Man-made global warming is set to accelerate desertification - some experts say this is probably already the case - yet paradoxically it also threatens deserts themselves, placing unique wildlife and cultures in peril.

Desertification is as old as human civilisation itself, dating back to earliest arable farming in Mesopotamia 5,000 years ago.

In the past quarter-century, though, the problem has become acute.

Drylands, areas with low rainfall and high evaporation that are on the fringes of the desert, account for 41 percent of Earth's land area, 43 percent of its cultivated surface and are home to more than two billion people, mostly in poor countries.

But between 10 and 20 percent of drylands are already classified as degraded, meaning that their ability to produce crops has been wiped out or severely reduced.

Land degradation causes crop losses of around 42 billion dollars a year, according to the UN Environmental Programme (UNEP). "The Sahel countries have had a drought that has now persisted for 25-35 years with between 10 and 20 percent less rain than before, and semi-desert regions have advanced southwards by 100 kilometres (60 miles)," said Gil Mahe, a desertification specialist at France's Institute for Development (IRD).

The cause lies mainly with dust storms that overwhelm dry vegetation (and, in the case of the advancing Gobi desert, can choke Beijing for days at a time). Salt deposited on fields through over-irrigation is another factor.

But the problem is set to worsen in many countries this century, as rainfall patterns shift, evaporation increases and water runoff from glaciers decreases.

"The probability of regions in the interiors of continents becoming desert will increase," says Ronald Prinn, New Zealand-born co-director of the Joint Program on the Science and Policy of Global Change (JPSPGC) at Massachusetts Institute of Technology (MIT).

According to the UN's science body, the Intergovernmental Panel on Climate Change (IPCC), temperatures in deserts could rise by an average of up to five to seven C (nine to 12.6 F) by 2071-2000 compared to the period 1961-1990.

Many deserts will suffer a decline in rainfall between five and 10 or even 15 percent, it predicts. Australia's Great Victoria desert, the Atacama and the arid Colorado Great Basin region of the southwestern United States will be most hit, although the Gobi is predicted to have rainfall increases of between 10 and 15 percent by the end of the century.

Mahe insists it is possible to fight desertification, provided countries "mobilise massively" to build stone dikes, plant coarse grass, hedges, shrubs and trees to act as natural barriers.

And they must also be patient - the results take between one and three decades to take effect - and willing to practise low-intensity agriculture in recovered land, he said.

"In northern Burkina Faso and Niger, people are growing crops in land that was abandoned in the past. In the same region, you can see stretches of land that have been abandoned and others that have been protected by good practices," said Mahe.

Saturday's World Day to Combat Desertification coincides with 2006 as the UN International Year of Deserts.

Marking that, a new UNEP report, Global Desert Outlook, says the cores of deserts represent some of the world's last pristine areas of total wilderness, yet many of its species could be imperilled by human incursion or development.

One such threat is the diversion of large rivers that cross deserts to help water-stressed cities and farms. These rivers create unique desert wetlands such as the Aral Sea and the marshlands of Iraq.

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