Researchers say they have the first evidence that climate in Africa is influenced by conditions in the northern hemisphere, a finding that contradicts the main theory about climate in the subtropics.  As VOA's Jessica Berman reports scientists say the discovery will help them make future climate predictions.  

The Mabira forest reserve in Uganda

Climatologists have long debated what forces make countries in the tropics hot and humid and those in the northern hemisphere relatively warm and mild.

Andy Cohen is a professor of geosciences at the University of Arizona in Tucson. "The tropics are the heat engine of our planet, if you will.  So it is really important for understanding climate history ... and it also gives us a background for understanding and evaluating the kind of climate changes we are seeing around the world today," he said.

In a study published this week in Science, researchers offer the first solid evidence that the climate in tropical Africa is influenced by conditions in the northern latitudes, not by solar radiation patterns along the equator as is generally believed.

Investigators base their conclusion on an analysis of a 60,000- year-old sediment core extracted from the bottom Lake Tanganyika in the East African Rift Valley, which borders Burundi, Democratic Republic of the Congo, Tanzania and Zambia.  The lake is the world's second deepest fresh water lake.

Researchers found evidence in the long core sample, including leaf wax, which traces Africa's climate to the northern hemisphere.

Jessica Tierney is with the department of geological sciences at Brown University in Rhode Island and lead author of the study.

Tierney says there are indications within the core sample that climate conditions in Africa alternated dramatically over tens and thousands of years. "In the past we see these very abrupt changes from conditions that are arid to conditions that are quite wet.  And these changes can happen within several hundreds of years. That's very fast on the geologic scheme of years...  It is very jumpy."

In modern times, Tierney says it appears most of the influence on Africa's climate comes from forces in and around the Indian Ocean. "Most of the rain that falls over Lake Tanganyika today, that water that came from the Indian Ocean, it was evaporated in the western Indian Ocean and transported to the lake."

Experts say the study, which involves ancient climate records, does not address the impact of global warming.

But the records could offer a template against which to measure global warming, according to Andy Cohen, a study co-author. "It allows us then to say how big are the changes that are occurring now in comparison with those natural changes that occurred before people were pumping carbon dioxide into the atmosphere."

Researchers are conducting similar long core studies of Lake Malawi in the Rift Valley to confirm their findings.