Why do scientists think we're nearing the end of the world?

HowStuffWorks "Why do scientists think we're nearing the end of the world?": "In a purely symbolic but still unsettling move, the Bulletin of Atomic Scientists has moved the minute hand on its Doomsday Clock two minutes closer to midnight. On the 60-year-old Doomsday Clock, midnight is nuclear destruction, the end of life as we know it"