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Last Updated: Tuesday, 14 October, 2003, 15:28 GMT 16:28 UK
Oceans linked to Sahel drought
A study has suggested that sea-surface temperature rises may have been to blame for the Sahel droughts that devastated West Africa in the 1970s and 1980s.

Map, BBC
The droughts, in which over a million people are thought to have died, were initially blamed on human degradation of the local environment.

But a large climate model at Columbia University in the US has seemingly found that ocean factors were more crucial.

"What we did was... to compare the model simulations with the real observations," Dr Alessandra Giannini told BBC World Service's Science In Action programme.

"We saw that the model was capable of reproducing the droughts in the Sahel in the long-term negative trend in rainfall.

"In this way we were able to attribute this long-term trend to the sea-surface change in temperatures."

Converging factors

The study looked at the Pacific - in particular the effect of the El Nino phenomenon and the effect on the Sahel rainfall - Atlantic, and Indian oceans.

It was found that the Indian Ocean was a powerful indicator of the level of precipitation in the Sahel over the long term, while the Pacific affected seasonal rainfall.

The waters around Africa were warmer than average, the study found, which weakened the "continental convergence associated with the monsoon" and left West Africa dry.

The drought pushed the Sahara into Nigeria, Niger and Mali
"The only thing that our model knows about the real world is the sea-surface temperatures," Dr Giannini explained.

"So anything that this model does that resembles the real evolution of our climate system that we've observed since the 1930s has to do with the sea-surface temperatures."

There were broadly two hypotheses to explain what happened in this semiarid region of Africa. One blamed the drought on changes brought about by human land use.

The expansion of farming and herding into marginal areas was said to have produced a spiral of changes, in which reduced vegetation led to reduced rainfall, producing further decreases in vegetation and still less rainfall.

The other hypothesis focused on temperature changes in the global oceans as the main culprit behind the drought.

"And as time went on and research has continued to study the problem, the hypothesis that this may have been due to larger scale - global scale - climate variability came out," said Dr Giannini.

"This was studied both with observational data - a statistical analysis - and also with some simple modelling experiments, by the UK Met Office in the mid-1980s."

Rainfall forecasts

It is hoped that, with such a link established, the model can now be used to predict future rainfall in the Sahel.

This could be crucial in giving time to plan for any forthcoming drought.

"We may be interested in seasonal or annual predictability - and in this case the fact that there is a connection between the sea-surface temperatures globally and the Sahel is a good sign," Dr Giannini said.

Dead cow in drought, BBC
If the sea temperature rise comes from global warming then we may be in dire straits
Dr Alessandra Giannini
"It means that if we can predict the surface temperatures with a season to a year of lead time then we should be able to say something about precipitation in the Sahel.

"That's the optimistic point of view."

However, she added that as yet scientists did not know what had caused the Indian Ocean to warm up.

But she said that it was possible it was due to global warming - in which case the Earth could be in "dire straits".

"The pessimistic point of view comes from looking at the longer term," she said.

"If we hypothesise that this trend in Sahel rainfall may be related to Indian Ocean sea-surface temperatures, then we have to ask ourselves where does this trend come from?

"If it comes from global warming then we may be in dire straits again - but that's all to be proven."

Dr Giannini's team published its work in the journal Science.

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