The GBR stretches from the Cape to Gladstone. Are you seriously suggesting that there is a uniform water temperature over that range that has NEVER varied over 600,000 years?
It would have begun as a humble fringing reef on the edge of the continental shelf during the glacial period, 70 km from our now coast and have grown upwards as the seas rose. If you had ever been there and THOUGHT you would wonder why reefs are vertical pillars of dead coral rising from the seabed 60m below where little light penetrates and the colourful corals we associate with "the reef" could never grow.
The GBR is very old and survived many changes. FWIW the crown of thorns is the current threat and that may [I don't know] be thriving because of something we are doing.
