In 4000 BCE, a lake Mega Chad larger than all five Great Lakes covered roughly 360,000 sq km of what is now the Sahara. 6,000 years later, only a tiny fraction, well under 1%, of that ancient lake remains

The Sahara that exists today is difficult to associate with a landscape dominated by enormous amounts of water. Yet the region's past was very different. Long before modern records, climate conditions allowed a huge lake to occupy a vast basin in what is now the southern Sahara. Its existence is preserved not in written history, but in shorelines, sediments and scientific reconstructions.

That ancient lake was Palaeolake Mega-Chad. At its largest, it covered roughly 360,000 square kilometres, although some studies have estimated its maximum area at more than 400,000 square kilometres. Even the lower estimate would make it larger than all five North American Great Lakes combined.

Today, the surviving southern portion of that ancient basin is occupied by Lake Chad, shared by Chad, Niger, Nigeria and Cameroon. Its modern open-water area is only a few thousand square kilometres less than one per cent of the ancient lake's estimated peak.

Reconstructing a lake that disappeared thousands of years ago

There is no written record explaining Mega-Chad at its greatest extent. Instead, scientists have had to reconstruct its history from physical proof left behind by the water.

Ancient shorelines remain in parts of the basin, while sediments deposited on the lake floor preserve evidence of earlier water levels. These features enabled researchers to determine where the lake once reached and how its size changed.

A 2015 study in PNAS by Simon Armitage, Charlie Bristow and Nick Drake used ancient shorelines together with sediment dating to reconstruct the lake's history. The researchers used optically stimulated luminescence, an approach that determines when buried grains were last exposed to sunlight.

Together, these methods generated what the research team called “a reconstructed lake level history for the ancient Lake Mega-Chad”, reaching back approximately 15,000 years.

The precise maximum size remains uncertain. Different studies have positioned the peak area somewhere between about 360,000 and more than 400,000 square kilometres. Regardless of the exact figure, the ancient lake was immense.

The wetter Sahara that made Mega-Chad possible

Mega-Chad could reach such a size because North Africa once experienced a much wetter climate.

Between roughly 11,000 and 5,000 years ago, the place went through the African Humid Period. During this interval, North Africa contained numerous lakes and much more vegetation than it does today.

One major influence was Earth's orbital movement.

In their review of the green Sahara, Peter deMenocal and Jessica Tierney explain that a slow wobble in Earth's orbit changed the amount of summer sunlight reaching the northern tropics. The additional solar heating strengthened seasonal rainfall and allowed the monsoon to reach farther into the continent.

The difference in rainfall was considerable. Their review notes that an almost 7 percent increase in summer sunlight of the kind experienced during that period translated into at least a 17 percent increase in African monsoonal rainfall, with ocean feedbacks potentially raising the increase to 50 percent.

Those wetter conditions were enough to maintain Mega-Chad's high water levels for thousands of years.

How the giant lake disappeared

The climate conditions that supported Mega-Chad did not last forever.

As Earth's orbital configuration changed, the seasonal distribution of sunlight changed. The rains weakened, and the water balance that had maintained the enormous lake changed with them.

The Armitage team interprets its evidence as featuring that the humid period ended fairly abruptly around 5,000 years ago.

The northern portion of the basin, the Bodélé Depression, lasted longer. It finally dried out around 1,000 years ago and became, in the words of the study, “the world’s greatest single dust source.”

There is still debate over exactly how rapidly the drying occurred. The rapid ending identified by the Armitage team represents their interpretation of the evidence rather than an uncontested scientific conclusion.

The modern decline of Lake Chad

The ancient disappearance of Mega-Chad took place over a very long climatic timescale. Modern Lake Chad, however, has faced major changes within just a few decades.

During the second half of the twentieth century, its surface area contracted dramatically. The UN Environment Programme, reporting in 2018, said that the lake's surface area had declined from about 26,000 square kilometres in 1963 to less than 1,500 square kilometres, a decrease of roughly 90 percent.

The lake is highly variable, however. Its surface region changes with seasons and rainfall, and NASA has noted some recovery and year-to-year fluctuations since its lowest point.

Several factors put pressure on the modern water supply that included drought, reduced rainfall and demand for water, especially for irrigation.

A basin that supports millions

The importance of Lake Chad extends far beyond its environmental history.

The wider Lake Chad Basin provides a lifeline to nearly 40 million people. According to the Wilson Center, between 80 and 90 percent of the basin's population depend on farming, herding and fishing.

That dependence makes changes in water availability particularly consequential.

Reduced or uncertain water supplies can place pressure on agriculture, livestock and fishing. Water scarcity and environmental stress have been connected with hunger and displacement, while competition in resources can intensify alongside existing political, economic and security problems.

For people living in the basin, the lake's changing water levels are therefore not simply measured on a map. They can directly impact livelihoods and access to essential resources.

The ancient and modern stories are not the same

It is easy to look at modern Lake Chad and imagine a healthy giant lake that simply collapsed. The longer climate record offers a more complicated scenario.

Modern Lake Chad occupies the surviving southern part of a basin that was transformed as the rains withdrew around 5,000 years ago. Its present condition is part of a much longer history of changing climate and water availability.

The timescales are mainly important.

The orbital changes that helped create the wetter conditions of the African Humid Period unfolded far too slowly to explain the dramatic water changes experienced within a human lifetime. On the modern timescale, rainfall variability and the demand placed on the remaining water are much more immediate factors.

That distinction offers the story of Mega-Chad a modern relevance. The ancient lake shows how dramatically the Sahara can change over thousands of years, while present-day Lake Chad demonstrates how changes in rainfall and pressure on water resources can impact millions of people much more quickly.

What looks today like an extremely dry basin was once home to one of the largest lakes known from Africa's past. Its shorelines and sediments remain as evidence of that lost world, while the much smaller Lake Chad continues to occupy part of the same ancient basin.

Source: SCIENCE BLOG

FAQs:

Q1. What was Mega-Chad?
Mega-Chad was an enormous ancient lake that once occupied a large basin in what is now the southern Sahara. It was much larger than modern Lake Chad.

Q2. Where was Mega-Chad located?
The ancient lake occupied the basin associated with present-day Lake Chad. Its surviving southern portion is now represented by modern Lake Chad.