Scientists say desertification of the mountain grasslands of the Tibetan plateau is accelerating climate change. 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. But in recent years the vegetation around his home, the Tibetan plateau, has been destroyed by rising temperatures, excess livestock and plagues of insects and rodents. The high-altitude meadows are rarely mentioned in discussions of global warming, but the changes to this ground have a profound impact on Tibetan politics and the world's ecological security. For Phuntsok Dorje, the issue is more down to earth. He is used to dramatically shifting cloudscapes above his head, but it is the changes below his feet that make him uneasy. "The grass used to be up to here," Phuntsok says, indicating a point on his leg a little below the knee. "Twenty years ago, we had to scythe it down. But now, well, you can see for yourself. It's so short it looks like moss." The green prairie that used to surround his tent has become a brown desert. All that is left of the grasslands here are yellowing blotches on a stony surface riddled with rodent holes. It is the same across much of this plateau, which encompasses an area a third of the size of the US.