Mongolian officials say that the construction of green wall corresponds with Beijing's pledge to stage a green and environmentally friendly Olympics next year. "This project will contribute significantly to an improved environment for the Beijing Olympic Games," said Inner Mongolia's information office director Meng Shude.

Between a small hill and the Gobi desert about five miles away, one can see young poplar trees, newly planted Mongolian pine about a meter high, as well as apricot bushes and grassland. Grass seed has been dropped from airplanes and millions of trees have been planted, while herdsmen have been banned from fragile grassland and thousands of families have been relocated.

"This area was in an atrocious state a few years ago," said Zhang Boawen, the deputy mayor of Taipusi, home to 230,000 people, mostly farmers or herdsmen.

Taipusi county is in Xilongol league, a province of central Inner Mongolia which was once more than 90 percent grassland but is now overrun by the Hunsandake desert, an eastward extension of the Gobi that now claims more than 30 percent of its land.

Overpopulation and over-grazing, as well as other factors including extended drought, have caused the desert to be expanding southeast at a rate of three kilometers a year, heading directly for Beijing. Though officials say that expansion has currently stopped.

Skeptics who oppose the Green Wall idea argue that the grass and trees alone will not correct the problems of overpopulation and unsustainable development. Jiang Gaoming, of the Institute of Botany at the Chinese Academy of Science, said that 60 billion yuan (7.6 billion dollars) spent on projects to control sandstorms hitting Beijing had been largely wasted.