Droughts lasting centuries in West Africa are commonplace

Jeremy Hance
April 16, 2009

New evidence shows that sub-Saharan West Africa has experienced megadroughts in recent history lasting hundreds of years, far worse than the Sahel drought of the 1970s and 80s which left 100,000 dead.

To uncover West Africa’s past drought patterns, researchers compiled a year-by-year record of the last 3,000 years of climate in West Africa by looking at annually-occurring layers of sediment in Lake Bosumtwi in Ghana.

"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," says lead author Timothy M. Shanahan, who conducted the research while he was a doctoral student at The University of Arizona in Tucson. According to the study published in Science some of these droughts lasted for centuries, most recently from 1400 to 1750.

Boys from nearby villages practicing traditional fishing methods on Lake Bosumtwi. Large tropical trees submerged in 15-20 meters of water provide evidence of severe, long lasting droughts just a few centuries ago.Image courtesy of J.T. Overpeck and W. Wheeler, University of Arizona.
"What's really striking about droughts in this area is that they last such a long time," Shanahan said. "You have droughts that last 30 to 60 years, and then some that last four times as long. 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," said Shanahan, now an assistant professor of geosciences at the University of Texas at Austin.

The researchers found that devastating droughts normally occurred every 30 to 65 years. A pattern that is expected to continue—and may worsen due to global warming.

Changes in the North Atlantic sea-surface temperature significantly affected the West African monsoon, which alters the climate between wet and dry periods, according to the scientists. By raising sea-surface temperatures, global warming could create more extreme drought conditions, with droughts lasting even longer than those on record.

View of Lake Bosumtwi, which fills a meteorite impact crater formed 1.09 million years ago and contains an invaluable record of past changes in the West African monsoon. Image courtesy of T.M. Shanahan, University of Texas at Austin.
"We know that Africa gets long, bad droughts. We also know that global warming will make these droughts a lot hotter. This could be devastating," says Jonathan T. Overpeck, professor of geosciences and co-director of UA's Institute for Environment and Society.

Focusing on Lake Bosumtwi, the researchers also looked at geological records and submerged forests, which grew during drought periods when the lake vanished completely for centuries.

Paul Filmer, program director at the National Science Foundation, which helped support the study says that the “project is a good example of how work in the tropics on sediment records provides more detailed insight into climate patterns that affect millions of people in a highly vulnerable area of the world."

CITATION: "Atlantic forcing of persistent drought in West Africa". T. M. Shanahan, J. T. Overpeck, K. J. Anchukaitis, J. W. Beck, J. E. Cole, D. L. Dettman,J. A. Peck, C. A. Scholz, J. W. King. Science, Vol 324. April 17, 2009.

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Jeremy Hance
mongabay.com (April 16, 2009).

Droughts lasting centuries in West Africa are commonplace.