Central equatorial Africa鈥檚 benign climate over the past century could be an aberration. A new study shows that for the past 1500 years the region has suffered from frequent droughts lasting anywhere from 10 to 100 years. With growing populations already straining today鈥檚 water supplies, a return to earlier conditions could be disastrous.
鈥淔or the past 1500 years the region has suffered from frequent droughts lasting anywhere from 10 to 100 years鈥�
Until recently researchers had thought that the climate in central equatorial Africa had been stable for the past millennium or two, although they had little evidence to support the claim. To better understand the region鈥檚 climate history, geologist James Russell of Brown University in Providence, Rhode Island, decided to study cores drilled from the bottom of Lake Edward, which is bordered by the Democratic Republic of the Congo and Uganda. Russell鈥檚 team measured the amount of magnesium in calcite deposits in the lake bed, which increases during droughts, and found that droughts had been common during the periods AD 540 to 890, 1000 to 1200, and 1400 to 1750 (Geology, vol 35, p 21). The dry spells sometimes lasted for up to a century. Before this study, the only detailed lake-bed measurements had come from Lake Naivasha in Kenya, about 700 kilometres to the east. While the climate in east Africa showed similar variations over time, it was wet when the central areas were dry between 1400 and 1750.
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The climate of both regions is affected by the intertropical convergence zone, the area near the equator where moist northern and southern air masses converge and rise, creating low pressure and often heavy rain. It migrates north and south seasonally with the sun, and its position can also change over periods ranging from decades to millennia.
That could explain the droughts between 1000 and 1200 in the Congo basin and Kenya. 鈥淏ut you can鈥檛 invoke north-south changes to explain an east-west gradient,鈥� says Russell. He suspects that the gradient came about as a result of changes in the flow of moist air over Africa from the Indian and Atlantic oceans.
Over the past century of stable climate, large populations have become dependent on rain-fed farming, and just one or two years of failed monsoon rains can cause famines. 鈥淵ou can just imagine what 10 or 20 years of monsoonal failure would mean,鈥� says Russell. While he cannot predict how global warming would affect climate in the region, he says it is likely to become more unstable.
The problem is a serious one because the demand for water 鈥渋s already approaching or exceeding the available supply in many areas鈥�, says Dirk Verschuren of Ghent University in Belgium, who studied Lake Naivasha.
