Australia's Great Barrier Reef underwent another mass bleaching event – the third in just five years.
Warmer sea temperatures – especially in February – are fearful of causing massive coral loss in the world's largest reef system.
Scientists say they have detected widespread bleaching, including extensive patches of serious damage. But they also found healthy pockets.
Two-thirds of the reef were damaged by similar events in 2016 and 2017.
The reef system, which covers more than 2,300 km (1,400 miles), is a World Heritage Site recognized for its "enormous scientific and intrinsic importance".
Last year, Australia was forced to reduce its five-year reef outlook from bad to very bad due to the impact of human-induced climate change.
On Thursday, the Great Barrier Reef Marine Parks Authority said its latest aerial surveys showed that the severity of bleaching varied between reefs.
But he said more areas were damaged than in previous events.
- Coral Barrier & # 39; Coral Babies & # 39; in mass decline
- Heat waves & # 39; cooking & # 39; barrier reef corals
"The reef had started to recover from impacts in 2016 and 2017 and now we have a third event," chief scientist David Wachenfeld told the BBC.
"Climate change is making the extreme events that drive these impacts more severe and more frequent, so the damage to an event is worse."
Previous events affected two-thirds of the coral system, destroying coral populations and destroying habitats for other marine species.
But Wachenfeld said that some reefs important for tourism – in the northern and central regions – have only been "moderately bleached" this year. That meant that corals were likely to recover, he added.
"The reef is still a dynamic and vibrant system, but overall, with all these successive events, the reef is more damaged than it was before," he said.
"We need to face these events, as the world calls for the strongest possible action on climate change," he said.
Global temperatures have increased by about 1 ° C since pre-industrial times. The UN warned that if temperatures rise 1.5 ° C, 90% of the world's corals will be eliminated.