By Richard Black
Environment correspondent, BBC News website
A partially-submerged tree shows Lake Bosumtwi was shallower in the past
Severe droughts lasting centuries have happened often in West Africa's recent history, and another one is almost inevitable, researchers say.
Analysis of sediments in a Ghanaian lake shows the last of these "megadroughts" ended 250 years ago.
Writing in the journal Science, the researchers suggest man-made climate change may make the situation worse.
But, they say, the droughts are going to happen again anyway, and societies should begin planning for them.
"It's disconcerting - it suggests we're vulnerable to a longer-lasting drought than we've seen in our lifetime," said Tim Shanahan from the University of Texas in Austin, who led the research team.
"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."
The region's most recent dry episode was the Sahel drought which claimed at least 100,000 lives, perhaps as many as one million, in the 1970s and 80s.
But the historical "megadroughts" were longer-lasting and even more devoid of precipitation, the researchers found.
The evidence comes from Lake Bosumtwi in southern Ghana, a deep lake formed in a meteorite impact crater.
Sediments laid down each year form neat, precise layers.
"Nothing lives at the bottom of the lake, so nothing disturbs these layers," said Professor Shanahan.
"Most lakes have this seasonal deposition, but it's rare in the tropics to find a lake where the bottom is undisturbed."
Wet and dry years are distinguished by the ratio of two oxygen isotopes in the sediment.
Layers of sediment preserve a record of rainfall
Droughts lasting a few decades occur regularly over the 3,000 years contained in this record.
They appear to be linked to the Atlantic Multidecadal Oscillation (AMO), a natural climatic cycle in which sea surface temperatures in the tropical Atlantic Ocean vary over time.
The Sahel drought coincided with a cool phase of the AMO. This changes wind patterns, and decreases the strength of the monsoon rains in this region.
However, the cause of the longer, multi-century droughts is not clear.
"That's one of the scary aspects - we have no idea what causes them," said Jonathan Overpeck from the University of Arizona, who oversaw the research effort.
"In Africa, we could cross the threshold, driving the system into one of these droughts, without even knowing why."
Michael Schlesinger, who first characterised the AMO a decade ago but was not involved in the current study, suggested a similarity between the outlook for West Africa and the southwestern portion of the US.
There, research has also shown a history of shorter and longer droughts.
"There are two things that need to be done, one of which California and Arizona and so on have done - and that is put in the water collection and distribution infrastructure to deal with the short periods of not very intense water stress," the University of Illinois at Urbana-Champaign scientist told BBC News.
The southwestern US has seen prolonged dry weather
"What West Africa won't handle - and neither will California - is the 100-year-long, deep megadrought.
"The only way I can see of dealing with that is desalination; if push comes to shove and these megadroughts appear - and they will, and it'll probably be exacerbated by man-made global warming - that will be the only thing to do."
Whereas the southwestern US could afford desalination, it is not clear that West African countries could - nor do they all have the infrastructure to move water inland.
The possibility of man-made climate change causing worse droughts is an example of the impacts that many developing countries fear, and which causes them to seek money from richer countries to protect their societies and economies.
Professor Schlesinger is at one with Tim Shanahan's team in suggesting that human-induced climate change would be likely to make droughts more severe, although computer models of climate produce varying projections for rainfall change over the West African region.
But even without changing the chances of drought, rising temperatures worsen the region's outlook, suggested Professor Overpeck.
"Even if we were able to reduce greenhouse gas emissions somewhat, we would still probably have warming in this region of about 2-4C over the century, and that could make droughts much harder to adapt to when they occur," he said.
"What it's pointing to is the need to reduce greenhouse gas emissions; but you can't do it all with mitigation, just as you can't do it all with adaptation."