In a paper appearing in the British journal Nature, researchers warn that much of southern Africa could be transformed into a roaming, Sahara-like desert as the dunes of the Kalahari become eroded and unstabilised by global warming.

For thousands of years, these dunes -- a swathe extending from 10 degrees to 30 degrees latitude south, from Angola to northern South Africa -- have been used for pastoral or agricultural farming.

The scientists assembled knowledge about the microdynamics of dunes, such as moisture and the percentage of vegetation that are needed to hold the sandy soil together.

They then combined this data with powerful computer simulations about the expected rise in temperatures over this century and its likely impact on rainfall.

Regardless of which scenario was crunched through the computer, the news was bad.

Within decades, higher temperatures and less moisture will start to destroy vegetation and erode the dunes, exposing them to wind shift.

In the southern dunefield (in eastern Namibia and northern South Africa), the dunes start to move "significantly" by 2039, a phenomenon likely to be followed in the eastern dunefields (eastern Botswana and western Zimbabwe) by

"By 2099, all dunefields are highly dynamic, from northern South Africa to Angola and Zambia," say the authors, led by David Thomas of Oxford University's Centre for the Environment.

"Our results suggest that dunefields are likely to be reactivated (the sand will become significantly exposed and move) as a consequence of 21st-century warming."

The other paper, likewise published in Nature, focuses on aerosols -- another term for dust, soot, particulates from industrial discharge, road traffic and volcanic eruptions that linger in the atmosphere.

Aerosols are like an invisible blocker: they intercept the sunlight and thus help to prevent Earth's surface from warming, although the picture is complex, as some types of particles also have a big effect on cloud and rainfall patterns.

One of the big unknowns is what happens when this accidental shield is removed by anti-pollution measures.

German and British scientists, led by Meinrat Andreae of the Max Planck Institute for Chemistry in Mainz, Germany, using what they admit is "a deliberately simplistic" method, estimate that as aerosols are cleaned up, Earth's mean global temperature could rise by more than 6 C (10.4 F) by 2100.

A rise of this magnitude is in the doomsday range, they admit.

"Such a degree of climate change is so far outside the range covered by our experience and scientific understanding that we cannot with any confidence predict the consequences for the Earth system," they write.

The figure even exceeds the most pessimistic estimate thrown up in 2001 by the top UN scientific authority on global warming, the Intergovernmental Panel on Climate Change (IPCC).

In its landmark report, the IPCC declared that the planet's temperature was rising as carbon gases, mainly emitted by oil, gas and coal, stored up heat from the Sun.

These estimates ranged from 1.4 to 5.8 C (2.5 to 10.4 C), according to C02 levels that ranged from 540 to 970 parts per million (ppm), which compares with 280ppm for pre-industrial times and around 380ppm today.

The greater the increase, the more dramatic the effect on Earth's climate, with the higher ranges predicting shrinkage of the polar ice caps, greater water stress, more frequent and more violent storms, and mean sea levels that would rise by up to 88 centimetres (2.9 feet).

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