By Deborah Zabarenko, Environment Correspondent
WASHINGTON, April 16 (Reuters) - The recent decades-long drought that killed 100,000 people in Africa’s Sahel may be a small foretaste of monstrous "megadroughts" that could grip the region as global climate change worsens, scientists reported on Thursday.
Droughts, some lasting for centuries, are part of the normal pattern in sub-Saharan Africa. But the added stress of a warming world will make these dry periods more severe and more difficult for the people who live there, the scientists said.
"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 University of Arizona climatologist Jonathan Overpeck, a co-author of the study published in the journal Science.
The Sahel is an area between the Sahara desert and the wetter parts of equatorial Africa that stretches across the continent from the Atlantic Ocean in the west to the Red Sea in the east.
Overpeck and his colleagues studied sediments beneath Lake Bosumtwi in Ghana that gave an almost year-by-year record of droughts in the area going back 3,000 years. Until now, the instrumental climate record in this region stretched back only 100 years or so.
The researchers found a pattern of decades-long droughts like the one that began in the Sahel in the 1960s that killed at least 100,000 people, as well as centuries-long "megadroughts" throughout this long period, with the most recent lasting from 1400 to 1750.
The scientists also described signs of submerged forests that grew around the lake when it dried up for hundreds of years. The tops of some of these tropical trees can still be seen poking up from the lake water.
RISING TEMPERATURES, NASTIER DROUGHTS
During the recent Sahel drought, the lake’s water level dropped perhaps 5 yards (meters). By contrast, during megadroughts the level fell by as much as 30 yards (meters).
"What’s disconcerting about this record is that it suggests that the most recent drought was relatively minor in the context of the West African drought history," said Timothy Shanahan of the University of Texas, a co-author of the study.
The most recent decades of data culled from Lake Bosumtwi show that droughts there appear to be linked to fluctuations in sea surface temperatures, a pattern known as the Atlantic Multidecadal Oscillation, or AMO, the researchers said.
"One of the scary aspects of our record is how the Atlantic ... changes the water balance over West Africa on multidecadal time scales," Overpeck said in a telephone briefing.
The cause of centuries-long megadroughts is not known, but he said the added burden of climate change could make this kind of drought more devastating.
Temperatures in this region are expected to rise by 5 to 10 degrees F (2.77 to 5.55 degrees C) this century, the scientists said, even if there is some curbing of the greenhouse emissions that spur climate change.
"We might actually proceed into the future ... we could cross a threshold driving the (climate) system into one of those big droughts without even knowing it’s coming," Overpeck said.
(Editing by Will Dunham)