The UN-sponsored conference - said to be the biggest environment meeting ever held in Africa - will try to reach a consensus on how to combat desertification, so that countries can get international financial backing to their efforts.

Figures released before the conference show that the phenomenon costs about $42bn a year and has serious social and economic consequences in the affected zones.

"Developed countries are spending billions to cut the effects of greenhouse gases," says Hama Armba Diallo, the executive secretary of the UN Convention to Combat Desertification and Drought

"We must show them that desertification is a global environmental problem and that if they don't do something, they'll feel the consequences themselves," he says.

The organisers say a large part of international funds for fighting environmental problems is given over to issues such as climate change and water management, at the expense of the desertification question.

Mostly man-made problem

Desertification reduces the land's resilience to natural climate variability.

Soil becomes less productive, as topsoil is blown away by the wind or washed away by the rainstorms. Vegetation is damaged.

Drought is part of the cause, but essentially desertification is a man-made problem.

It arises from placing too much pressure on the land, often because of overgrazing.

Many environmentalists also blame the phenomenon on the destruction of forests for short-term gain.

The Dakar conference will also address the link between El Nino phenomenon and desertification and drought.

Billion people at risk

The organisers of the conference estimate that the spread of barren land has an impact on 250 million people, and could eventually threaten a billion people.

In North America, 74% of the dryland is already "seriously or moderately" affected by desertification.

Africa is a close second at risk, with 73% of the dryland damaged.

In the immediate sub-Saharan Sahel region, experts put the death rate from the related impact on living conditions at 200,000 people per year.

Desertification has also hit Asian and Pacific nations, as well as Europe, notably Italy and Spain.

Developed countries as a whole - and more favoured areas of developing countries - are also being affected indirectly as people migrate to them after being unable to live off their degraded land.