An unprecedented coral bleaching episode has spread to 84 percent of the world's reefs in an unfolding human-caused crisis that could kill off swathes of the essential ecosystems, scientists warned Wednesday.

Harmful bleaching of the world’s coral has now grown to include 84% of the ocean’s reefs in the most intense event of its kind in recorded history.

Bleached coral dies when exposed to heat stress for too long, threatening the bountiful marine ecosystem that depends on it for survival.