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Changes in sea surface temperatures were the main cause of shifting rainforest patterns in southern Africa in the past, according to new research.

The study looked at marine deposits dating from 450,000 to 1.2 million years ago, and suggests that fluctuations in sea temperature in the tropical Atlantic determined the expansion and shrinkage of rainforests and dry savanna belts in southern Africa over that period.

"The realisation that shifts in the vegetation zones are largely independent of other climate factors is completely new," says one of the researchers, Enno Schefuss, who is currently at the DFG Research Center Ocean Margins in Bremen, Germany.

His team's findings — which are published in this week's Nature — can be explained by the influence sea surface temperature has on the interplay between evaporation and rainfall.

When the sea is cooler, less water evaporates, resulting in reduced rainfall and the replacement of rainforests by dry savannas. When the sea is warmer, there is more rainfall and the rainforest zone can flourish and expand.

"We can now verify long-term processes that, until now, meteorologists and climate modellers could only provide presumptions for," says Schefuss. "It is also an indication of the importance of the influence of ocean margin regions on our land climate."

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Link to Nature research paper