They are moving southward at a velocity of 2.1 miles every year. That means the first dunes will reach Beijing in around 16-17 years from now. Many environmentalists hold that desertification does not result from the spread of existing deserts but from fragile ecosystems caused by human practices. Yet historical and archaeological evidence indicates that desertification has been going on relentlessly for some 5000 years. This means efforts to stop it have little chance of succeeding.
Global maps indicate that most of the world's deserts are contained within two bands.
Going from west to east the first band lies between the latitudes of 20 and 40 degrees. At its western extremity it includes the deserts of the American southwest and northern Mexico. Most of the rest of the Western Hemisphere is water or grassland. In the Eastern Hemisphere the band includes the Sahara, Arabian, Iranian, Baluchistan, Sindh and Rajasthan deserts. At this band's eastern border a similar desert band appears to the north between latitudes 30 and 50 degrees.