The Sahara once had a lake 48% larger than the five Great Lakes combined, but Greater Chad disappeared about 5,000 years ago.
Nearly 6,000 years ago, the center of what is now the Southern Desert contained a lake larger than all five of North America’s great lakes combined, spread across about 361,000 square kilometers of open water. Today, only Lake Chad remains the remaining surface water remnant of that much larger system, shared by Chad, Niger, Nigeria and Cameroon, while the ancient lake basin extends far beyond it. At its highest peak, Lake Palaeolak Mega Chad was about 48 percent larger than the surface area of the five Great Lakes combined, which together cover 244,106 square kilometres. The story of how a lake of this size formed, and how it disappeared, comes from ancient beaches etched into the desert landscape and sedimentary deposits that scientists have dated using optically stimulated luminescence and other geological methods, revealing a climate shift far greater and faster than most people would expect from a region now largely defined by sand. Its disappearance has also left a landscape that records how powerful rainfall can reshape entire regions over relatively short geological timescales.
How scientists reconstructed the size of an ancient lake buried under the sands of the Sahara
Reconstructing a lake that dried up thousands of years before written history requires tracking physical evidence still preserved throughout the landscape. Ancient shorelines surround the basin like bathtub rings indicating past water levels, while lake sediments preserve evidence of the environments that existed there. Study entitled West African monsoon dynamics derived from sudden fluctuations in Lake Mega Chad, published in Proceedings of the National Academy of Sciences Simon Armitage, Charlie Bristow and Nick Drake reconstruct changes in the lake using the optically stimulated luminescence of sand dunes, beaches and river lake sediments. This technique estimates the last time mineral grains were exposed to sunlight.The researchers found evidence of wet conditions that began about 15,000 years ago. About 11,500 years ago, Lake Mega Chad reached a high level that lasted until approximately 5,000 years ago, when lake levels rapidly declined. A 2015 reconstruction estimated a maximum area of about 361,000 km2, while previous geomorphological work based on ancient beaches produced estimates of more than 400,000 km2.Both estimates make the Greater Chad region much larger than the five Great Lakes of North America, which have a combined area of 244,106 square kilometers. Using the 361,000 square kilometer reconstruction, the area of Greater Chad is about 1.48 times the area of the five Great Lakes combined.
Why was the Sahara Desert once green enough to support a lake this size?
The lake exists because the Sahara and Sahel were wetter during the African Humid Period. About 15,000 to 5,000 years ago, large areas of North Africa had lakes, wetlands, rivers, and much more vegetation than today, although the timing of wet conditions varied across the region.The main orbital mover was not simply a change in the Earth’s tilt. Instead, changes in orbital motion altered the seasonal distribution of sunlight, producing stronger summer insolation in the Northern Hemisphere during the early Holocene. Higher summer temperatures in North Africa have enhanced the West African monsoon and helped push the monsoon belt northward into the now arid regions. Changes in vegetation, soil moisture, surface water, and ocean conditions can amplify the primary tropical forcing.The study found that Lake Mega Chad reached its highest level about 11,500 years ago, and remained high until about 5,000 years ago, when lake levels declined rapidly. The researchers interpreted this as evidence that the African humid period ended abruptly in the Greater Chad Basin, suggesting that the African monsoon can respond nonlinearly to gradual changes in tropical forcing.The northern part of the former lake basin, the Budelli Depression, did not become completely dry at exactly the same time. The study places its final drying at about 1,000 years ago. Today, the depression is the largest single source of mineral dust in the world, because sediments deposited by the former lake are easily eroded by strong winds.
Why does what happened 5,000 years ago still matter to millions living near Lake Chad today?
Modern Lake Chad has undergone significant contraction, but this should not be presented as a mere continuation of an ancient drought event. Its recent changes have occurred over decades and have been affected by rainfall variability, drought and human use of water, including irrigation.According to the United Nations Environment Programme, the surface area of Lake Chad has decreased from about 26,000 square kilometers in 1963 to less than 1,500 square kilometers at a documented low point, a decline of more than 90 percent. Because Lake Chad is so shallow, its area can change dramatically with fluctuations in rainfall and river flow, so the exact modern figure depends on the period and definition used.The Lake Chad Basin provides a lifeline to about 40 million people, according to the United Nations Environment Programme. Agriculture, livestock raising and fishing are among the activities associated with the basin’s water resources.Understanding the ancient collapse of Greater Chad does not in itself explain the modern crisis. The two events occurred on very different time scales and had a different set of causes. What the ancient record shows is the extraordinary sensitivity of the region’s landscape to changes in rainfall and the strength of monsoons and how a basin that once supported one of the world’s largest lakes could eventually become one of the most important sources of dust.
