The Sahara Was Once Green
What causes the Sahara to switch between arid and humid phases?
How does increased solar radiation affect the Sahara?
What evidence supports the idea of alternating dry and wet periods in the Sahara?
The Sahara, which covers the bulk of North Africa, is the world’s largest hot desert. It is a vast, desolate region of sand dunes, rocky canyons and ridges, dry lake and riverbeds, scattered oases, and salt lakes. However, not so long ago—well, geologically speaking—the Sahara was green and verdant, filled with woodlands, grasslands, lakes, and rivers, as well as plants and animals that are not found in the region today. Evidence collected by paleontologists and archaeologists note that the Sahara’s green period (officially called the African Humid Period) began shortly after the end of the last ice age, about 11,700 years ago, and lasted until about 5,000 years ago.
Yet this green period was not the region’s only humid phase. Scientists have collected evidence of at least 20 such swings between arid and humid phases over the most recent 800,000 years. The secret to this climatic pendulum lies in Earth’s precession—that is, the wobble on its axis that completes a cycle roughly every 26,000 years (see also Milankovitch cycles). Today Earth’s axis of rotation points about 23.5° away from the plane of the planet’s orbit around the Sun, and its closest approach to the Sun (perihelion) takes place during the winter in the Northern Hemisphere, when the rays of the Sun that fall upon the Sahara and the rest of the Northern Hemisphere are more indirect than during the summertime (see also season). During green periods, however, Earth’s precession is such that the direct rays of summer sun and perihelion occur at the same time, giving the Sahara a 7-percent rise in overall solar radiation compared to today.
Scientists suggest that this increase in the region’s heating affected regional weather patterns; a warmed-up North Africa became a vast area of rising warm air capable of drawing in humid tropical Atlantic air. Today the southern part of the Sahara receives summer rain driven by the West African monsoon, but these rains do not penetrate the desert very far. Scientists argue that the energy boost caused by the change to Earth’s rotation supercharged the West African monsoon, making it strong enough to advance across the whole of the Sahara. The rains that followed were enough to transform the arid landscape into a lush region of rivers, fields, and forests.
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This hypothesis is supported by several pieces of evidence. The presence of depressions and basins across the present-day landscape are strong indicators of lakes that later dried up. Deposits of wind-driven mineral dust that appear in separate layers of marine sedimentary rock found near the North African coast point to alternating periods of higher and lower dust production. Clues also come from bits of fossilized waxy parts cast off from plants. Waxy leaves help to control water loss in drier environments, so the discovery of alternating sediments in Lake Tanganyika and marine sediments off northwestern Africa with higher and lower amounts of hydrogen isotopes associated with leaf wax also supports the notion of alternating dry and wet periods in the Sahara, as well as parts of sub-Saharan Africa.