Australia's Great Barrier Reef has lost more than half its coral in the last three decades due to warmer seas driven by climate change, a new study has found.
Marine scientists at the ARC Centre of Excellence for Coral Reef Studies in Queensland assessed the health and size of coral colonies across the reef from 1995 to 2017 and found that all types of corals had suffered a decline across the world's largest reef system.
Their study, which was published in the journal Proceedings of the Royal Society B, found the sharpest declines along the reef followed mass bleaching events in 2016 and 2017.
The team said they now fear the sweeping losses, caused by frequent bleaching, will compromise the ecosystem's ability to recover.
Record-breaking temperatures that triggered the bleaching events in 2016 and 2017 have meant fewer small, baby corals and breeding adult ones, said Terry Hughes, a co-author of the study and professor at the ARC Centre of Excellence for Coral Reef Studies.
Another bout of bleaching earlier this year could meanwhile have caused yet more damage to the reef's southern region, Mr Hughes added.
"That means the resilience of the reef, its ability to bounce back from recurrent mass bleaching events, has been compromised," he said.
"We used to think the Great Barrier Reef is protected by its sheer size, but our results show that even the world's largest and relatively well-protected reef system is increasingly compromised and in decline."
The researchers found populations had dropped by more than 50 per cent in all coral sizes and species, but that branching and table-shaped corals had been particularly badly hit by the back-to-back mass bleachings, which damaged two-thirds of the reef.
Branching and table-shaped corals are large, structural species which usually provide habitats for fish and other marine life. The loss of these corals, in turn, diminishes fish abundance and the productivity of coral reef fisheries.
The bleaching events such as those which affected these species occur when corals under stress from overheating or too much light expel the algae - known as zooxanthellae - that give them colour.
Corals can recover if normal conditions return, but it can take decades.
A 2019 study found that damaged coral colonies had struggled to regenerate because most of the adult corals had died.
Dr Andy Dietzel, the study's lead author, said: "A vibrant coral population has millions of small, baby corals, as well as many large ones."
He added: "Our results show the ability of the Great Barrier Reef to recover - its resilience - is compromised compared to the past, because there are fewer babies, and fewer large breeding adults."
The Great Barrier Reef runs 2,300 km (1,429 miles) down Australia's northeast coast spanning an area half the size of Texas.
It was world heritage listed in 1981 by UNESCO as the most extensive and spectacular coral reef ecosystem on the planet.
But in the past decade, in particular, it has suffered extensive damage due to a warming of the world's seas caused by a rise in global temperatures by about 1C since pre-industrial times and which has killed off coral, dispersed other sea life and sped up the growth of algae and other contaminants.
The reef suffered its most extensive bleaching event ever in March, the third one in five years.
“There is no time to lose – we must sharply decrease greenhouse gas emissions asap,” the report's authors warned.
