The Great Barrier Reef is known as the largest biological organism in the world which we were able to see when visiting the reef. The ecology of the Great Barrier Reef is one that is vulnerable to even slightest human influence. In these times, human activity has intensified to a level where our actions have extended to the coral reefs on a global scale. We took this picture where you can see the effect of coral bleaching. 

(https://theconversation.com/great-barrier-reef-dying-beneath-its-crown-of-thorns-6383)

Question: What causes coral bleaching?