African Droughts In Perspective
By Paul Homewood
The submerged trees on Lake Bosumtwi, Ghana, stand in 15 to 20 metres of water
With recent news that, despite IPCC claims to the contrary, there has been little change in drought conditions in the past 60 years, it is perhaps time to stand back a little and look at droughts in Africa from a slightly longer term perspective.
According to this article in New Scientist, for instance:-
The infamous 1970s drought of the African Sahel region, which lasted several decades and killed more than 100,000 people, was actually a "minor" event, say researchers who have uncovered evidence that such droughts occur cyclically in the region and can be much more severe.
Timothy Shanahan at the University of Texas, Austin and colleagues analysed the first rainfall dataset that spans several millennia. "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," he told New Scientist.
The researchers analysed a sediment core pulled from the bottom of Lake Bosumtwi, Ghana’s only natural lake. The lake is an ancient meteorite impact crater, making its levels very dependent on rainfall.
By studying the relative amounts of different oxygen isotopes in the sediment core, the team could reconstruct rainfall dating back 3000 years. Higher concentrations of the slightly heavier – and therefore harder to evaporate – 18O indicate periods of drought.
Dry for decades
They found that the region’s history was punctuated by droughts lasting several decades, every 30 to 60 years. Each was comparable to the drought of the 1970s, which killed more than 100,000 people, according to UN estimates.
Alessandra Giannini of the International Research Institute for Climate and Society at Columbia University says that historical accounts of how centres of political power moved throughout the region over the millennia are consistent with periodic periods of drought.
But the sediment cores also revealed a more alarming pattern. As well as the periodic droughts lasting decades, there was evidence that the Sahel region has undergone several droughts lasting a century or more.
The most recent mega-drought was just 500 years ago, spanning 1400 to 1750 and coinciding with Europe’s Little Ice Age. At the time, Lake Bosumtwi dropped so low for so long that a forest sprouted on the crater’s edges. Those trees now stand in 15 to 20 metres of water (see images, right).
The reference to the LIA, of course, is interesting because the 1970’s drought occurred at the same time as the Northern Hemisphere was cooling down.
More in Part II tomorrow.