Desert dunes set to roam
Climate change will shift sands and destroy desert ecosystems.
Sand dunes in the Kalahari Desert in Africa, which have been immobile for thousands of years, will soon start to move again, researchers warn. The wandering dunes may affect hundred of thousands of people in southern Africa.
Researchers have long warned that some of the driest and poorest parts of the world are getting drier, causing deserts to grow. But David Thomas, a geographer at the University of Oxford, UK, and his colleagues wondered whether the anticipated climate change might also affect the movement of dunes within deserts.
Dunes move when sand grains on one side are picked up by the wind and deposited on the other. But the speed of movement varies greatly depending on factors including the shape and size of the dunes, moisture content in the sand, and wind speed.
Dune movement can be dramatically slowed or prevented by sparse vegetation. When vegetation cover drops below 14%, erosion speeds up significantly. The result is a self-perpetuating system in which the blown sand smothers remaining plants, destroying ecosystems and prompting further erosion.
On the move
Using data collected from 1960 to 1991, Thomas and his team applied a climate model to investigate the effects of anticipated loss of vegetation cover, reduced moisture content, and increased wind energy on African desert regions.
Their simulations revealed a significant increase in dune activity in the southern Kalahari by 2039. By 2099, sand dunes throughout South Africa, Angola and Zambia will be on the move, they predict; such a phenomenon has not occurred in the past 14,000 to 16,000 years. These shifting sands are likely to destroy local ecosystems, making any kind of farming or other use of nearby land even more difficult, they report in Nature1.
The model included seasonal variations in annual rainfall and the likely impact of an increase in atmospheric greenhouse gases on temperature. It did not, however, include the influence of higher carbon dioxide levels on plant productivity, nor human impacts such as increased agriculture in surrounding areas.
Local people are aware of the problem, says Thomas, but often act to worsen it. Large-scale sheep farming in the northern part of South Africa, for example, reduces available ground water, because it requires extensive well digging.
"Fighting the process of vegetation loss and dune movement would require major adaptations," says Thomas. One possible solution would be to plant new vegetation. But dune ecosystems are very sensitive and differ greatly from region to region. It would take many years of careful tending to stop a moving dune from wandering around, he explains.
- Thomas D. S. G, et al. Nature, 435. 1218 - 1221 (2005).