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| This Chinese restaurant worker wears a plastic bag over her head as she delivers fast food in a sandstorm near Beijing's Tiananmen Square. Experts blame the city's poor air quality on industrial pollution -- coal smoke, factory exhaust, and emissions from an increasing number of automobiles. But some of Beijing's bad air sweeps in from the north, where the cutting of forests has hastened the southward spread of the Gobi desert. Nearly one million tons of fine sand blow into Beijing annually. According to the UN Convention to Combat Desertification, 70 percent of the world's "dryland" eco-systems (which comprise one-third of the world's total land area) have been degraded. The resulting "desertification" is largely due to over-cultivation, over-grazing, deforestation or poor irrigation practices.
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