Sunday, April 19, 2009

Drought in Africa - Not Caused by Global Warming?

Well, it seems that the severe drought in Africa was not caused by AGW. How can that be? Well the eggheads have determined that this is part of a long term climate cycle. Its happened before and will happen again. Think this will be widely reported? Not!

Africa trapped in mega-drought cycle

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 and colleagues at the University of Texas, Austin, 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).

Close to the edge

"Clearly much of West Africa is already on the edge of sustainability," says Jonathan Overpeck of the University of Arizona, Tuscon, who was Shanahan's doctoral supervisor while the Lake Bosumtwi study was carried out. He believes the situation could worsen with climate change.

Several studies have suggested that fluctuations in the surface temperature of the North Atlantic Ocean are partly responsible for shifts in the African monsoon. Shanahan and colleagues found more evidence in support of that when they compared sea temperature records with the patterns in their sediment samples and found a strong correlation.

Some models forecast that changes to North Atlantic temperatures caused by global warming will dry out the Sahel even more. "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," says Shanahan.

But Reindert Haarsma, a meteorologist at the Royal Netherlands Meteorological Institute, points out that there is still disagreement among climate scientists on whether the Sahel will become wetter or drier with climate change. African weather is among the least studied globally, so forecasts are extremely uncertain.

Journal reference: Science (DOI: 10.1126/science.1166352)

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