china_warming01_ph001.jpg Namgyal Tsering, a 22-year-old nomadic shepherd, watched his flock of sheep above the wetlands around Gyaring Lake. At 14,000' elevation, the lake forms the source of the Yellow River, which is China's second-longest river and which waters the dry northern grainbelt. Tsering said the poor rains of recent years has made it harder to make a living from his sheep (down the hill, in background) and yaks. Robert Collier / The Chronicle Ran on: 08-01-2007 Namgyal Tsering, 22, a nomadic shepherd, watches his sheep above the wetlands around Gyaring Lake. At 14,000 feet elevation, it is the source of the Yellow River, Chinas second longest.