Somewhere in my home, I have a memory stick with a 2017 GBR bleaching pdf report and a pile of raw data that I got from a RAN officer who was diving with us on the Mike Ball boat Spoilsport. She'd been collating environmental reef studies, and said it was bad all along the thing. On our trip, we even saw bleaching as far out as Osprey, about halfway between Cape York and PNG. My take is that a lot of it comes from chemical runoff from industrial processes, landscaping and indiscriminate pesticides, particularly in QLD as they service agricultural regions.