"It's clear now that desertification is amongst the greatest challenges that humankind faces," Crown Prince Felipe, heir to the Spanish throne, said in an opening address to the forum.

"We should never forget that the impact of desertification is not only felt in zones where the problem originates, but also in areas much further away," he said.

The 12-day gathering of senior politicians and experts from nearly 200 countries hopes to come up with a new 10-year "strategic plan" to stem desertification and set measurable objectives with a timeline for achieving them.

Around 200 million people live in desert areas while just over two billion -- or one-third of the world's population -- live on arid land that makes up 41 percent of the earth's surface, according to a study by the United Nations University.

The areas of the planet that are forecast to get drier due to climate change are also those with the highest population growth rate, the study's lead author and the director of the UN University's Canada-based International Network on Water, Zafar Adeel, told AFP.

"This is a recipe for trouble brewing if we continue on our present path," he said.

Desertification is blamed for forced migration, conflict and growing food insecurity.

Unless action is taken to curb the problem, some 50 million people could be displaced within the next decade, according to the study which was prepared by over 200 experts from 25 countries.

Sub-Saharan Africa and Central Asia are the regions most vulnerable to desertification but the problem affects all continents, it said.

The annual loss of income as a result of land degradation is estimated at 65 billion US dollars (47.6 billion euros).

Excessive cultivation and grazing, deforestation, unsustainable irrigation practices, overpopulation and climate change have been identified as the main causes of desertification.

Experts urge tighter land use management policies that protect existing vegetative cover from overgrazing and a ban on unsustainable irrigation practices as a means to curb desertification.

"We know why soils erode and what to do to avoid it. Now we must move from knowledge to action," Spain's Environment Minister Cristina Narbona told a news conference in Madrid on Thursday called to discuss the gathering.

The gathering in Madrid will be the eighth conference of the 191 nations that have signed the Convention to Combat Desertification (UNCCD), which was adopted in Paris in June 1994.

The event is held every two years in a different city. The last UN conference on desertification was held in the Kenyan capital Nairobi in October 2005.

More than one-third of Spain's surface faces a "significant risk" of desertification, according to the Spanish environment ministry which is helping to organize the conference.

The rural, southeastern region of the country and Spain's Canary Islands off the coast of Morocco are the most affected areas.

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