The Atlantic Ocean and Africa

THE TROPICAL ATLANTIC CONTROLS AFRICA´S CLIMATE - Nature article emphasizes the importance of ocean margin regions

EMBARGO: March 26, 2003, 20:00

Our impressions of the great African landscape are dominated by images of deserts, savannas, and rain forests. Over long periods of time these vegetation zones are characterized by extreme change. In the upcoming volume of the journal "Nature" (Mar. 27, 2003) Dr. Enno Schefuß of the DFG Research Centre Ocean Margins, along with Dutch colleagues, explains why African rain forests have expanded and shrunk through time, and why the dry savanna belts have grown and receeded. They report that the rise and fall of sea temperature in the tropical Atlantic is the controlling factor. The results are based on investigations of marine deposits dating back to 1.2 million years.

According to the marine scientist's investigations, temperature developments in the upper water layer of the tropical Atlantic control the interplay between evaporation and precipitation. These, in turn, have a long-term influence on the development of plant life in tropical-to-subtropical Africa. When the tropical Atlantic is cooler less water evaporates, resulting in a decrease in precipitation over the continent, and transformation of the moist rain forests into dry savannas. When the upper water layer is warmer the rain forest zone can flourish and expand again.

The research team sampled deep-sea sediment cores retrieved in 1998 during a cruise of the international Ocean Drilling Program, with Bremen scientists participating, from the Angola Basin off the west coast of central Africa. The marine sediments document environmental and climate conditions within the time frame of 450,000 to 1.2 million years ago. The researchers were searching for particular waxy compounds that originated from land plants and were blown into the sea by the trade winds. These compounds can be clearly distinguished as originating from either the rain forest trees or the savanna grasses. These waxes can therefore be used to reconstruct the changes in the African landscape for this ancient time frame.

In addition to the wax compounds, the drilling cores contain the remnants of single-celled algae. From their organic remains, Schefuß & Co. were able to glean information about changes in the surface-water temperatures in the tropical Atlantic. Comparison of the two data sets reveals a close link between the rain forest/grassland fluctuations and the temperature development of the surface waters of the Angola Basin between 1.2 million and 450,000 years ago. "These results indicate that the levels of precipitation on the African continent are dependent on temperature changes in the Atlantic warm-water reservoir," states the Bremen geochemist.

"The realization that shifts in the vegetation zones are largely independent of other climate factors is completely new. Even the growth of the ice shields in far northern Scandanavia and Greenland is not the ultimate deciding factor in the development of rain forests and savannas," says Dr. Enno Schefuß. "With this we can now verify long-term processes that, until now, meteorologists and climate modelers could only provide presumptions for. It is also an indication of the importance of the influence of ocean margin regions on our land climate," says the recently appointed scientist at the DFG research center.

Further information:
Kirsten Achenbach
Public Realtions
DFG Research Center Ocean Margins
Tel. 0421 - 218-9000
mail:
www.oceanmargins.de