<P> Coral bleaching occurs when coral polyps expel algae that live inside their tissues . Normally, coral polyps live in an endosymbiotic relationship with this algae crucial for the health of the coral and the reef . The algae provides up to 90% of the coral's energy . Bleached corals continue to live but begin to starve after bleaching . Some corals recover . </P> <P> Above - average sea water temperatures caused by global warming is the leading cause of coral bleaching . 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 . In 2017, the bleaching extended into the central region of the reef . The average interval between bleaching events has halved between 1980 and 2016 . </P> <P> The corals that form the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae - like single - celled flagellate protozoa called zooxanthellae that live within their tissues and give the coral its coloration . The zooxanthellae provide the coral with nutrients through photosynthesis, a crucial factor in the clear and nutrient - poor tropical waters . In exchange, the coral provide the zooxanthellae with the carbon dioxide and ammonium needed for photosynthesis . Negative environmental conditions thwart the coral's ability to provide for the zooxanthellae's needs . To ensure short - term survival, the coral - polyp then expels the zooxanthellae . This leads to a lighter or completely white appearance, hence the term "bleached". As the zooxanthellae provide up to 90% of the coral's energy needs through products of photosynthesis, after expelling, the coral may begin to starve . </P>

What is the main cause of coral reef bleaching