The steady retreat of glaciers on China's Qinghai-Tibet plateau during the past 40 years is troubling, scientists in the Asian nation said Wednesday.

Xin Yuanhong, senior engineer in charge of a three-year field study, said the glaciers at the headwaters of the Yangtze, China's longest river, now cover 406 square miles, down from 482 square miles in 1971, Xinhua reported. The scientists said the melting, enough to fill Beijing's largest reservoir, was occurring at a "worrisome speed," the Chinese state-run news agency.

"The reduction means more than 989 million cubic meters (1.3 billion cubic yards) of water melted away," said Xin.

Xin said the accelerated melting -- a rate close to that of the Quelccaya Glacier in Peru, the world's largest tropical ice mass -- is attributable to global warming and will have long-term affects.

"Melting glacier water will replenish rivers in the short run, but as the resource diminishes, drought will dominate the river reaches in the long term," he said.

The information gathered during the study will be used by the China Geological Survey Institute under the Ministry of Land and Resources to draft water-preservation policies.