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.
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.
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