"The waters from Sanjiangyuan sustain life for 600 million people downstream but in recent years this vast water tower has been under threat," says Marc Foggin, a conservation biologist at the NGO Plateau Perspectives, who has studied life on the plateau for 15 years. "And what affects China, affects the world."

That threat is environmental degradation. In 2000, officials panicked when they found dried-up lakebeds and grasslands turning to desert near the source of the Yellow River, in Guoluo county, Qinghai. The Yellow and Yangtze rivers function as China's two major arteries, flowing through its industrial heartland. The third of the "three rivers", the Mekong, also flows through Myanmar, Laos, Thailand, Cambodia and Vietnam. The future of China and Southeast Asia depends on how this water source is managed.

Climate change is thought to be the main cause of rangeland degradation, with the plateau warming at twice the world average, helped in part by China having overtaken the United States as the world's largest polluter in 2007. Officials, however, also blame a burrowing mammal called a pika - and the overgrazing of nomadic herds.

By next year, more than 530,000 nomads in Qinghai province are to be resettled under the "ecological resettlement programme", although the trend towards sedenterisation of the nomads began in the 1960s, as part of Mao Zedong's Great Leap Forward. Ecological resettlement is the latest and possibly last phase of a process threatening to end this ancient way of life.

The Tibetan identity is rooted in the land and the herding of yak. Traditionally, this would have meant moving tents with each season, but today it tends to involve shifting from a permanent winter home to a summer tent.

Motorbikes and smartphones have become essential tools for the modern nomad but - although resettlement is producing an educated but rurally ignorant generation - for the time being, yak herding remains at the heart of the Tibetan identity.

"The yak is intimately associated with the religion and culture of this region," says Gerald Wiener, a British-based animal geneticist who co-authored the book for the United Nations. "Yak are the most wonderful animals for sustaining life in the region. They provide milk for butter, yogurt and cheese, hair for weaving into tents and rope, with the finer fabric made into clothing, and are a source of meat."

The late Cai Li, a professor and yak expert from Chengdu, Sichuan province, believed that, without yak dung - the only fuel available - no people or civilisation could have colonised these vast mountainous regions.

"Grassland life is a lot of hard work," though, says nomad turned conservationist Tsera, who goes by one name. "Nomads have started thinking the grass is greener in the city."

Under the resettlement programme, families are provided with a home and a living subsidy in return for moving their children to towns for education.

"But [in town, people] have no jobs and no money. They have an identity crisis," says Tsera. "They always ask me, 'What shall I do?' At first people think it's a good choice but after a while they realise they have to buy meat and yak dung, pay to build a toilet and don't have enough money to move back."

Those who resent the resettlement villages have labelled them "thief schools" and "ghettos on the grasslands". In addition to education and health care, they have introduced inner-city problems to the rural landscape. Seventy per cent of resettled Tibetans remain unemployed, relying on government handouts, stripped of hope and suffering culture shock. This sense of despair has been used to account for a spate of self immolations by Tibetans since 2009.

In 2010, an earthquake destroyed Yushu, the largest town in the Three Rivers Reserve, with the loss of more than 2,600 lives. While most buildings crumbled, a statue of Tibet's mythic King Gesar remained standing in the town centre. Beijing has pledged to rebuild Yushu as an ecological city centred on King Gesar Square and the rebuilt hill-top monastery: twin totems of Tibetan culture. An airport and 800 kilometres of dual carriageway from the provincial capital will bring in tourists seeking an authentic cultural experience.

Horse festivals on the plains near Yushu have again become popular, despite government restrictions on large gatherings in the province. Resettlement villages have their own regular races - a reaction, perhaps, to "settled" life.

"No culture is a museum piece," says Foggin. Plateau Perspectives works with nomads to sustain livelihoods and conserve local wildlife. "Tibetan pastoralism will continue to create and recreate itself. There is a creativity in us that has allowed humans to survive in harsh habitats."

Lifestyles become ancient, he reasons, by having adapted to changing environments. For millennia, the nomads have looked for their survival to their wildlife and it is likely that at least some of them will do so again.

New transport infrastructure will not only help herding co-operatives get their produce to the lucrative eastern markets, it could also facilitate eco-tourism.

The Three Rivers Reserve is a haven for rare creatures such as the snow leopard, the Tibetan antelope and the black-necked crane. Last November, the authorities announced paid positions for nearly 10,000 "keepers of the grassland" - nomad monitors tasked with gathering scientific data and deterring poachers - an official endorsement of the value of herders.

So it seems today, as in the past, that Tibetan culture must rely on the wildlife that inhabits this stunning but harsh landscape.

"There is no need to be nostalgic for a past time," says Dorje, himself a product of voluntary resettlement. "Whether the culture survives depends on the people. Every individual has the ability to do something to save traditional culture."