The Sahara, known to be the world's largest desert, used to be home to the largest freshwater lake in the world.

Lake Chad is key to finding out more about what used to be the world's largest freshwater lake, Mega-Chad, since Lake Chad is now only a remaining fraction of it.

Researchers from Royal Holloway, Birkbeck and Kings College, University of London confirm this in a study published in the Proceedings of the National Academy of Science of the United States of America.

Some 700 million people depend on Lake Chad for water, but this lake is now just a tiny part of a larger body of water, Paleolake Mega-Chad. The 137-square-mile Lake Chad was only a fraction of the 139,000-square-mile Mega-Chad. Even combining together the five Great Lakes of North America will not equate to what used to be the largest lake in the world.

By examining the borders of Lake Chad, the researchers found evidence of a significant change in climate over the past 6,000 years. Mega-Chad had possibly shrunk from drying out.

To explain how the shrinkage and drying out could have occurred, Dr. Simon Armitage of the Department of Geography at the Royal Holloway and first author of the study used the example of a hanging basket.

The Bodélé depression of Sahara, the site once occupied by Mega-Chad, would possibly have been a source of nutrients maintaining soil fertility, heavily washing out soluble materials from the Amazon basin. As in a hanging basket, daily washing removes soluble nutrients from the soil.

Changes in rainfall from the West African monsoon may have caused the abrupt and dramatic change in the southern Sahara that took place in just a few hundred years and also, eventually, the drying out of Mega-Chad from a giant lake to desert dunes.

These changes that occurred in the giant lake due to climate change over 15,000 years is mapped in the study. According to the authors, the optically stimulated luminescence dating of dunes, shorelines and fluviolacustrine deposits was used to reconstruct fluctuations in Lake Mega-Chad. Lake sediments also helped them identify the ages of the shore lines.

The most recent shrinkage is now caused by deforestation and human consumption; however, for the most part, climate change is to blame. The study confirmed that, because of this, the African Humid Period ended abruptly.

The study was funded by Royal Geographical Society grants for Chad expeditions in 2005 and northern Nigeria expeditions in 2008.

Photo: NASA Goddard Space Flight Center | Flickr

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