query: How much of the Great Barrier Reef is bleached?
passage: Coral bleaching
Above-average sea water temperatures caused by global warming is the leading cause of coral bleaching.[2] According to the United Nations Environment Programme, between 2014 and 2016 the longest recorded global bleaching events killed coral on an unprecedented scale. In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef's coral.[3][4][5] In 2017, the bleaching extended into the central region of the reef.[6][7] The average interval between bleaching events has halved between 1980 and 2016.[8]
passage: Cyclone Hamish
Along the storm's track parallel to the Queensland coastline, the eye passed over a substantial portion of the Great Barrier Reef, resulting in some of the worst damage to the area in recent history. Unlike most cyclones which travel from east to west in the region, impacting only a small area of the reef, Hamish moved alongside the reef for nearly its entire existence. The Bureau of Meteorology estimated that about a quarter of the area was impacted by the storm, with some parts being within of Hamish's eye when it was a Category 5 cyclone. According to post-storm surveys of the reef, the damage done to the coral was extensive, with upwards of 70% losses in the hardest hit spots. Nearly all of the exposed coral was destroyed by turbulent waters; however, in places where the coral was slightly sheltered, there was little to no damage. Some areas were completely stripped of all living tissues, leaving only bare limestone. According to preliminary estimates, it would take the reef between eight and fifteen years to recover from Hamish if nothing hampers growth.