Wielding the theme "Don't desert drylands!", the United Nations said that up to a fifth of the world's land surface is desert -- from the Sahara to the Gobi -- and that other regions are at risk of turning arid.

"Across the planet, poverty, unsustainable land management and climate change are turning drylands into deserts, and desertification in turn exacerbates and leads to poverty," UN Secretary General Kofi Annan said in a statement.

"There is also mounting evidence that dryland degradation and competition over increasingly scarce resources can bring communities into conflict."

"Dryland degradation is a serious obstacle to eradicating extreme poverty and hunger," he said, adding that environmental and economic refugees were straining cities in developing nations.

Drylands -- including many of the world's crop growing regions -- cover about 41 percent of the planet's land surface and are home to two billion people.

"It is estimated that between 10 and 20 percent of drylands are already degraded," Annan said. "The problem is particularly acute in sub-Saharan Africa and South Asia."

FARM LOSSES

The United Nations said that land degradation causes an estimated loss of US$42 billion a year from agricultural production -- without counting human suffering from famine.

Annan said he was urging governments to "focus on the challenges of life on the desert margins" to slow degradation.

Among pressures on land, the world's population has surged to 6.5 billion from about 2.5 billion in 1950.

And many scientists say that heat-trapping gases released by burning fossil fuels are driving up world temperatures, bringing more heat waves, droughts, floods and rising sea levels.

World Environment Day marks the date of the first UN environmental summit held in Stockholm in 1972. In the UN calendar, 2006 is also the year of deserts and desertification.

Around the world, tree planting to slow erosion were planned in countries from Bhutan to Algeria, the main host of World Environment Day.

Algerian President Abdelaziz Bouteflika said destruction of natural ecosystems and desertification "aggravate conditions of poverty across the world, deepening the crisis on a global scale."

He urged the adoption of a World Charter on Deserts to help achieve a Millennium Goal of halving poverty by 2015.

In Mauritius, one group planned to plant vegetation on dunes to protect beaches from erosion. In Churchill, Australia, activists would collect computer parts for recycling. A group in Zambia would hold a "Miss Environment" beauty pageant.