Why it rains in Africa or not

Close climate coupling of land and ocean controls precipitation

Africa from space. © DLR
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The rainfall in tropical Africa is closely related to the temperature difference between the land and the tropical Atlantic. Dutch and German researchers are now publishing these results in the journal Science. For this purpose, they used a new method with which for the first time detailed past temperature developments for the country can be reconstructed. Their results allow conclusions to be drawn about future climate scenarios in tropical Africa.

"Our work shows that over the past 25, 000 years, the temperature in Central Africa has risen far more than the temperature of the tropical Atlantic Ocean, " explains dr. Enno Schefuß, who was a significant contributor to the study as an employee of the DFG Research Center Ocean Margins in Bremen. While today's land and sea are about equally warm, the temperature difference in the last ice age was much greater. The country was four degrees Celsius (° C) colder than today, the ocean water, however, only about 2.5 ° C.

This stronger temperature contrast had major consequences for the climate, especially rainfall in Africa. A comparison with existing hydrological data showed that the colder the land was compared to the ocean, the less rain fell. Because: If the air cools off over the land, it sinks down and hinders the transport of moist sea air to the countryside. Conversely, warming the country causes more rainfall.

To decode the hard-to-access temperature data from the past for the country, it took some detective work. In the sea, it is above all the remains of algae deposited on the seabed that have stored information about environmental conditions.

Sediments neatly stacked

"The sediment layers on the sea floor are neatly stacked and can be read like a book about the past, " explains Dr. Enno Schefuß, who has since moved to the University of Kiel. "On land, however, such climate archives are rare and usually incomplete where available. Soils contain information about the climate, but erosion tears pages out of the book, so to speak. "Display

However, soil material and the information contained therein are constantly being transported by large rivers into the sea. Thus, not only extensive and consistent information about the marine environment can be found on the seafloor, but also what happened in the catchment area of ​​the rivers on land. The researchers used this circumstance with a newly developed method that allows for the first time to reconstruct the temperature development on land for long periods from marine sediments.

The lead author of the study, Johan Weijers, found that soil bacteria produce certain organic compounds whose structure is essentially temperature-dependent. The method was developed at the Royal Netherlands Institute for Sea Research in Texel and thus allows to directly compare the temperatures in the sea with the climate data of land.

More rain or not?

The results of the study also show that the temperature difference between sea and land was not only responsible for the rainfall during the long-term development of the climate, but also worked during rapid climatic changes.

"This is followed by two scenarios of how the climate in this region can continue to develop in the future, " continues Schefu . Further global warming alone would heat up the country more than the ocean it would bring more rain. However, if there is a further weakening of the Gulf Stream, the tropical ocean would heat up considerably. This would result in much less rainfall in Central Africa, with all the consequences for local living conditions.

(idw - MARUM_Forschungszentrum Ozeanr nder / Kirsten Achenbach, 23.03.2007 - DLO)