"Vegetation has a big influence, especially in semi-arid regions, in determining weather patterns," said Foley. "The loss of vegetation in itself contributed to a drier climate."
Human and animal populations came to rely more and more on water from the lake. Massive irrigation projects to combat the drier climate diverted water from both the lake and the two main rivers that empty into it, the Chari and the Logone.
The situation is a "domino effect," the researchers say. Overgrazing reduces vegetation, which in turn reduces the ecosystem's ability to recycle moisture back into the atmosphere. That contributes to the retreat of the monsoons. The consequent drought conditions have triggered a huge increase in the use of lake water for irrigation, while the Sahara has gradually edged southward.
Lake Chad is not likely to be replenished to its former size in our lifetime, the researchers say.
The lake's decline and the climate change have had an enormous impact on the 9 million farmers, fishermen, and herders living in the region. They have experienced crop failures, dying livestock, collapsed fisheries, and the continuous draining of the lake.
"The problem is expected to worsen in the coming years as population and irrigation demands continue to increase," Foley warned. "It shows how vulnerable our water resources can be."
Coe says the situation illustrates the urgent need to better manage water resources, which are limited. "There are enough people in the world now that we need to start planning and looking at fresh water as a finite resource or we're going to be in trouble," he said. "We don't get any more."
Lake Chad was one of the largest freshwater lakes in Africa, about the size of the state of Vermont. It is 820 feet (250 meters) above sea level and is fed by the Chari and Logone river system.
Lake Chad has always undergone seasonal and inter-annual fluctuations because it is less than 23 feet (7 meters) deep.
In recent decades, the lake expands during wet periods up to 10,000 square miles (25,900 square kilometers). The warming climate and increasing desertification in the surrounding Sahel region have caused water levels to decline much below the average dry season level of 4,000 square miles (10,000 square kilometers) to only 839 square miles (1,350 square kilometers).
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