"There is a chain reaction. It leads to social turmoil," Zafaar Adeel, the study's lead author and head of the UN University's International Network on Water, Environment and Health, said.

The study urged governments to work out ways to slow the advance of deserts, from the Sahara to the Gobi, caused by factors such as climate change and land over-use. Better plantings of crops and forests in nearby drylands were simple measures to help.

"Desertification has emerged as an environmental crisis of global proportions, currently affecting an estimated 100 to 200 million people, and threatening the lives and livelihoods of a much larger number," the study said.

"The loss of soil productivity and the degradation of life-support services provided by nature pose imminent threats to international stability," it said. The report drew on the work of 200 experts from 25 nations.

It said 50 million people were at risk of being forced from their homes by unchecked desertification in the next decade -- equivalent to the population of South Africa or South Korea.

"The largest area is probably sub-Saharan Africa, where people are moving to northern Africa or to Europe," said Adeel.

"The second area is the former Soviet republics in central Asia," he told Reuters. Adeel said it was hard to isolate desertification from other factors making people move, such as poverty or armed conflicts.

Sudan's Darfur region was an example, he said. International experts reckon 200,000 people have died and 2.5 million have been driven from their homes in four years of strife. Sudan says 9,000 have died.

CHAIN REACTION

The report urged governments to link up often scattergun efforts by environment, agriculture or economy ministries to fight desertification.

Improved crop and forestry plantings on drylands, which cover more than 40 percent of the world's land area, could slow desertification and also help fight global warming widely blamed on gases released by use of fossil fuels.

Plants absorb carbon dioxide, the main industrial greenhouse gas, as they grow and release it when they are burnt or rot. Carbon markets might even develop financial mechanisms to promote more vegetation in drylands.

Among efforts, China is planting a 700-km "Great Green Wall" of trees and enclosed grassland to slow the advance of deserts. Algeria is also putting up a "green wall" against the Sahara.

"Such plans can work, but can also lead to problems," Adeel said. He said China was in some cases planting trees that needed large amounts of water, aggravating shortages.