Released on Monday, the first comprehensive look at the world's parched areas, Global Deserts Outlook said these regions, their wildlife and, most of all, their scarce water supplies are facing dramatic change.
"Deserts are the last great wildernesses and the Cinderellas of the conservation world - out of sight, out of mind," said Nick Nuttall, a spokesman for the United Nations Environment Programme.
"Everybody cares about the mountains. Everybody is worried about the oceans ... But nobody has really thought about the deserts before. They need help."
The report depicts deserts as rich cultural and biological worlds far more complex than the common image of them as mere seas of sand.
Desert areas make up a quarter of the Earth's land surface, or 33.7 million sq km, and are home to about 500 million people, more than previously thought.
Most of the 12 desert regions, including the Great Victoria desert of Australia, for which the future climate has been modelled in a computer, face a drier future, the report said.
Scientists predicted that rainfall would fall by as much as 20% by the end of the century.
Compounding the threats to these vast areas are the melting of glaciers that sustain many deserts, such as the Atacama and Monte deserts in South America, the report said. Because of global warming, the glaciers have been failing to re-form after the annual thaw.
The world's deserts are home to
some 500 million people today
Andrew Warren, one of the report's authors and an emeritus professor of geography at University College London, said: "When the glaciers disappear, you are in serious trouble."
The glaciers on the Tibetan plateau, for example, may decline by as much as 80% by the end of the century, according to the Intergovernmental Panel on Climate Change, a group of scientists advising the UN.
The impact of such changes will be felt around the world. "A large fraction of the water used for agricultural and domestic purposes in the arid Southwest of the United States, the deserts of Central Asia and the Atacama and Puna Deserts on both sides of the Andes is drawn from rivers that originate in the glaciated/snow-covered mountains," the report said.
The report said that renewable water supplies fed to deserts by large rivers are also in danger. It cited the Rio Grande and Colorado Rivers in North America, the Tigris and Euphrates in southwestern Asia and the Amu Darya and Indus Rivers in Central Asia as being under threat.
The report's authors urge more comprehensive water policies that manage the world's precious supplies more effectively.
The report said that Chad, Iraq, Niger and Syria, for example, could experience scarcity in water supplies by 2050 if nothing is done.
Warren also said urgent action was need to protect wildlife, noting that the increasing wealth in Arab lands has led to convoys of hunters sweeping through the arid landscapes of Arabia, Kazakhstan and Sudan in search of prey, "shooting what they can or running it down in Jeeps".
Several species of gazelle, Barbary sheep and the falconer's favourite prey, the Houbara, are among the species considered under threat, Warren said.
Future of world's arid regions chronicled in landmark UN environment report - UNEP news centre, June 5, 2006