Passage: 'The Great Barrier Reef, spanning over two thousand kilometers off the northeastern coast of Australia, is the world's most extensive coral reef ecosystem. Composed of billions of tiny organisms known as coral polyps, the reef provides habitat for thousands of marine species. In recent decades, elevated ocean water temperatures have caused mass coral bleaching events, during which corals expel their symbiotic zooxanthellae algae, leaving the calcium carbonate skeletons stark white and vulnerable to starvation.' What causes coral bleaching according to the passage?