Nairobi — Countries in the Sahel region of Africa will receive more rainfall and floods, while southern Africa will experience persistent drought in the coming decades, say researchers.
James Hurrell of the US National Centre for Atmospheric Research and Martin Hoerling of the US National Oceanic and Atmospheric Administration say the changes are linked to temperature changes in the Indian and Atlantic oceans and are partially caused by greenhouse gas emissions.
"Changes in the Indian and Atlantic oceans are causing climate change in Africa and will have ripple effects on people and the environment," says Hurrell.
The researchers drew their conclusions, which they presented at the annual conference of the American Geophysical Union on 24 May, after analysing 60 computer models that imitate global climate.
The findings indicate that the warming of the Indian Ocean is responsible for the current drought in southern Africa, while temperature changes in the Atlantic have generated more rainfall in the Sahel.
Since 1970, recurrent droughts have caused crop failures in Angola, Botswana, Lesotho, South Africa, Swaziland, Zambia and Zimbabwe. Hurrell and Hoerling attribute these droughts to warming of the nearby Indian Ocean.
"In our models, the Indian Ocean shows very clear and dramatic warming into the future, which means persistent drought for southern Africa [and parts of the Horn of Africa]."
Hurrell and Hoerling predict that these conditions will intensify in the 21st century.
The researchers compared model results from 1950-2000 to several control runs that omitted the Indian Ocean warming. None of these showed the magnitude of drying that has actually occurred in southern Africa.
However, when the models did include the Indian Ocean warming, southern Africa consistently dried out, matching reality. The experiment suggests that the warming Indian Ocean is responsible for the drought in southern Africa.
The researchers add that there is a strong suggestion that greenhouse gas emissions from human activity are responsible for the warming of the Indian Ocean.
The models showed that by 2050, the monsoon winds that bring seasonal rain to sub-Saharan Africa could be 10-20 per cent drier than the 1950-2000 averages in southern Africa.
For much of 1950-2000, the southern Atlantic Ocean was warmer than the northern Atlantic. This, say the researchers, drew rain-bearing monsoon winds away from the Sahel, contributing to the very dry conditions there.
From the 1990s, however, the situation switched and the northern Atlantic became warmer than the southern ocean, partly because of higher levels of greenhouse gases in the atmosphere.
As a result, say the authors, we are witnessing more rainfall in the Sahel.
The study also showed that climate changes to Africa's monsoons have occurred in the distant past because of variations in solar output.
From this long-term perspective, says Hurrell, Africa's recent droughts "appear to be neither unusual nor extreme".
But he added that whereas such changes might be considered minimal on a global scale, food production and distribution would be disrupted and the most severe impact is likely to be felt in sub-Saharan Africa.
John Ng'ang'a, a professor in the department of atmospheric science at the University of Nairobi, Kenya, says the change would increase the area of dry land in southern Africa, adding that: "The effect of climate change on farming communities will be very severe".
Last month at a conference in London, United Kingdom, researchers warned that global warming will have a big impact on agriculture and food security in parts of Africa and Asia.
Noting that rising temperatures have led to nearly all of the snow on Mount Kenya and Mount Kilimanjaro in eastern Africa, Ng'ang'a said: "These are signals that climate change has occurred much faster in sub-Saharan Africa than what scientists anticipated."
Hurrell and Hoerling's results will be published in the Journal of Climatology.