TUNIS (AFP) -- World experts seeking to stem the merciless advance of parched land that drives millions of people to flee famine in poor countries meet here for a United Nations desertification conference.

Desertification threatens 1.2 billion people in 110 countries, according to the UN, while two billion -- or a third of the world's population -- live in arid and semi-arid regions.

Crop losses from the problem stand at an estimated 42 billion dollars (33 billion euros) a year, while the advance of desert land causes famine, insecurity, social tension and mass migration from southern countries to the north, the organization says.

Organized by UNESCO, the three-day conference -- part of the UN's year of focus on deserts and desertification -- unites 100 world experts from Monday to discuss ways in which political leaders can fight the spread of the deserts.

The experts will discuss endangered ecosystems and research on sustainable development and protection of arid land. Among other topics, they will discuss water shortages and management, environmental disasters and conservation.

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 civilization 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.

"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.