West Africa is already living on the edge, and new research indicates that even worse droughts are possible than the one that devastated the region in the late 20th century.
As many as 100,000 people died in the multi-decade Sahel drought that began in the 1960s, and a detailed look at lake sediments in Ghana indicates that such dry periods have occurred periodically, punctuated by occasional century-long droughts.
Changing climate could cause even more such disastrous droughts, according to researchers who report their findings in Friday's edition of the journal Science.
"Clearly, much of West Africa is already on the edge of sustainability, and the situation could become much more dire in the future with increased global warming," said lead researcher Jonathan Overpeck of the University of Arizona.
Timothy Shanahan, an assistant professor of geosciences at the University of Texas at Austin, added: "Even more disconcerting ... is that it suggests the region is vulnerable to longer and more severe droughts."
"What's really striking about droughts in this area is that they last such a long time," he said.
The researchers looked at sediments from Lake Bosumtwi spanning about the last 3,000 years.
As global warming progresses, the increases in temperature may make the normal climate pattern more extreme, producing even more severe and prolonged droughts than those of the past, Overpeck said.
"We also know that global warming will make these droughts a lot hotter. This could be devastating," said Overpeck.
Shanahan noted in a teleconference that there have been six large, century-scale droughts in the last 2,700 years in the region, including two "really large ones" in the last 1,000 years.
In the two most recent large droughts the lake level dropped about 81 feet and 97 feet respectively, while in the Sahel drought it dropped just over 16 feet, Shanahan said.
The researchers were able to tell wet from dry years by analyzing the amounts of different forms of oxygen in the lake sediment.
In addition, they found remains of a partially submerged forest, which grew during a centurylong drought only a few hundred years ago when the lake was much lower, added Shanahan.
The team found a correlation between wet and dry periods and a shift between warmer and cooler ocean water called the Atlantic Multidecadal Oscillation.
"Temperatures are rising because of human causes and that will have a knock-on effect by creating more evaporation from the soil and vegetation," Overpeck said.
The research was supported by the National Science Foundation.