A new study shows that about eleven thousand years ago Africa went through a natural global warming process that led to constant rainfall. Back then, wide areas of desert were turned into grasslands and savannas. Scientists believe that the same process is about to repeat.
“The future impact of greenhouse gases on rainfall in Africa is a critical socioeconomic issue. Africa’s climate seems destined to change, with far-reaching implications for water resources and agriculture,”
Bette Otto-Bliesner, researcher at the National Center for Atmospheric Research (NCAR) and lead author of the study, said.
The study revealed that 14.700 years ago, two major African regions – the southern equatorial and northern part – had increasingly more rainfall than it has today. Researchers used a computerized climate model that simulated how Africa’s weather looked like thousands of years ago. The computer used analyzes of sediments and other clues about past climate.
Researchers also tried to explain why Africa had such dramatic climate changes during its history. Otto Bliesner and her US and Chinese colleagues tried to find a coherent explanation for this. It seems that 21.000 years ago the ice sheets covering most of America and northern Europe began to melt. At first, Africa had a very dry weather but after 4.000 years it became very humid. Between 12.700 BC and 3.000 BC a lot of rain occurred in Africa. During this period (called African Humid Period or AHP) deserts became covered with grasslands and savannas.
Previous research says that AHP was caused by a shift in Earth’s orbit that led to hotter northern regions during summer time. This heating must have somehow influenced the monsoons triggering rainfall.
However, Dr Otto-Bliesner said the orbital shift would not suffice to explain why Africa was so rainy its southern part since the shift in Earth’s orbit led to less summertime heating in this region. The study showed there are two more factors that caused AHP – a change in the Atlantic Ocean circulation and a natural greenhouse effect.
Eleven thousand years ago, greenhouse gases (especially Co2 and methane) increased to pre-industrial levels, but scientists don’t know the exact reason for this natural global warming.
Also, 17.000 years ago, the melting ice sheets brought some extra water into the Atlantic Ocean and disturbed the way it transported heat and salinity. This circulation now shifted precipitation toward South Africa, leaving the northern, eastern and equatorial parts dry. When the global melting stopped, the circulation brought back rainfall in northern and southeastern parts of Africa. This meant that desert regions of Africa had constant rainfall.
These three factors combined – the orbital shift, Atlantic shift and natural global warming – made Africa a rainy continent between 12.700 BC and 3.000 BC , but the new research shows that it will rain more in Africa due to global warming effect.