When the Sahara Was Green

When the Sahara Was Green

Among the foothills of the Tassili Mountains in southern Algeria, near the heart of the Sahara Desert, stand several tall, narrow rock pillars of reddish sandstone. One of the pillars is unique; 8,000 to 10,000 years ago a human passed by and carved a picture on it. The composition shows three life—sized cattle and a calf drinking at a waterhole.

Many other rock carvings and paintings were done in nearby areas during the years that followed. These ancient artworks, combined with discoveries of hippo bones and traces of fishing villages in dried lakebeds, clearly indicate that a rather green Sahara existed over 8,000 years ago.

The area was then a broad savanna, populated by gazelles, giraffes, zebras, elephants, and other animals that inhabit the plains of present—day sub—Saharan Africa. Crocodiles and hippos wallowed in numerous lakes, and the adjacent mountains, from which many rivers sprang, were covered with forests.

As the Sahara began to dry, the ancient cattlemen were forced to move their herds to the higher elevations of the mountains, which remained cool and moist. But the spreading desert was relentless and moved steadily up the slopes, drying out cattle pastures and waterholes. The drying process, which went on for centuries, was dramatically recorded by the cattlemen in the rock carvings and paintings of the Tassili Mountains.

More recent paintings record battle scenes between warriors wielding spears and bows and arrows. The men probably fought over the depleted grazing lands and waterholes. But in the end the desert was victorious, and the people who remained became nomads. The latest of the Tassili Mountains paintings depict palm trees and camel caravans. These pictures mark the end of a painting tradition thousands of years old.

The fact that the Sahara was covered by savannas and forests as recently as 8,000 years ago has led many people to the false conclusion that the Sahara is only a few thousand years old. It actually began to develop about twenty—four million years ago, when a rift widened between Africa and Arabia, forming the Red Sea. The massive shifting raised the Atlas Mountains on the northwestern edge of Africa. As they approached 10,000 feet in height, the mountains interfered with the supply of moisture blown in from the Atlantic Ocean. By about 2.5 million years ago rivers and streams had dried up, winds piled sand in the valleys, and the Sahara Desert began to form.

The natural aridity of North Africa was interrupted about two million years ago by the last Ice Age. During this cycle of earth refrigeration the ice sheets advanced and retreated several times. The last glacial advance, about 20,000 years ago, reached far to the south. With great portions of Europe, Asia, and North America covered by ice sheets, the low—pressure systems that bring rain originated in the middle Atlantic rather than in the north Atlantic as they do today. These rain—soaked low—pressure systems crossed the Sahara region, and the result was a green North Africa, uninterrupted by deserts.

A recent radar image taken from the space shuttle Columbia probed beneath the dry desert façade, exposing a complex of ancient river valleys. These were present during the last cold phase of 8,000 years ago, when many of the rock carvings and drawings were created. With the final retreat of the northern ice sheets the world's climate slowly began to return to present conditions.

The parching of North Africa was a relatively slow process, and the continuing extension of the Sahara's boundaries shows that desertification is by no means complete. The desert advance, not as dunes over—running a village but as bare patches of land breaking out, vegetation withering, and thin topsoil blowing away.

The Sahara Desert is expanding both into Europe and into grasslands to the south. And it is not alone. Scientists note that the arid lands of the entire world are expanding collectively at the rate of about forty square miles per day. Natural forces of wind and water receive much help from the human population in speeding up the process, with 15 million acres of useful land being lost each year. We contribute greatly to the expansion of the world's desert through deforestation, destruction of the vegetable cover, and overgrazing and generally by consuming rather than replenishing the treasures of the earth.

A hostile, windswept field of rock and sand photographed by a Viking landing vehicle on the surface of Mars bears a striking resemblance to many hyperarid deserts of Earth. Does Mars present a picture of things to come, or do we reverse the tide of sand and restore our fragile, wounded ecosystems to their former vitality? We are the primary villains, but we are also the ultimate victims.

From the book: 
Our Fascinating Earth