Perhaps the clearest and most visible sign of climate change in America won’t be around for much longer: The glaciers of Glacier National Park in Montana are melting and will be gone within 30 years, scientists say. Dan Fagre, the 49-year-old leader of the U.S. Geological Survey team studying the problem, says, “It’s not just going to happen in my lifetime. It’s going to happen during my career.” More than a century ago, there were 150 glaciers in the high alpine-valley park; today, there are only 35. Ice fields are vanishing from mountain ranges everywhere, from the Andes to the Himalayas. Still, with all the data amassed by scientists, mystery abounds: For example, even as most glaciers in Glacier head toward extinction, a few still cling inexplicably to their mountain peaks.