The Sahara desert contained the world's largest freshwater lake until it evaporated in just a few hundred years, a new study has found.
The Sahara desert contained the world’s largest freshwater lake until it evaporated in just a few hundred years, a new study has found.
Researchers used satellite images to map abandoned shore lines around Palaeolake Mega-Chad, and analysed sediments to calculate the age of these shore lines, producing a lake level history spanning the last 15,000 years.
At its peak around 6,000 years ago, Palaeolake Mega-Chad was the largest freshwater lake on Earth, with an area of 360,000 square km.
Now today’s Lake Chad is reduced to a fraction of that size, at only 355 square km, researchers said.
The drying of Lake Mega-Chad reveals a story of dramatic climate change in the southern Sahara, with a rapid change from a giant lake to desert dunes and dust, due to changes in rainfall from the West African Monsoon, researchers said.
The research confirms earlier suggestions that the climate change was abrupt, with the southern Sahara drying in just a few hundred years.
Part of the Palaeolake Mega-Chad basin that has dried completely is the Bodele depression, which lies in remote northern Chad.
The Bodele depression is the world’s single greatest source of atmospheric dust, with dust being blown across the Atlantic to South America, where it is believed to be helping to maintain the fertility of tropical rainforests.
However, the University of London team’s research shows that a small lake persisted in the Bodele depression until about 1,000 years ago.
This lake covered the parts of the Bodele depression which currently produce most dust, limiting the dust potential until recent times.
“The Amazon tropical forest is like a giant hanging basket,” said Dr Simon Armitage from the Department of Geography at University of Royal Holloway.
“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.
“As the World’s most vigorous dust source, the Bodele depression has often been cited as a likely source of these nutrients, but our findings indicate that this can only be true for the last 1,000 years,” Armitage added.
The study was published in the journal PNAS.