Almost half the coral at the Great Barrier Reef is dead or dying, according to Australian scientists.
The world's largest coral reef system is a major tourist attraction, and brings around £2.7 billion into the economy each year.
Warmer water is forcing the coral to expel living algae, causing it to calcify and turn white.
The process, known as bleaching, is now thought to affect 93 percent of the Great Barrier Reef.
Mildly bleached coral can recover if the temperature drops, but scientists say nearly half of the coral is already dead or dying.
Scientists believe climate change is the underlying cause of the bleaching, and that the impact has been exacerbated by El Nino, one of the strongest weather systems in nearly 20 years.
The Australian Environment Minister, Greg Hunt, says this is a "significant event", and that the government is "taking it seriously".