoon retreated south to roughly where it sits now, and the desert gradually returned. Marine cores from the north tropical Atlantic record the shift in wind-blown mineral dust, which suddenly plummeted around 12,500 BCE during the Bølling-Allerød phase as wetter conditions took hold. The moister climate peaked during the Holocene thermal maximum around 4000 BCE, when mid-latitude temperatures appear to have run 2 to 3 degrees warmer than in the recent past. At present the desert sits in a dry period, but the Sahara is expected to become green again in 15,000 years.

• 6. People Of The Wet Sahara

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  A triple burial at Gobero, dated to 5,300 years ago, holds an adult female and two children, estimated by their teeth to be five and eight years old, hugging each other. Pollen residue indicates they were buried on a bed of flowers. The three are assumed to have died within 24 hours of each other, yet their skeletons show no trauma and no sign of plague, leaving the cause of their deaths a mystery. Gobero lies in Niger, in the Ténéré Desert, and was discovered in 2000 during an expedition led by Paul Sereno that was searching for dinosaur remains.

  The Kiffian culture existed between 10,000 and 8,000 years ago, during the Neolithic Subpluvial. Gobero is the largest and earliest grave of Stone Age people in the Sahara, and the Kiffians who hunted there were tall, standing over six feet in height. Bones of large savannah animals nearby suggest they lived on the shores of a lake. Traces of the Kiffian culture vanish after 8,000 years ago, when a dry period set in, and the Tenerian culture later colonized the area. Some 200 skeletons have been found at Gobero. The Tenerians were shorter and less robust, and they buried their dead with jewelry made of hippo tusks and clay pots.

  Uan Muhuggiag holds the well-preserved mummy of a young boy whose organs were removed and replaced with an organic preservative, then wrapped in an antelope-skin sack insulated by leaves. An ostrich eggshell necklace was found around his neck. Radiocarbon dating placed the mummy at about 5,600 years old, roughly 1,000 years older than the earliest previously recorded mummy in ancient Egypt. An expedition led by Antonio Ascenzi in 1958-59 conducted the anthropological, radiological, histological, and chemical analyses.

  Further east, megaliths at Nabta Playa stand among probably the world's first known archaeoastronomy devices, predating Stonehenge by some 2,000 years. The complexity expressed there likely shaped both the Neolithic society at Nabta and the Old Kingdom of Egypt. A 2025 study of individuals from Takarkori, dated to 7,000 years before present, found ancestry from an unknown ancestral North African lineage, distinct from both sub-Saharan Africans and Eurasians.

• 7. Empires Of Sand

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  Around 500 BCE the Garamantes built a prosperous empire in the heart of the desert, in a valley now called the Wadi al-Ajal in Fezzan, Libya. They dug tunnels far into the mountains flanking the valley to tap fossil water and carry it to their fields. As they grew populous and strong, they conquered their neighbors and enslaved many people, forcing them to extend the tunnels. The Greeks and Romans regarded the Garamantes as uncivilized nomads, yet traded with them, and a Roman bath has been found in their capital of Garama. Their civilization eventually collapsed once they had drained the aquifers and could no longer extend the tunnels.

  The Phoenicians, who flourished from 1200 to 800 BCE, built a chain of coastal settlements and traded with the ancient Libyans, ancestors of today's Berber speakers. The Libyco-Berber alphabet seems to have been based on Phoenician, and its descendant Tifinagh is still used by the Tuareg of the central Sahara. By 500 BCE Greeks reached the desert and set up trading colonies along the Red Sea, while raids from nomadic Berber people stayed a constant worry for those on the desert's edge.

  After the Muslim conquest of North Africa in the mid-7th to early 8th centuries, Islamic influence spread rapidly, and by the end of 641 all of Egypt was in Muslim hands. Trade across the desert intensified, and a slave trade ran north at an estimated 6,000 to 7,000 people each year from the 10th to 19th centuries. The Beni Hassan and other nomadic Arab tribes came to dominate the Sanhaja Berber tribes after the Char Bouba war of the 17th century, bringing Arabization. From 1517 Egypt became a valued part of the Ottoman Empire, giving the Ottomans control over the Nile Valley and the trade in spices, gold, silk, and enslaved people.

• 8. Colonial Lines And Modern Resources

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  France conquered the regency of Algiers from the Ottomans in 1830, and French rule pushed south and east to reach Timbuktu in 1893, Tunisia in 1881, and Morocco in 1912. In 1902 French forces penetrated the Hoggar Mountains and defeated the Ahaggar Tuareg in the battle of Tit. The French Colonial Empire became the dominant presence in the desert, establishing air links from Toulouse, home of the famed Aéropostale, over the Hoggar to Timbuktu and on to Bamako and Dakar. A film shot by the aviator Captain René Wauthier in 1933 documented the first crossing by a large truck convoy from Algiers to Tchad.

  Egypt, under Muhammad Ali and his successors, conquered Nubia in 1820-22, founded Khartoum in 1823, and conquered Darfur in 1874. Egypt and Sudan became a British protectorate in 1882. Spain captured present-day Western Sahara after 1874, and Italy took parts of what became Libya from the Ottomans in 1912. Most Saharan states won independence after World War II: Libya in 1951; Morocco, Sudan, and Tunisia in 1956; Chad, Mali, Mauritania, and Niger in 1960; and Algeria in 1962.

  Libya's Great Man-Made River now stands as the world's largest irrigation project, pumping fossil water from the Nubian Sandstone Aquifer System to coastal cities including Tripoli and Benghazi. The desert holds large deposits of oil and natural gas in Algeria and Libya, and phosphates in Morocco and Western Sahara. Several Trans-African highways have been proposed across the Sahara, among them the Cairo-Dakar Highway along the Atlantic coast and the Trans-Sahara Highway from Algiers to Kano in Nigeria. Each remains partially complete, broken by significant gaps and unpaved stretches.