Even though coral reefs have recently gone through a record breaking bleaching events, bleaching is not the same as the death of a coral reef. Bleaching is a survival tactic from which a reef can recover.
Nonetheless, too many coral bleaching events can put a reef in danger of dying and is the biggest threat to the Great Barrier Reef. Coral polyps of the type that build reefs create a skeleton of calcium carbonate and the living coral sit in little cups on top of a ever-expanding skeleton.
Coral polyps get the majority of their energy from single celled algae that live inside their body. During a bleaching event the polyps excrete the algae cells causing the coral structure to lose its color.
The recent El Ni�o event as well as rising ocean temperatures due to global warming are considered the primary cause of recent bleaching events. Still, it's too early for obituaries.