NOTE TO EDITORS: The national contact for this story is Marc Airhart, Jackson
School of Geosciences, University of Texas, (512) 471-2241,
mairhart@jsg.utexas.edu.
A team of scientists from several major research universities, including Syracuse
University, has discovered that sub-Saharan West Africa has been plagued by
droughts much more severe than the infamous Sahel drought of the 1970s and 1980s,
which killed more than 100,000 people. The research, reported in the April 17 issue of
the journal Science, also suggests that rising global temperatures could provide the
conditions needed to trigger a prolonged, severe drought that could devastate the
region.
Using data collected from samples of sediment cores extracted from the basin of Lake
Bosumtwi in Ghana, the scientists developed the first, almost year-by-year record of
the last 3,000 years of West African climate. During that period, droughts lasting 30-
60 years were common. Surprisingly, however, these decades-long droughts were
dwarfed by much more severe droughts lasting three to four times as long-more
than a century. Their findings are published in the study "Atlantic Forcing of
Persistent Drought in West Africa."
"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 first author Timothy M. Shanahan, assistant professor of geosciences at the
University of Texas at Austin. "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."
Shanahan conducted the research while he was a doctoral student at the University
of Arizona. He is part of a research team that includes SU earth sciences professor
Christopher Scholz and geosciences professor Jonathan Overpeck of the University of
Arizona. Scholz and Overpeck are lead investigators on a large-scale study of Lake
Bosumtwi, funded by the National Science Foundation and the International
Continental Scientific Drilling Program.
Lake Bosumtwi formed 1.1 million years ago when a meteor several times the size of
the Carrier Dome crashed into the Earth's surface. The lake has no natural outlets. It
fills and dries with changes in precipitation and in the tropical climate. As a result, the
sediment below the lake, deposited over the past million years by inflowing streams,
contains some of the best-preserved records of climate change in the tropics.
"The lake is one of the best sites in the world for the study of tropical climate," Scholz
says. "To understand the current global climate, we need to have records of the
climate conditions that existed in the past."
Ten years ago, Scholz designed a special boat and scientific equipment to explore the
surface below Lake Bosumtwi. The boat was constructed in modules and shipped to
Africa, where it was reassembled. During the early expeditions, the research team
first gathered seismic reflection data that enabled its members to initially create maps
of the lake's subsurface and then collected cores of undisturbed lake mud to analyze
for detailed climate changes.
The geophysical data collected from the seismic maps suggested the lake's water
levels significantly fluctuated during the past 2,500-3,000 years. Shanahan's study
combines the early findings with a detailed study of sediment cores he collected on
subsequent expeditions. The result is a detailed look at geological records of the lake
level and other climate indicators, including submerged forests, which grew around
the lake during past droughts when the lake dried up for hundreds of years.
The new findings suggest that changes in sea surface temperatures of the Atlantic
Ocean have played a key role in sustaining drought over the sub-Saharan region for
decades to centuries. The findings support other recent research using climate models.
On 30- to 60-year intervals, sustained periods of wet and dry conditions appear to be
directly linked to a hypothesized mode of climate variability termed the Atlantic
Multidecadal Oscillation. Because long, high-resolution records of ancient climate
from the Atlantic are sparse, scientists had questioned the existence of such a mode.
"This paper provides a long-term context suggesting that the Atlantic Multidecadal
Oscillation actually exists," Shanahan says. "Our rainfall records are strongly related
to these really distant sea surface temperature reconstructions, at least on this
multidecadal time scale. It suggests that the rainfall patterns are being generated by
the sea surface temperature patterns and not by some other influence."
Armed with this new information, scientists who create climate models will be better
equipped to sort out some of the conflicting climate predictions for West Africa. Some
climate models have forecast wetter conditions for West Africa, while others have
forecast drier conditions. The results of this study highlight the susceptibility of this
system to sustained dry conditions and suggest that climate records from the past 100
years do not capture the severity of natural droughts that have been common in the
sub-Sahara over the last 3,000 years.
"The study suggests that rising air surface temperatures could cause the climate
pattern in sub-Saharan West Africa to revert back to this century-scale drought mode
without too much difficulty," Scholz says. "The drought that occurred 30 years ago
in the region would be dwarfed by the kind of prolonged, decades-long drought that
has occurred during the past few thousand years."
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