An enormous lake called Mega Chad in what is now the Sahara desert took just a couple of hundred years to shrink to a fraction of its size, British scientists have found.

Mega Chad was once the biggest freshwater lake on earth covering 139,000 sq miles (360,000 sq km) of Central Africa – and rapidly shrunk to a tiny fraction of its former size 1,000 years ago.

The discovery sheds light on how the Amazon rain forest grows – because dust from the remains of the dried-up lake blows across the Atlantic to help fertilise the jungle.

Mega Chad was the biggest freshwater lake on earth covering 139,000 sq miles (360,000 sq km) of Central Africa. The graphic above shows how it has shrunk to just 137 sq miles

All that remains now is Lake Chad (pictured), which at 137 sq miles (355 sq km) is still large but a fraction of its former scale

'A reconstructed lake level history for the ancient Lake Mega-Chad, once the largest lake in Africa, suggests that a North African humid period, with increased precipitation in the Sahara region, ended abruptly around 5,000 years ago, and that the lake’s Bodélé basin, now a large source of atmospheric dust, may not have dried out until around 1,000 years ago,' the team wrote.

But the researchers' discovery shows this fertilisation could only have happened 1,000 years ago – leaving a riddle as to how the jungle received vital nutrients before then.

The researchers found that the change took place in just a few hundred years – much more quickly than previously considered.

All that remains is Lake Chad, which at 137 sq miles (355 sq km) is still large but a fraction of its former scale.

The lake, which crosses the borders of Chad, Niger, Nigeria and Cameroon, has been further reduced in size by humanity siphoning off fresh water from it.

The researchers explained the significance of the finding. The dried up remains of the lake is the biggest source of dust in the world, the Bodélé depression.

HOW DUST FROM THE SAHARA HELPS TO BREATHE LIFE IN TO THE AMAZON They are thousands of miles apart, on different continents and separated by an entire ocean. But dust from Africa's Sahara desert plays a key part in maintaining the lush greenery of the Amazon rainforests. Millions of tons of dust containing rich nutrients is carried across the Atlantic every year and end up fertilising the soils of South America. Crucially, it contains phosphorus and other fertilisers, which replace the vital components washed away every year through heavy rain in the dense jungle. Dust from Africa's Sahara desert plays a key part in maintaining the lush greenery of the Amazon rainforests Millions of tons of dust containing rich nutrients is carried across the Atlantic every year and end up fertilising the soils of South America According the Tech Times, 22,000 tons of phosphorus from the Sahara is deposited on the Amazon annually. Dr Simon Armitage of Royal Holloway’s geography department said dust from the dried-up Mega Chad is part of the process. ‘The Amazon tropical forest is like a giant hanging basket. 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. Advertisement

Dust from Bodélé blows across the Atlantic to help fertilise the rainforests of the Amazon.

Dr Simon Armitage of Royal Holloway’s geography department said: ‘The Amazon tropical forest is like a giant hanging basket.

‘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.

Millions of tons of dust from the Sahara blows across the Atlantic to help fertilise the rainforests of the Amazon (pictured)

'As the World’s most vigorous dust source, the Bodélé 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.’

To analyse Mega Chad’s decline, researchers from Royal Holloway, Birkbeck and Kings College, University of London used satellite images to map abandoned shore lines.

They also analysed lake sediment to calculate the age of these shore lines, producing a lake level history spanning the last 15,000 years.