the reef is dying off XL Catlin Seaview Survey unprecedented bleaching event Corals are dying on the Great Barrier Reef global bleaching crisis bleaching the once-thriving reef climate change 1,200-mile (2,000 kilometer) unprecedented die-off bleached bone white Coral reefs are dying decimating corals climate change National Geographic will kill off many coral reefs frightening rate Reefs National Geographic Great Barrier Reef coral bleaching global bleaching crisis coral reefs are in peril opposite effect on marine life global bleaching crisis Coral reefs climate change corals Grande Barreira de Coral bleaching unprecedented die-off National Geographic Reefs verbleekt the reef is dying off corals National Geographic