AS an expedition from Chinese state television worked its way across the remote Tibetan plateau earlier this year, the explorers were amazed by what they found.
The plateau has been called the world's third largest ice store after the North and South Poles. Yet according to Chinese scientists, the "third pole" is warming up faster than anywhere else on earth.
The TV team found bare rock where glaciers had retreated. Lakes had dried up. Lush grassland had turned to desert. The livestock was dead, the farmers impoverished.
They brought back a visual lesson in global warming so stark that censors allowed the programme makers to broadcast a frank exposé. Their film attracted the attention of the Communist party's leaders and has put climate change at the centre of a remarkably open debate in China ahead of a summit on the issue in Copenhagen next month.
It means that when President Barack Obama arrives in China next weekend he will find his hosts ready to talk about dozens of measures to slow the rate of global warming. He will not find them willing to agree to calls by rich countries for Beijing to accept a binding cap on carbon emissions - a condition that commentaries in the Chinese media have defined as politically unacceptable.
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Any compromise might break an international deadlock and allow a treaty to be signed. However, even if that now looks unlikely to happen - and the United Nations official leading the talks accepts this - the fact is that China has woken up to the damage in an unprecedented way.
The speed and scale of change on the Tibetan plateau have made Chinese leaders react to something they understand - a potential threat to the future of China itself.
They are clearly seeking to mould opinion in favour of "greener" policies after decades of a highly polluting dash for economic growth that has poisoned China's rivers and darkened its skies.
Last month, for example, researchers discovered that levels of black carbon in the ice core of the Tibetan plateau had soared since the 1990s because of smokestack industries and coal fires in millions of homes.
The plateau's 36,000 glaciers, which once extended for 18,000 square miles, could vanish before mid-century if present rates of warming persist. More than 80% of them are in retreat. The overall area has shrunk by 4.5% in the past 20 years.
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Most ominous of all, in the area that Chinese know as Sanjiangyuan, where three mighty rivers rise - the Yangtze, the Yellow and the Mekong - the headwaters run shallow and weak, threatening the water supplies for hundreds of millions of people.
"In the 1970s and 1980s, here was rich grassland and sheep grazed everywhere, but the weather has become hotter and drier," a Tibetan herder, Sonarenqin, 39, told the TV crew.
"Five years ago my family had 300 sheep and 30 yaks. Now I have no sheep at all and merely a few yaks," an 80-year-old Tibetan named Seluo added. "Our life has become so hard that we live on handouts."
In the past 30 years the thawing of permafrost, a layer of soil that is usually frozen all the year round, has changed the landscape profoundly.
"There were 4,077 lakes and now 3,000 of them have disappeared," said Xin Hongyuan, a geologist in Qinghai, which shares the huge expanse of plateau with the Tibet autonomous region and the provinces of Sichuan and Gansu.
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"The snow is thawing and the snowline has risen from 4,600 metres to 5,300 metres. The Jianggendiru glacier, which is the main water supply of the Yangtze, has been degenerating fast since 1970, and when the glaciers shrink there will be a water crisis in the Yellow and Yangtze rivers."
The Yellow river, for example, supplies water to a fifth of China's 1.3 billion population and serves 50 big cities along its 3,395 miles.
In recent years it has sometimes slowed to a trickle. Once it virtually stopped flowing for 226 days, causing urban waterpipes to run dry and confronting downstream provinces with huge financial losses.
Qin Dahe, an eminent scientist and explorer, has been permitted to disclose alarming official assessments of the causes to Xinhua, the state news agency. "Owing to global warming, glaciers on the QinghaiTibet plateau are retreating extensively at a speed faster than in any other part of the world," he said.
Temperatures on the plateau have risen by an average of 0.32C every 10 years since 1961, about six times as fast as in the rest of China. In Tibet, it is hotter than at any time in the past half century, while in the south and west of Tibet there is between 30% and 80% less rainfall.
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Qin said floods, mudslides and other calamities would become more frequent.
It is not just China that is at risk. Its giant neighbour, India, also faces the consequences of climate change in Tibet, whose glaciers nourish the Indus, Brahmaputra and Ganges rivers. Qin said the breakdown of permafrost would eventually destroy the ecological balance and change the summer monsoons, to devastating effect. There would be droughts in northern India, more intense floods in southern China and parched lands in the wheat-growing north, he predicted.
Greenpeace said recently that if the glaciers vanished, it would put at risk water supplies for 900m people in China and India alone.
At present, there is no sign of co-operation between the two countries to meet the threat. Instead they are frozen in a diplomatic and military dispute about the Himalayan border, over which they went to war in 1962. Indeed, tension has intensified in recent weeks because of a visit by the Dalai Lama to a monastery in a contested area.
If Obama cannot save the climate in his visit to China, he might at least be able to lower the temperature between the Asian giants and start them talking about the diminishing "third pole".