The Great Barrier Reef keeps bleaching. Why are we told corals are thriving?
By Mike Foley
Global warming is heating the ocean so much that mass coral bleaching is now a yearly threat to the Great Barrier Reef, including a severe event this past summer. But some will cite the latest scientific surveys, released on Wednesday, to claim the reef is in rude health.
So what’s actually going on under the water?
The latest results from the Australian Institute of Marine Science’s (AIMS) long-term monitoring program, which began in 1985, show the reef has some of its highest levels of coral cover on record up and down the 2300-kilometre-long ecosystem.
Coral cover is the proportion of the reef structure with live, hard corals. It has fluctuated dramatically in the survey’s 38 years, with strong regrowth often following big declines due to cyclone damage or mass bleaching events.
Mass coral bleaching events hit the Great Barrier Reef in 1998, 2002, 2016, 2017, 2020, 2022 and in the summer of 2024. AIMS’s most recent coal survey was largely completed last year, before the most recent bleaching event.
Sceptics of the mainstream narrative have cited strong regrowth figures to claim that scientists and green groups overstate the threat from global warming, arguing the results show the reef can bounce back.
But scientists warn of intense stress on the reef ecosystem, as the frequency and severity of bleaching events increases. AIMS said the reef’s most dramatic losses and gains have occurred in the past 15 years, which has seen both historic highs and lows in coral cover.
What’s more, the regrowth is being dominated by a relatively narrow range of coral types.
“Much of the recent hard-cover increase was driven by increases in the fast-growing Acropora corals, which have proliferated across many Great Barrier Reef reefs,” AIMS said of its survey results.
The Acropora genus includes table and staghorn branching corals. Many other corals, like the massive boulder corals of the Porites genus, take decades to regrow.
The ARC Centre of Excellence for Coral Reef Studies has found that the number of corals across the Great Barrier Reef had halved in the past 20 years, and the reduction in coverage and diversity of corals had shrunk crucial habitat for species that comprised the reef’s vibrant marine life.
It also said mass coral bleaching had “cascading effects on community composition and ecosystem functioning”. In other words, the nature of the ecosystem could be turned on its head.
Australia’s climate has already warmed by an average of 1.47 degrees over the past century, and sea surface temperatures have increased by an average of 1.05 degrees.
AIMS survey leader Dr Mike Emslie said bleaching was the greatest risk to the reef and reducing greenhouse emissions was the best way to protect it.
“We’re already seeing biennial bleaching events. If we keep pushing through that 1.5 [degrees] envelope, the predictions are these bleaching events are going to become a lot more frequent than that and there’s only so much resilience inherent in the system.”
AIMS research director Dr David Wachenfeld said the strong regrowth figures could already have been undone by last summer’s bleaching event.
“We are only one large-scale disturbance event away from a reversal of the recent recovery. The 2024 bleaching event could be that event – almost half of the 3000 or so reefs that make up the marine park experienced more heat stress than ever recorded,” Wachenfeld said.
Bleaching occurs when corals are put under severe stress due to high water temperatures, which causes the organisms to expel the algae living in their tissues and turn white.
Not all corals that turn white die. They can recover if sea surface temperatures return to normal quickly enough.
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