Instrumental and observational records of climate in West Africa
suggest that this region may be susceptible to abrupt, decades-
long drought events, with potentially catastrophic impacts for
the people living in this region. However, because of the dearth
of long, continuous and high quality climate records from sub-
Saharan Africa, little is known about the long-term frequency
and persistence of drought events in this region. It is also
unclear whether observed 20th century droughts are natural or
due to human impacts. In the present study, we use several
complementary approaches to develop a high-resolution record
of paleoclimatic changes in West Africa from the geological
record preserved at Lake Bosumtwi, Ghana.

Our results suggest that West Africa has undergone significant
hydrologic variations over the last ca. 10,000 years. The
dominant influence on hydrologic changes over this interval was
changes in northern hemisphere summer insolation and the
associated feedback processes acting in the oceans and on land.
This led to a more northerly position of the Intertropical
Convergence Zone (ITCZ) and increased precipitation. In the late
Holocene, a secondary increase in precipitation occurred as a
result of the southward migration of the ITCZ. Over the last ca.
3 ka, the evolution of the West African monsoon has been more
gradual, and consistent with changes seen in the Asian and
Indian monsoon regions, presumably as a result of large scale
forcing of tropical convection.

The West African monsoon also varies on timescales from
millennia to decades. Millennial and century-scale variations
appear to be paced by changes in solar irradiance, either directly
or indirectly. On decadal timescales, variability appears to be
dominated by changes in Atlantic sea surface temperatures. The
dominant mode is a ca. 40 year oscillation, which in strongly
coherent and in phase with the Atlantic Multidecadal Oscillation
(AMO). These data suggest that the West African monsoon is
affected by significant, natural climate variability on timescales
of decades to centuries. In this context the Sahel drought may
represent the most recent of these events forced by the Atlantic,
and does not necessarily require an anthropogenic cause.