The Sahara: The World's Largest Hot Desert Was Once a Lush Green Savanna

Not Always a Desert

Rock art found on the walls of caves and sheltered rock faces across the Sahara depicts a world that bears almost no resemblance to the current landscape. Elephants, giraffes, hippopotamuses, cattle, crocodiles, and human figures fishing and herding animals appear in these images, created between roughly 10,000 and 5,000 years ago when the Sahara was wetter and far more hospitable than today.

This "Green Sahara" or "African Humid Period" occurred because of a mechanism called Milankovitch cycles — periodic changes in Earth's orbital geometry that affect the distribution of solar radiation reaching different latitudes at different times of year. Between approximately 11,000 and 5,000 years ago, the Northern Hemisphere's summer received more solar energy than it does today, intensifying the West African Monsoon and pushing rainfall much further north and into regions that are now hyper-arid.

This greener Sahara supported human populations and facilitated connections between sub-Saharan Africa and the Mediterranean world. The drying that began roughly 5,000 years ago forced population movements southward and possibly contributed to the concentration of human activity along the Nile Valley that produced ancient Egyptian civilization.

The Geography of the World's Largest Hot Desert

At 9.2 million square kilometers — approximately 31 percent of Africa's total land area — the Sahara encompasses an enormous variety of terrain that popular imagination tends to reduce to endless sand dunes. Ergs, the sandy desert landscapes with dunes, cover only about 25 percent of the Sahara. The majority of the desert is rocky hamadas (flat, wind-scoured rock pavements) and reg (gravel plains), far less visually dramatic but equally hostile to life.

The Sahara contains dramatic mountain ranges — the Ahaggar and Tibesti highlands rise above 3,000 meters — and even small areas of permanent vegetation around underground water sources. The Sahara is not lifeless: it supports several dozen species of mammals, including the fennec fox, the dromedary camel (long domesticated but originally from this ecosystem), and the critically endangered Saharan cheetah and addax antelope. Roughly 500 plant species survive in the desert's most hospitable microenvironments.

Temperatures in the Sahara represent some of the most extreme on earth. The highest reliably recorded air temperature ever measured was at Aziziyah in Libya in 1922: 58°C (136.4°F), though this record has been disputed. Ground surface temperatures can exceed 70°C in direct sunlight. At night, particularly in the drier upland areas, temperatures can drop below freezing — a 40°C daily temperature range is not unusual.

The Desert That Is Growing

Desertification — the process by which arid land expands at the expense of surrounding less arid zones — is advancing the Sahara's southern margin at a measurable rate. The Sahel, the transitional belt between the Sahara and the wetter savanna regions of sub-Saharan Africa, has experienced significant land degradation over the past century, driven by a combination of climate variability, population pressure, overgrazing, and deforestation.

The Great Green Wall initiative, an African Union project established in 2007, aims to plant a barrier of trees and restore vegetation across the Sahel from Senegal to Djibouti — roughly 8,000 kilometers — to halt and reverse desertification. Progress has been uneven, but the project represents the largest ecological restoration effort in history and, if successful, would demonstrate that human action can reverse the processes driving the Sahara's expansion.

The Milankovitch cycles that dried the Sahara 5,000 years ago will eventually return it to a greener state — but on timescales of tens of thousands of years that offer no comfort to the people whose livelihoods are being consumed by advancing sand today.

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