Tens of millions of people could be driven from their homes by encroaching deserts, particularly in sub-Saharan Africa and Central Asia, a report says.
Desertification could displace up to 50m people over the next decade
The study by the United Nations University suggests climate change is making desertification "the greatest environmental challenge of our times".
If action is not taken, the report warns that some 50 million people could be displaced within the next 10 years.
The study was produced by more than 200 experts from 25 countries.
This report does not pull any punches, says BBC environment reporter Matt McGrath.
One third of the Earth's population - home to about two billion people - are potential victims of its creeping effect, it says.
Tree-planting schemes may put pressure on scarce water resources
"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 overexploitation of land and unsustainable irrigation practices are making matters worse, while climate change is also a major factor degrading the soil, it says.
People displaced by desertification put new strains on natural resources and on other societies nearby and threaten international instability, the study adds.
"There is a chain reaction. It leads to social turmoil," said Zafaar Adeel, the study's lead author and head of the UN University's International Network on Water, Environment and Health.
The largest area affected was probably sub-Saharan Africa, where people are moving to northern Africa or to Europe, while the second area is the former Soviet republics in central Asia, he added.
The UN report suggests that new farming practices, such as encouraging forests in dryland areas, were simple measures that could remove more carbon from the atmosphere and also prevent the spread of deserts.
"It says to dryland dwellers we need to provide alternative livelihoods - not the traditional cropping based on irrigation, cattle farming, etcetera - but rather introduce more innovative livelihoods which don't put pressure on the natural resources," Mr Adeel said.
"Things like ecotourism or using solar energy to create other activities."
Some countries like China have embarked on tree-planting programmes to stem the advance of deserts.
But according to the author, in some cases the trees being planted needed large amounts of water, putting even more pressure on scarce resources.