"This wasn't here seven or eight years ago," said Longben, 22, his feet digging into the stony sand. "The water levels just keep on dropping."

Lake Qinghai, holy to ethnic Tibetans such as Longben, is shrinking, hit by declining rainfall and desertification partly caused by overgrazing. Some also blame global warming.

Over the past three decades, water levels have dropped almost four metres (13 ft) and an area half the size of Singapore has been turned into dry land, according to state media.

Perched more than 3,000 metres (10,000 feet) above sea level, the salt water lake is more than 100 km (60 miles) long, but the number of rivers feeding it has halved in the past 50 years and salinity has risen markedly.

Despite that, it remains more than twice the size of London and its surface area has historically waxed and waned.

Although far from becoming a Chinese version of the Aral Sea, which was virtually drained to feed Soviet cotton fields, smaller lakes are already breaking off from the main body of water, and at the eastern edge sand dunes tower at the water's edge.

Other Chinese lakes face similar problems, such as Poyang in the southeastern province of Jiangxi.

In Qinghai, the situation seems so serious that the Erlangjian sand spit now appears to be a permanent feature of the lake, known in Mongolian as Kokonor, or blue lake.

The spit has electricity, and huts have been built for birdwatchers. Boats take tourists across for day trips and there are even camp sites.

Further west sits Bird Island, now an isle in name only.

"Our environmental consciousness here used not to be very good," said Longben, as his weather-beaten face looked out at the azure water rippling in the breeze.

BATTERED ENVIRONMENT

Qinghai province, which stands on the northeastern part of the Tibetan Plateau and takes its name from the lake, has had its environment battered by mankind for the past half century.

China's first atomic bomb was detonated at a site on the northeastern shores of the lake in 1964, and vast salt and potash plants pit the province's desolate central plains.

A naval torpedo testing station in the lake had to be closed in the 1980s as water levels dropped, residents say.

It was turned into a museum, but the water dropped further and the wharf is now too high up for boats to disembark passengers safely. Today it sits abandoned 100 metres offshore, still sporting the red star of the People's Liberation Army.

Apart from the military, fishing was another pillar of the lake's economy.

Sun Zhi, 47, came 25 years ago from the frozen northeastern province of Heilongjiang to work the boats fishing for the unusual scaleless naked carp that are found in the lake.

That was when Lake Qinghai was full of fish. Now their numbers have fallen so catastrophically that all fishing has been banned, although tourists are still offered carp at lakeside restaurants once the owners are sure the police are not looking.

"I have to rely on tourism now, and that's only for a couple of months of the year," said Sun, who works the tourist boats out to Erlangjian and blames the lake's problems on "rising temperatures".

GLOBAL WARMING

Some say that the shrinking lake is the price China is paying for not taking global warming seriously.

China is the second largest emitter of greenhouse gases after the United States. Despite recent pledges to make its economy more sustainable, the country burnt nearly 6.5 million barrels of oil per day in the first quarter of 2006, releasing vast quantities of heat-trapping carbon dioxide into the atmosphere.

"The climate is becoming warmer, which increases evaporation and has caused the water level to go down," environmental activist Dai Qing told Reuters.

"Plus the rivers around the lake and even underground water supplies are drying up. It's a really terrible problem. It shows that global warming is going to affect China too," she said.

On the lake's southeastern shores, close to a tacky tourist development where Tibetans dance in front of gawping Chinese crowds, the effects of the shrinking water are obvious.

A boat sits rusting above the water line of a small port once home to fishermen but now serving only tourist vessels.

"Every year the water gets lower," said Wang Jianming, a native of the nearby provincial capital, Xining, who has been visiting the lake since the 1980s.