Extreme droughts lasting centuries are the norm for West Africa, scientists have found.
Researchers analysed 3,000 years-worth of sediments in Lake Bosumtwi, Ghana, and found that the last such 'megadrought' ended 250 years ago. Droughts of this severity dwarf the region's driest season to date — the Sahel drought, which killed at least 100,000 people in the 1970s and 80s.
"If the region were to shift into one of these droughts it would be very difficult for people to adapt; and we need to develop an adaptation policy," says Tim Shanahan at the University of Texas in the United States.
There is also concern that man-made climate change could worsen the situation — but the droughts will happen either way, scientists warn.
Megadroughts not only last longer than other droughts but are also drier.
Those lasting a few decades may be linked to the Atlantic Multidecadal Oscillation (AMO) — a climatic cycle in which Atlantic Ocean sea surface temperatures vary over time — but the cause of the centuries-long droughts are not known.
Michael Schlesinger, who first characterised the AMO, told BBC Online: "The only way I can see of dealing with [a 100-year long drought] is desalination; if push comes to shove and these megadroughts appear — and they will, and it will probably be exacerbated by man-made global warming — that will be the only thing to do".
Science 324, 377 (2009)