And yet, the deserts keep growing. In Central Asia alone, for instance, scientists estimate that 8,000 to 10,000 square kilometers of desert is created every year where cattle once grazed or lakes once existed.
In neighboring China, the problem continues to grow in severity, with sand dunes advancing to less than 100 kilometers from the capital Beijing. The annual spring dust storms that used to be a minor local annoyance have become an international problem, darkening skies across the sea in Korea and Japan.
So what is responsible for desertification, and can anything be done to prevent it?
RFE/RL spoke to an expert on arid regions at the UN's Food and Agriculture Organization (FAO) in Rome. Michel Malagnou says a combination of factors, including global warming, is to blame, but that the biggest culprit is man.
"Climate change and change in rain patterns can induce desertification," Malagnou said. "But more often, it is created by man. Desertification is caused by overexploitation of the natural resources, whatever they are -- range [land] or forest or even soil fertility which is overexploited."
Soil in dry regions tends to be fragile and offers limited yields. That means that overgrazing or intensive farming can quickly strip it of its nutrients. When a patch of grassland has been stripped, rain and wind will wash the topsoil away, leading eventually to desertification. The year-round grazing of cattle in Turkmenistan and the Kazakh steppe is contributing to desertification in those regions.
Paradoxically, too much water can also cause desertification. A major factor cited by a recent U.S. study in Turkmenistan was water leaking from old irrigation canals. It is estimated that half the water in the Kara Kum canal seeps into land along its path. This excessive irrigation brings salt to the surface, creating salt marshes that then dry into infertile clay flats.
The overuse of chemicals in cotton harvesting is another factor that has contributed to desertification in countries such as Turkmenistan and Uzbekistan. After several years of higher yields, the soil is drained of nutrients and becomes toxic, turning into sterile desert.
Once desertification has occurred, it is extremely expensive or impossible to reverse, said Malagnou, which is why sensible land management is the best policy to pursue.
"It is much better to prevent by better management, by decreasing the pressure on the land, by rotation, for instance -- you have cattle or wood exploitation on a piece of land and then you keep the land resting for some years," Malagnou said. "If you do that, you will restore the cover on the land. If you don't do that, you will reach a point where it will be expensive to reverse it."
Nick Nuttall, at the United Nations Environment Program, made a similar point, in a phone interview from his office in Nairobi: "We have to be more careful about what crops we are growing, where we are growing them, and also we have to be sensible about how we manage forests so that we contain water in the land rather than allowing the soil to dry out."
The African continent offers a stark example of the social and economic impact of desertification at its most extreme. Experts estimate that 60 million people will be forced to move from newly desertified areas of sub-Saharan Africa by the year 2020 if present trends continue.
Those people are likely to move to already crowded cities, leading to increased water use, pollution, and a further degradation of the environment. It is a vicious circle.
Malagnou said the problem is getting worse, but that over the past decade, since the United Nations adopted its Convention to Combat Desertification, governments have become more aware of the need to act.
"On the ground, in many places, it is getting worse," Malagnou said. "But, in fact, the convention has made the decisionmakers, let's say, more and more aware of the problem, and now countries are tackling the problem more than before."
In the wake of devastating floods in 1998, for example, China embarked on a massive reforestation campaign to stem the advance of the desert. Other countries have cut back on the use of fertilizers and encouraged better water management. Time will tell if those efforts can reverse current trends.
(Biloliddin Hasanov of RFE/RL's Uzbek Service contributed to this report.)