Half the corals on Australia’s Great Barrier Reef have died over the past 25 years, scientists said Wednesday, warning that climate change is irreversibly destroying the World Heritage-listed underwater ecosystem.
A study published in the Proceedings of the Royal Society Journal found an alarming rate of decline across all sizes of corals since the mid-1990s on the vast reef that lies off the country’s northeastern coast.
Larger species, such as branching and table-shaped corals, have been worst affected – almost disappearing from the far northern reaches of the reef, researchers found.
“They’re typically depleted by (up to) 80 or 90 percent compared to 25 years ago,” report co-author and James Cook University professor Terry Hughes told the AFP news agency.
“They make the nooks and crannies that fish and other creatures depend on, so losing big three-dimensional corals changes the broader ecosystem.”
Aside from its inestimable natural, scientific and environmental value, the 2,300-kilometre-long (1,400-mile-long) reef was worth an estimated $4bn a year in tourism revenue for the Australian economy before the coronavirus pandemic struck.
The reef is at risk of losing its coveted World Heritage Status because of ocean warming, fuelled by climate change, which is damaging its health.
Changes in ocean temperatures stress healthy corals, causing them to expel algae living in their tissues and draining them of their vibrant colours in a process known as bleaching.
Back-to-back mass bleaching events in 2016 and 2017 prompted the government to downgrade the long-term outlook for the world’s largest living organism to “very poor”.
Mass bleaching was first seen on the reef in 1998 – at the time, the hottest year on record – but as temperatures continue to soar its frequency has increased, making it harder for the reef to recover from each incident.
The ‘big mamas’
“A vibrant coral population has millions of small, baby corals, as well as many large ones — the big mamas who produce most of the larvae,” the study’s lead author Andy Dietzel, also of James Cook University, said.
“Its resilience is compromised compared to the past because there are fewer babies and fewer large breeding adults.”
On top of long-term ocean warming and associated bleaching, the reef has been battered by several cyclones and two outbreaks of crown-of-thorns starfish, which eat the coral, since 1995.
Our latest research published today shows drastic changes on the #GreatBarrierReef over the past 25 years – almost every coral species has declined, the mix of species has changed, colony sizes are smaller, and there are fewer juvenile corals. https://t.co/YPL5yW1GTy @_ADietzel https://t.co/YKlx2lSAvc
— Terry Hughes (@ProfTerryHughes) October 14, 2020
When the starfish occur in small numbers, they are considered part of the natural ecosystem, but when a large outbreak happens, they can rapidly destroy parts of the reef.
While four mass bleaching events up to 2017 were covered by the latest research, the damage to coral species from bleaching in early 2020 is yet to be assessed.
It was the most widespread bleaching on record, impacting swathes of the southern reaches of the reef for the first time.
Hughes said scientists expected corals to continue dying off unless nations met their Paris Agreement commitment to keep the increase in global average temperature less than 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels.
“It takes about a decade for a half-decent recovery for the fastest-growing species, so the chances of us getting decades between the future sixth, seventh and eighth bleaching events is close to zero because temperatures are going up and up and up,” he said.
If temperatures do stabilise later this century under the Paris target, it is hoped that corals will be able to reassemble and rebuild their numbers.
Even then, Hughes said: “We don’t think they’ll rebuild into the mix of species that we’ve known historically”.
If the rise is as much as three or four degrees Celsius, “forget it”, he said.
“The trajectory is changing very, very quickly – we’re shocked and surprised by how quickly these changes are happening – and there’s further change ahead.”