Climate change is likely to 'activate' sand dunes across southern Africa, with potentially disastrous consequences for herding communities, report researchers in Nature today.
As the Kalahari desert gets hotter and drier in the 21st century, dunes will become unstable and vegetation for grazing scarce.
"By 2070, the models all predict a significant increase in dune activity," says Thomas.
Dune activity depends on two main factors: soil moisture and wind. With little moisture and high winds, vegetation declines, allowing sand to be picked up and blown around.
The higher temperatures caused by global warming will lead to a long-term decline in soil moisture, longer and more frequent droughts, and stronger winds, say the researchers.
|The spreading dunes could
have big impacts on goat
They say the expected loss of vegetation could be disastrous for pastoral farmers, whose livestock depend on plants for grazing.
"It's going to be a lot harder to keep livestock alive and therefore harder for people to live on a sedentary basis in large parts of the Kalahari," says Thomas.
He foresees a possible return to the nomadic lifestyle that was the norm before large-scale pastoral farming.
The Kalahari is an immense region of arid and semi-arid land covering about 70 per cent of Botswana, and parts of Angola, Namibia, South Africa, Zambia and Zimbabwe.
The research predicts that by 2040 the southern dunes of Botswana and Namibia will be 'activated'; by 2070, the more northern and eastern dunes of Angola, Zambia and Zimbabwe; by 2099, all the dunes from Angola and Zambia in the north to those in South Africa.
Some climate models do forecast a seasonal increase in rainfall, but even these conclude that this would be more than compensated for by increased evaporation because of higher air temperatures.
Enhanced levels of carbon dioxide, which in theory could stimulate plant growth, are unlikely to have much positive impact in practice.
"There's a fair body of evidence which says greater levels of carbon dioxide will lead to a more vigorous plant system and that's true, but only if the plants are not stressed, which they will be if there's a moisture deficit," says Thomas.
The models used by Thomas's team did not consider how human activities would affect the Kalahari dunes.
The potential for human activities to affect the shifting dunes should not be underestimated, says David Niemeijer, co-author of a recent report on desertification published by the Millennium Ecosystem Assessment (see Desertification 'a threat to two billion people').
"Conserving water and planting trees could make things better," he says, "or improper husbandry could make things worse."
Niemeijer emphasises that Thomas's study is purely theoretical and its predictions might not come to pass. The models could also be rendered inaccurate by advances in climate change research.
He adds that farmers might be able to take some degree of climate change in their stride — they are used to coping with seasonal dune activity. But recent decades have also seen an unprecedented intensification of grazing that has led to local desertification.
The simulation models in the study were calibrated by checking their predictions for past weather against existing climate records in sub-Saharan Africa over the last twenty years.
By using monthly rather than annual data, the models took into account the high seasonality of the sub-tropical climate.