Scientists say corals in the northern Great Barrier Reef experienced a 'catastrophic die-off' during a record-breaking marine heatwave two years ago.
New research suggests almost a third of corals died off in the reef system - which is located off the coast of Australia - between March and November 2016.
The study, published in the Nature journal, shows that 29% of the 3,863 reefs lost two-thirds or more of their corals.
Researchers say the situation means diverse and mature areas of the reef are becoming 'more degraded' - meaning only tough species are able to survive.
They are now calling for urgent action to combat climate change, which they say will "radically alter tropical reef ecosystems".
The different colour morphs of Acropora millepora, each exhibiting a bleaching response during mass coral bleaching event. Credit: ARC Centre of Excellence for Coral Reef S Studies / Gergely Torda
Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies, explained: "When corals bleach from a heatwave, they can either survive and regain their colour slowly as the temperature drops, or they can die. Averaged across the whole Great Barrier Reef, we lost 30% of the corals in the nine month period between March and November 2016.
"That still leaves a billion or so corals alive, and on average, they are tougher than the ones that died. We need to focus urgently on protecting the glass that’s still half full, by helping these survivors to recover."
He added that while the reef is threatened by climate change and global warming, a quick response to greenhouse gas emissions would mean "it is not doomed".
The Great Barrier Reef covers a 344,400 km² area in northeastern Australia and is around 2,300 km long.
It is a World Heritage Site and is considered one of the world's natural wonders for its colourful coral and the unique surrounding ecosystems.
Data released last year revealed that two-thirds of the massive reef system had been hit by 'severe' coral bleaching.
Bleaching occurs when coral is 'stressed', which is often caused when the water temperature becomes particularly hot.
Such abnormal conditions cause the coral to 'expel' tiny and colourful algae known as zooxanthellae.
In the absence of the algae - which produce food for the coral - coral turns white and 'bleaches'.
