The Sahara has crossed the Mediterranean, forcing thousands to migrate as a lethal combination of soil degradation and climate change turns parts of southern Europe into desert.

A major UN conference was told yesterday that up to a third of Europe's soil could eventually be affected.

A fifth of Spanish land is so degraded that it is turning into desert, according to figures released for the first time yesterday, and in Italy tracts of land in the south are now abandoned and technically desert.

Portugal, Spain, Italy and Greece are the four EU countries already so badly affected that they have joined the United Nations Convention to Combat Desertification (CCD) which is meeting in Bonn this week.

One expert, Maurizio Sciortino, said that there were many causes of the soil degradation, including changing weather patterns and the rise of global farming, which is making it uneconomic to run smallholdings and is driving people from the land.

"Land that has been carefully cultivated and preserved for 2,000 years, with terracing for soil conservation and careful irrigation to keep up productivity, is being abandoned and lost," he said. "The walls of the terracing break down, the soil is washed away and we are left with bare rock. Once that happens there is no way back.

"The conditions are particularly bad in southern Italy, Spain and Greece. Even southern France is not immune but so far they do not admit it for political reasons."

The problem is not confined to the EU. Bulgaria, Hungary, Moldova, Romania and Russia have all reported signs of desertification. Experts say Moldova in particular is "highly vulnerable" to desertification, with about 60% its farmland degraded.

Beyond the Black Sea, there are belts of fast-degrading land stretching as far as Mongolia. China, for example, has said that land deterioration in its northern provinces is costing its economy £4bn a year.

In places such as drought-stricken Sardinia and Sicily, economic conditions are accelerating the problem. "In many places tourism is making things far worse," Mr Sciortino said.

"Water is pumped from below ground, pulling salt water from the sea into the aquifers. Imagine how much water it takes to keep an 18-hole golf course going for the tourists. The trouble is they use the money to buy petrol to drive the desalination plants for more water and that makes global warming worse. In the end it means more deserts. We have to stop this cycle."

Italy has a programme of helping the countries of North Africa to combat desertification, partly in order to stem the increasing tide of refugees attempting to reach Europe.

Valareo Calzolaio, the Italian environment minister, said that at first people migrated from the country to the cities and then "toward remote western mirages - migrants fall to the network of criminal traders in human beings."

He is also concerned about the a rise in the number of refugees from the east: "If we do not take urgent action then within 10 years millions will be forced to migrate from their degraded land and they will be heading for Europe."

Klaus Töpfer, the former German environment minister who is now executive director of the UN Environment Programme, said: "Although often overlooked, soil is a natural resource that is no less important to human wellbeing and the environment than clean water and clean air, the two things that most people in the EU have been focusing on.

"The sustainable use of soil is one of Europe's greatest environmental, social and economic challenges."

Domingo Jimenez-Beltran, executive director of the European Environment Agency, said: "In some parts of Europe, the degradation is so severe that it has reduced the soil's capacity to support human communities and ecosystems and resulted in desertification. Because it can take hundreds or thousands of years to regenerate most soils, the damage occurring today is, for all purposes, irreversible."

In a report published yesterday the agency said the first stages of serious soil degradation were being noted in parts of Europe and 150m hectares are at a high risk of erosion. Deterioration is at a critical point in Mediterranean countries.

Meanwhile, the situation is no better in eastern Europe, where 41% of agricultural land in Ukraine is at risk of erosion.

Currently, 172 countries have ratified the the CCD, as it is known in UN jargon, and this is the fourth annual meeting of members. It is a sister organisation to the Climate Change Convention - which recently had a disastrous meeting in the Hague - and the Biodiversity Convention, which attempts to preserve habitats and species.

Hama Arba Diallo, the executive secretary of the convention, said most schemes to com bat desertification were small-scale and concentrated in border areas in Africa to prevent migration and conflict.

"Most of our schemes cost between $20,000 and $50,000, and involve local communities identifying what needs to be done and doing it. I can take people to schemes in China where $20,000 has turned 15 hectares of desert into valuable agricultural land. If we had 100 times as much money we could do 100 similar schemes to push back the deserts."

However, Mr Diallo said it was difficult to attract finance for such small local schemes.

Action Plan

At this year's Convention to Combat Desertification (CCD), countries in southern Europe produced action plans to deal with:

 Seasonal droughts, very high rainfall variability and sudden high-intensity rainfall

 Poor, highly erodable soil, prone to surface crusts

 Crisis conditions in traditional agriculture with associated land abandonment

 An increase in forest fires

 Concentration of economic activity in coastal areas