Photo taken on July 15, 2014 shows a Yak skull with Purog Kangri Glacier in its background in Shuanghu County, Nagqu, southwest China's Tibet Autonomous Region. As the highest terrain in the world's mid-latitude regions, China's Qinghai-Tibet Plateau is now under negative influence of the global warming. Purog Kangri Glacier, the third largest in the world, has shrunk by 50 meters over the past 30 years. [Photo/Xinhua]