Tibetan nomads struggle as grasslands disappear from the roof of the world

Tibetan nomad Phuntsok Dorje outside his tent. The green prairie that used to surround it has become a brown desert. Photograph: Jonathan Watts for the Guardian
Like generations of Tibetan nomads before him, Phuntsok Dorje makes a living raising yaks and other livestock on the vast alpine grasslands that provide a thatch on the roof of the world.