A leading agricultural economist warned China on Tuesday to tackle the causes, not symptoms of stifling sandstorms and rapid soil erosion as the country launches a billion-dollar battle against desertification.

Lester Brown, president of watchdog Earth Policy Institute, said the dust bowl area in China, Mongolia and Central Asia was likely bigger than that in the 1930's United States.

In the worse scenario it could lead to tens of millions of "environmental refugees", he said.

Brown raved over pilot initiatives he visited in the Inner Mongolia region and Gansu province, but put down what he called Beijing's "Great Wall mentality" to fend off sandstorms with green belts.

"Planting the trees is a useful thing to do but controlling the dust storms is going to take work in Inner Mongolia and Gansu and Xinjiang and in Mongolia itself," Brown said, adding that that problems such as over-grazing should be overcome.

"One has to be careful that we don't treat the symptoms of the problem, rather than the causes," he told reporters at the end of a tour by train across northern and western China.

Brown, who upset Beijing in 1994 by predicting a global grain shortage as the country's population swelled, has begun to look at the problem of sandstorms that have turned China's skies orange and crossed the Pacific to lay dust blankets on the western United States.

He said Chinese desertification experts in the Gansu capital of Lanzhou told him the ecological crisis + widely attributed to over-logging, over-grazing, over-ploughing and over-use of water resources -- was 80-90 percent man-made.

He traced the blinding storms to retreating farm and grasslands in northern and western China, Mongolia and the former Soviet republics in Central Asia.

"Probably it's much larger than the dust bowl that affected the United States in the 1930s," he said.

Too soon to tell

A top forestry official last week announced China's most ambitious plans yet to halt the desert's onslaught, pledging several hundred billion yuan in spending to protect forests and plant grasslands and orchards over the next decade.

It was the latest sign of what Brown dubbed the "conceptual breakthrough" in Beijing's environmental policy, spearheaded by Premier Zhu Rongji and sparked when deadly floods in 1998 led to a ban on logging near the upper reaches of the Yangtze River.

"The remaining question is whether even that substantial increase in resources will be enough to turn the tide," Brown said.

"We now have some local responses that are working, but the local efforts in Inner Mongolia and Gansu and elsewhere in the country are still relatively small compared to the scale of the problem."

China's 1.3 billion population was eight times that of the United States in the 1930s, when around three million American farmers were driven from the Great Plains states, raising the possibility of millions more environmental refugees, he said.

But Brown, who called China's Lanzhou-based institute the best research centre on desertification in the world, said the country could lead the world in the fighting the scourge.

Experiments in Lanzhou ranged from identifying desert shrubs and trees for reforestation to helping farmers convert to green house cultivation.

"The risk with that sort of thing is you'll get too many greenhouses and prices of fresh produce will fall," he said. "But it seemed to me to be moving in the right direction."

Outside the Inner Mongolian capital of Hohhot, planting projects were aimed at halting desertification and restoring the land while raising farmers' incomes.

Farmers were feeding cows the roughage from a legume similar to alfalfa used to stabilise the soil. "They're fed in an enclosed area. They're not permitted to wander."