The sands of the Sahara seem endless – in time as much as distance – but the world's largest desert is of surprisingly recent origin. Lake Chad's borders reveal just how dramatically the region's climate has changed; only 6,000 years ago this inland sea was larger than North America's five Great Lakes combined, making it the largest freshwater lake in the world.
Lake Chad still exists, indeed 70 million people depend on it for water. However, it is a shadow of its former self, having declined dramatically since the 1960s. Even the mid-20th century version was small compared to its ancestor, known as Palaeolake Mega-Chad.
The existence of a giant paleolake is well established, but a paper in the Proceedings of the National Academy of Sciences has mapped its changes over a period of 15,000 years, revealing the North African climatic history in the process.
“We use optically stimulated luminescence dating of dunes, shorelines, and fluviolacustrine deposits to reconstruct the fluctuations of Lake Mega-Chad,” the authors report. The lake peaked in size 6,000 years ago, but was close to its largest extent from 11,500-5,000 years ago.
At its largest, the paleolake was 360,000 square kilometers (139,000 square miles) in size, a hundred times its current extent. The Caspian Sea, the world's largest enclosed body of water, is 371,000 square kilometers (143,000 square miles).
Although the most recent shrinkage is a result of deforestation and human consumption, most of the reduction is a consequence of changes in the West African Monsoon. This occurred astonishingly rapidly, with the authors concluding that the southern Sahara dried out in only a few hundred years, roughly 5,000 years ago.
“This record provides strong terrestrial evidence that the African Humid Period ended abruptly,” the paper notes, “supporting the hypothesis that the African monsoon responds to insolation forcing in a markedly nonlinear manner.” Such non-linearity increases the difficulty of predicting the effects of human-induced global warming.
Even after rainfall dropped, the region remained wet enough to suppress dust for four millenia. The findings undermine a theory on another of the Earth's great geographic features: the Amazon rainforest.
"The Amazon tropical forest is like a giant hanging basket," said first author Dr. Simon Armitage of the Royal Holloway, University of London. "In a hanging basket, daily watering quickly washes soluble nutrients out of the soil, and these need to be replaced using fertiliser if the plants are to survive. Similarly, heavy washout of soluble minerals from the Amazon basin means that an external source of nutrients must be maintaining soil fertility.”
Unsurprisingly, the Sahara – specifically the Bodélé depression once filled by the paleolake – is the world's largest source of dust. Prevailing winds from north Africa flow southwest, bringing that dust to the Amazon, which has been suggested to be the source of half of the replacement nutrients. However, “Our findings indicate that this can only be true for the last 1,000 years," said Armitage. As of now, the previous source of nutrients is unknown.