Two billion people live in drylands vulnerable to desertification, ranging from northern Africa to swathes of central Asia, he told Reuters. And storms can lift dust from the Sahara Desert, for instance, and cause respiratory problems for people as far away as North America.

Over-grazing and over-planting of crops, swelling human populations and misuse of irrigation were contributing to desertification, the report said. It estimated that 10-20 percent of drylands were already degraded.

Global warming, widely blamed on human emissions of heat-trapping gases from cars, factories and power plants, was likely to exacerbate the problems in coming decades by triggering more floods, droughts and heatwaves.

"Growing desertification worldwide threatens to swell by millions the number of poor forced to seek new homes and livelihoods," according to the report, part of a Millennium Ecosystem Assessment led by UN agencies and other groups.

"Desertification is potentially the most threatening ecosystem change impacting livelihoods of the poor," it said.

The report said 41 percent of the world's land area was dryland, including most of Australia, the western part of North America and much of the Andean region of South America.

DUST WORSENS POVERTY

Desertification meant increasing health problems linked to dust, reduced farm production and poverty.

Infant mortality in drylands in developing nations averaged 54 children per 1,000 live births in 2000, double the rate in other poor regions and 10 times the rate in industrial nations.

"An increase in desertification-related dust storms is widely considered to be a cause of ill-health -- fever, coughing, sore eyes -- during the dry season," it said.

And dust from the Gobi Desert in Mongolia could affect people as far away as Japan or Hawaii. Some scientists estimate that a billion tonnes of dust can be lifted from the Sahara region into the atmosphere every year.

Dust particles can also carry bacteria and fungi. Dust-borne microrganisms from Africa were believed to have damaged coral reefs in the Caribbean, said Uriel Safriel, another of the report's lead authors.

"Bedouins in Israel are known to be infected by spores of fungi and bacteria transported by dust," he told Reuters. Some dust carries toxins like pesticides from around the Aral Sea.

And dust storms from Africa can damage plants' ability to grow as far away as Florida by muting the sunlight. African dust, however, can also carry nutrients and is credited with helping forests to survive in the Amazon.

The report said that better management of crops, more careful irrigation and strategies to provide non-farming jobs for people living in drylands could help mute problems. But it was easier to prevent desertification than to reverse it.