The desert encroaching onto fertile planting fields, about 100 kilometers west of Yin Chuan, capital of Ningxia Hui Autonomous Region, in northwest China in this July 31, 1996 file photo. China has lost 92,100 square kilometers (36,000 square miles), an area the size of Portugal, to desert, since the 1950s.(AP Photo/MaxVision)

UNITED NATIONS (AP) - The world is turning to dust, with lands the size of Rhode Island becoming desert wasteland every year and the problem threatening to send millions of people fleeing to greener countries, the United Nations says.

One-third of the Earth's surface is at risk, driving people into cities and destroying agriculture in vast swaths of Africa. Thirty-one percent of Spain is threatened, while China has lost 36,000 square miles to desert - an area the size of Indiana - since the 1950s.

This week the United Nations marks the 10th anniversary of the Convention to Combat Desertification, a plan aimed at stopping the phenomenon. Despite the efforts, the trend seems to be picking up speed - doubling its pace since the 1970s.

"It's a creeping catastrophe," said Michel Smitall, a spokesman for the U.N. secretariat that oversees the 1994 accord. "Entire parts of the world might become uninhabitable."

Slash-and-burn agriculture, sloppy conservation, overtaxed water supplies and soaring populations are mostly to blame. But global warming is taking its toll, too.

The United Nations is holding a ceremony in Bonn, Germany, on Thursday to mark World Day to Combat Desertification, and will hold a meeting in Brazil this month to take stock of the problem.

The warning comes as a controversial movie, "The Day After Tomorrow" is whipping up interest in climate change, and as rivers and lakes dry up in the American West, giving Americans a taste of what's to come elsewhere.

The United Nations says:

-- From the mid-1990s to 2000, 1,374 square miles have turned into deserts each year - an area about the size of Rhode Island. That's up from 840 square miles in the 1980s, and 624 square miles during the 1970s.

-- By 2025, two-thirds of arable land in Africa will disappear, along with one-third of Asia's and one-fifth of South America's.

-- Some 135 million people - equivalent to the populations of France and Germany combined - are at risk of being displaced.

Most at risk are dry regions on the edges of deserts - places like sub-Saharan Africa or the Gobi Desert in China, where people are already struggling to eke out a living from the land.

As populations expand, those regions have become more stressed. Trees are cut for firewood, grasslands are overgrazed, fields are over-farmed and lose their nutrients, water becomes scarcer and dirtier.

Technology can make the problem worse. In parts of Australia, irrigation systems are pumping up salty water and slowly poisoning farms. In Saudi Arabia, herdsmen can use water trucks instead of taking their animals from oasis to oasis - but by staying in one place, the herds are getting bigger and eating all the grass.

In Spain, Portugal, Italy and Greece, coastal resorts are swallowing up water that once moistened the wilderness. Many farmers in those countries still flood their fields instead of using more miserly "drip irrigation," and the resulting shortages are slowly baking the life out of the land.

The result is a patchy "rash" of dead areas, rather than an easy-to-see expansion of existing deserts, scientists say. These areas have their good times and bad times as the weather changes. But in general, they are getting bigger and worse-off.

"It's not as dramatic as a flood or a big disaster like an earthquake," said Richard Thomas of the International Center for Agricultural Research in the Dry Areas in Aleppo, Syria. "There are some bright spots and hot spots. But overall, there is a trend toward increasing degradation."

The trend is speeding up, but it has been going on for centuries, scientists say. Fossilized pollen and seeds, along with ancient tools like grinding stones, show that much of the Middle East, the Mediterranean and North Africa were once green. The Sahara itself was a savanna, and rock paintings show giraffes, elephants and cows once lived there.

Global warming contributes to the problem, making many dry areas drier, scientists say. In the last century, average temperatures have risen over 1 degree Fahrenheit worldwide, according to the U.S. Global Change Research Program.

As for the American Southwest, it is too early to tell whether its six-year drought could turn to something more permanent. But scientists note that reservoir levels are dropping as cities like Phoenix and Las Vegas expand.

"In some respects you may have greener vegetation showing up in people's yards, but you may be using water that was destined for the natural environment," said Stuart Marsh of the University of Arizona's Office of Arid Lands Studies. "That might have an effect on the biodiversity surrounding that city."

The Global Change Research Program says global warming could eventually make the Southwest wetter - but it will also cause more extreme weather, meaning harsher droughts that could kill vegetation. Now, the Southwest drought has become so severe that even the sagebrush is dying.

"The lack of water and the overuse of water, that is going to be a threat to the United States," Thomas said. "In other parts of the world, the problem is poverty that causes people to overuse the land. Most of these ecological systems have tipping points, and once you go past them, things go downhill."