New York Oct 22 : Contradicting the prevailing notion that the African continent has been getting progressively drier over time, a new study has found that drought in the continent has actually decreased over the past 1.3 million years.
The study, published in the journal Nature, also found that the continent is on a 100,000-year cycle of wet and dry conditions. Josef Werne, Associate Professor at University of Pittsburgh, along with colleagues from other universities in the US, Australia, Chile, and the Netherlands, made the discovery by examining core samples extracted from the bottom of Lake Malawi, one of the world's largest lakes, located between Malawi, Mozambique, and Tanzania in southeastern Africa.
These new findings challenge the savannah hypothesis - one of the keys to human evolutionary theory -- which states that the progressively drier conditions in Africa led to prehuman ancestors migrating from forests and moving into grasslands.
Previous studies of the climate of Africa focused on the northern part of the continent, and were responsible for the origin of the savannah hypothesis that the continent was getting drier, Werne explained.
Lake Malawi had not been explored previously because the depth of the waters - 700 feet - exceeded researchers' ability to get core samples from the bottom.
The researchers were able to overcome that limitation by using a barge and modifying oil-rig equipment to obtain a 380-meter-long sediment core sample.
The core was dated using a combination of radiocarbon, volcanic ash, and magnetic polarity reversals and examined for "molecular fossils" indicating changing temperature and rainfall.
By noting the changes in temperature records and especially rainfall, the team determined that the continent was getting wetter over time in southern East Africa.
"Climate in this sector of eastern Africa (unlike northern Africa) evolved from a predominantly arid environment with high-frequency variability to generally wetter conditions with more prolonged wet and dry intervals," the study said.