T. Dressler/UNEP/Topham  
  frica's Kalahari desert - already much larger than California or Pakistan - is expected to double in size as global warming takes hold, covering hundreds of thousands of square kilometres of farmland with shifting sands.

Scientists say that climate change will stir up the desert's giant dune fields, and send them marching over much of South Africa, Botswana, Namibia, Angola, Zimbabwe and Zambia.

Around 10,000 to 20,000 years ago the dunes - made of sand created by the crumbling of soft rock in the area - stopped roaming around Southern Africa and settled down. They stabilized, and plant life grew on them, anchoring them in one place. But this, it seems, is about to change. Researchers at Oxford University in the United Kingdom have found that as wind speeds in the area accelerate, the sands will start moving again. At present the winds are relatively light, but climate change is expected to increase them.

As they get stronger they will start picking up sand and blowing it over the landscape. Sand drifts will bury vegetation - already weakened by lower rainfall - killing it and making the dunes even more unstable. The desert will start to expand.

Professor David Thomas - who has been studying the desert for 20 years - says that the effects will be 'drastic'. He adds: 'These landscapes are potentially very dynamic and they can kick in with a form of activity that is rather hostile to farming. The Kalahari is a large area supporting a reasonably big rural population that lives by farming. It is these people who are vulnerable to their environment becoming a rather more hostile, active, dune landscape than it is today.'

He urges local governments and aid donors not to make things worse. But present policies and projects are enormously boosting the number of cattle in the area, threatening to turn already arid land into yet more desert.

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