- Scientist have identified hitherto unknown climate characteristics of West Africa; returning "megadroughts" that last for a century. The last one ended 250 years ago, and the next is sure to come, they conclude.
A new study of lake sediments in Ghana, published in the latest issue of the journal 'Science', suggests that severe droughts lasting several decades, even centuries, were the norm in West Africa over the past 3,000 years - which is the era more or less consistent with the region's current climate.
The earlier dry spells dwarfed the well-documented drought that plagued West Africa in the late 20th century. The returning "megadroughts" are both heavier and longer lasting, the US researchers believe.
The scientists believe the megadroughts are driven in part by circulation of the ocean and atmosphere in and above the Atlantic - and possibly beyond. If climate models for such circulation patterns hold true, the study suggests global warming could create conditions that favour such extreme droughts. Other studies however suggest that global warming could provide West Africa with a moister climate.
The findings emerged from sediments that lie at the bottom of Lake Bosumtwi in Ghana, deposits of soil and organic matter that contain annual bands of light (winter) and dark (summer) layers that stretch back more than three millennia.
"Lake Bosumtwi is really unique in that its one of the few locations in tropical West Africa where varves, annual sediment layers, are preserved. This allows us to look at changes in climate at very high resolution," explains lead author Timothy Shanahan, now an assistant professor at the University of Texas.
"Support for our geochemical interpretations also came from evidence for past lake stands during drought periods, including a partially submerged forest, which grew during a century-long drought only a few hundred years ago when the lake was much lower," added Mr Shanahan.
Data sets from the lake show periods of "normal" droughts in a 30-40 year cycle. These correlate to fluctuations in sea surface temperatures, a pattern called the Atlantic Multidecadal Oscillation. Earlier, tree-ring variations from sites around the West Atlantic, have hinted at the same scenario.
The lake's sediment record is also punctuated by less frequent, but much more severe, century-long drought events. Because of the size and duration of those events, their impact would have been much more severe than the multi-decade droughts linked to the Atlantic Multidecadal Oscillation.
"What is disconcerting about this record is that it suggests the most recent drought was relatively minor in the context of the West African drought history," said Mr Shanahan. "If we were to switch into one of these century-scale patterns of drought, it would be a lot more severe, and it would be very difficult for people to adjust to the change."
The researchers were not able to make predictions on when the next "megadrought" could be expected to hit West Africa. But the last being "only" 250 years ago, there could still be many generations enjoying the region's current climate conditions.
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