Catastrophic droughts in Africa are the norm, claim scientists

Catastrophic droughts in Africa such as the those which devastated the Continent in the late 20th century are the norm and not due to human activity, claim scientists.

By Richard Alleyne, Science Correspondent
Last Updated: 6:14PM BST 16 Apr 2009

In the past, many scientists thought the drought in the Sahel zone – a band that runs just below the Sahara – was caused by humans overusing natural resources in the region.

In a new study in the journal Science shows that they are a natural part of weather pattern of the area for the last 3,000 years. If anything the droughts were less severe than those seen historically, with previous periods without rain lasting more than a century.

The earlier dry spells dwarfed the well-documented drought that plagued West Africa in the late-20th century, and as the planet warms, the study's authors believe the region's rainfall patterns will have an even greater impact.

During the 1970s and 1980s the drought pushed the Sahara desert south, destroying farmland. It had a major impact on many countries including Nigeria, Niger and Mali.

"Clearly, much of West Africa is already on the edge of sustainability and the situation could become much more dire in the future with increased global warming," said Jonathan Overpeck of the University of Arizona.

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.

The study shows regular periods of dryness, particularly droughts in the 30-40 year range. Some have even lasted centuries.

The team, which reported its findings in Science, believe the arid periods correspond with fluctuations in sea surface temperatures, a pattern called the Atlantic Multidecadal Oscillation (AMO).

Previous researchers produced a computer model that included ocean surface temperature, the amount of moisture in the soil, and loss of vegetation.

With all those conditions, the computer model behaved just like the Sahel drought – producing a long period of dry, cool weather.

"What's 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 co-author Timothy Shanahan, of the University of Texas,

"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."