Half of the Great Barrier Reef’s corals have died over the past 25 years, scientists said yesterday, warning that climate change is irreversibly destroying the 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 World Heritage-listed reef off Australia’s northeastern coast.
Larger species, such as branching and table-shaped corals, have been affected hardest — almost disappearing from the far northern reaches of the reef.
“They’re typically depleted by [up to] 80 or 90 percent compared to 25 years ago,” report coauthor and James Cook University professor Terry Hughes said. “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 2,300km reef brought in an estimated A$4 billion (US$2.9 billion) a year in tourism revenue for the Australian economy before the COVID-19 pandemic.
The reef is at risk of losing its coveted world heritage status because of ocean warming — fueled by climate change — damaging its health.
Changes in ocean temperatures stress healthy corals, causing them to expel algae living in their tissues — draining them of their vibrant colors in a process known as bleaching.
Mass bleaching was first seen on the reef in 1998 — at the time, the hottest year on record — but as temperatures continue to soar it frequency has increased, shrinking the reef and making it harder to recover.
“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,” said study lead author Andy Dietzel, also of James Cook University. “Its resilience is compromised compared to the past, because there are fewer babies and fewer large breeding adults.”
While four mass bleaching events up to 2017 were covered by the latest research, the damage to coral species from bleaching earlier this year 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 that scientists expected corals to continue dying off unless nations met their Paris Agreement commitment to keep the increase in global average temperature under 2°C above preindustrial 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 stabilize later this century, it is hoped that corals would be able to reassemble and rebuild their numbers.
Even then “we don’t think they’ll rebuild into the mix of species that we’ve known historically, Hughes said.
If the rise is as much as 3°C or 4°C, “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,” he added.
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