By Richard Black
Environment Correspondent, BBC News website
Two new studies predict that climate change will make dry regions of Africa drier still in the near future.
Computer models of the global climate show the Sahel region and southern Africa drying substantially over the course of this century.
Sahel rainfall declined sharply in the late 20th Century, with droughts responsible for several million deaths.
The research comes just after the latest United Nations summit on climate change opened in Montreal.
"Our model predicts an extremely dry Sahel in the future," said Dr Isaac Held of the US National Oceanic and Atmospheric Administration (Noaa), whose team publishes its research in the scientific journal Proceedings of the National Academy of Sciences (PNAS).
"If we compare it against the drought in the 1970s and 80s, the late 21st Century looks even drier - a 30% reduction in rainfall from the average for the last century," he told the BBC News website.
Sahel rainfall fell dramatically in the second half of the 20th Century; since 1970, about half of the region has been in severe drought.
In the late 1980s, a recovery began, but rainfall is not back to pre-1970 levels.
Southern Africa has fared better than the Sahel, but research by another Noaa group led by Marty Hoerling also projects a drier future for this region.
"Between 1950 and 1999, there has been about a 20% decline in summer rainfall over southern Africa," he told the BBC News website.
"Our modelling indicates much more substantial ongoing drying, with the epicentre for drought in Africa effectively moving further south."
Dr Hoerling's study has been submitted to the Journal of Climate for publication.
This latest research may help pin down the physical processes which determine African rainfall.
"What we do know from observations is that if you have a warm north Atlantic and a cool south Atlantic you'll get increased Sahel rainfall, and vice versa," said Professor Chris Folland from the UK Meteorological Office.
"But even temperatures in the Mediterranean sea can affect it as well."
The theory is that if the North Atlantic warms more than waters further south, the rain belt is pulled north over the Sahel; if the southern waters warm more, rain retreats south again, leaving the Sahel dry.
The key to southern African rainfall, meanwhile, may be temperatures in the Indian Ocean, according to Marty Hoerling's results.
Rising concentrations of greenhouse gases in the atmosphere are projected to increase temperatures in the Indian Ocean and the differential between temperatures in the north and south Atlantic.
Projecting future climate change is far from an exact science, and other computer models of African regions have come up with different results.
But these latest results demonstrate how severe the impacts of human-induced global warming may be for some of the poorest countries on the planet.
The fact that their predictions contrast with other models of the same regions also indicate the problems which policymakers face in trying to adapt to the local consequences of global climate change.
Climate modelling is done on powerful supercomputers
Scientists attempt to validate the various models by seeing how well they simulate the climate of the recent past - the climate we know - when all the key data is fed in.
"Our simulation of the 20th Century is closer to what was observed in Africa than other models," said Isaac Held.
"That's why we're giving this model credence, though it's not enough to be certain."
The key, according to Chris Folland, is to develop better models which can tie local details into global simulations; but he fully rejects the conclusion drawn by some climate change sceptics that models are so unreliable as to be next to useless.
"No model has ever been run of an atmosphere with increased greenhouse gas concentrations that hasn't produced a warming," he said.
"They produce different amounts of warming, but they do all produce warming and that's a universal result."