Large parts of Australia's Great Barrier Reef are struggling to recover from damage caused by climate change, and baby corals needed to restock and rebuild the reef are in dramatic decline, a study shows.
Published today in the journal Nature, the study found that new corals settling on the reef declined by 89 per cent following the loss of about half of all adult shallow-water corals between 2016 and 2017.
"Dead corals don't make babies," Professor Terry Hughes, the lead author, told The Straits Times.
Prof Hughes and his colleagues from the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Queensland found a direct correlation between the loss of adult corals and the dramatic decline in young corals.
Co-author Andrew Baird said: "The number of coral larvae produced each year and where they travel to before settling on a reef are vital components of the resilience of the Great Barrier Reef.
"Our study shows reef resilience is now severely compromised by global warming."
The Great Barrier Reef, the world's largest, comprises hundreds of different coral species. Some produce tiny larvae directly, others (produce) eggs and sperm in annual mass-spawning events. Ultimately, the tiny larvae settle on the reef and begin to grow, taking several years to become young adults.
But global warming is threatening this natural cycle. Sudden spikes in ocean temperatures can literally cook the corals, badly damaging or killing them. This leads to the images of ghostly white coral skeletons caused by bleaching events, which are triggered by marine heatwaves often lasting weeks.
Scientists say the gap between bleaching events is shrinking, greatly reducing the reef's capacity to recover.
Prof Hughes and his team measured the decline in baby corals by counting the number of juveniles that settled on experimental tiles placed across 17 reefs last year, following the 2016 and 2017 bleaching events. They then compared these figures with measurements of young coral numbers in the years prior to 2016-17.
The biggest decline in young coral numbers occurred in the dominant branching and table corals, called Acropora, that are key to the reef's structure. The scientists recorded a 93 per cent drop in replenishment for Acropora species.
Prof Hughes said: "We expected the loss of adult corals in 2016-17 would reduce the number of offspring they produce, but the 89 per cent decline in rates of replenishment last year surprised us."
He said the mix of species on the Great Barrier Reef was changing very rapidly for two reasons.
"One is due to the bleaching, which is very selective - we found that some species are more susceptible to extreme temperatures in 2016 and 2017 than others.
"The other is the radical shift in the mix of baby corals we saw in 2018.
"So the reef is moving to a new configuration, with a greater proportion of the species that are resistant to bleaching, or that are capable of bouncing back the fastest."
While the results are worrying, Prof Hughes said the reef is not dying. "Certainly, our study shows that the resilience of the reef has been compromised by global warming. It will take longer for it to recover than before and it will recover to a different kind of reef."
The southern section of the reef remains in good condition because it escaped the 2016-17 bleaching. But it is too far south to replenish the damaged northern and central sections, the scientists said.
"There's only one way to fix this problem," said Prof Hughes, adding that greenhouse gas emissions have to be slashed to zero as soon as possible.