[darkseas]

The Great Barrier Reef formed over the last 14,000 years since the last Ice Age rather than million years as you suggest. And while the vast colonies may die, and their calcareous skeletons may dissolve as the ocean acidifies, the polyps themselves will probably survive fine in different forms in other ecological niches. This will be no help for the diverse array of creatures that rely on the reefs, or associated fisheries and tourism industries however.