Coral Coverage at Great Barrier Reef Plummets Following Unprecedented Heat Events

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The latest survey from The Australian Institute of Marine Science (AIMS) shows coral cover on parts of the Great Barrier Reef has reduced by as much as a third from record high levels following a global mass bleaching event.

The heat-vulnerable tropical ecosystem off the coast of Queensland has experienced its sharpest decline in hard coral prevalence in four decades, with a 2024 spike in ocean temperatures largely to blame.

The survey does not include the latest bleaching event from early 2025, which impacted the Great Barrier Reef and ecosystems along the Western Australian coast.

The growing prevalence of bleaching events

Report co-author Daniela Ceccarelli was concerned about the growing prevalence of bleaching events.

“These back-to-back events were previously completely unheard of,” Ceccarelli said.

Heat stress events in quick succession were worrying, given the emerging dominance of “fast to grow and first to go” Acropora coral species.

Capable of bouncing back quickly after a destructive event when given more space to expand, the fast-growing varieties were largely responsible for the previous survey’s record-high coral coverage rates.

Ceccarelli likened the fast-growing corals to grasses and bushes that shoot up first after a bushfire.

“And if you were to fly over, you’d go ‘it’s nice and green, it’s great’,” Ceccarelli said.

“But the trees are not there yet.”

Heat stress threatening coral recovery

With heat stress events coming too often for hardier, slow-growing corals to get a foothold, Ceccarelli warned such cycles of crash and rapid recovery were becoming more common.

“The question is, how long can this go on before we reach a low from which recovery is not possible?” she said.

“We don’t know that, but it’s worrying we aren’t getting a lot of time between heatwaves anymore.”

A marine researcher wearing a wetsuit is looking at a fish underwater.

Marine experts fear corals may not adapt to and recover from the impacts of climate change. Credit: AAP

Particularly sensitive to heat stress, corals expel the algae living in their tissues when water is too warm, causing the coral to turn completely white.

Coral can recover from bleaching, but it is a sign of stress and can kill the organisms if severe enough.

While cyclones and outbreaks of crown-of-thorns starfish also harm reefs, AIMS indicated that the primary factor driving the 2024 decline in coral coverage was ocean warming due to climate change.

Marine experts say coral can recover from bleaching but it can be fatal if severe enough Credit: AAP

Fears over further decline in reef health

The entire tropical ecosystem recorded falls in coral coverage — an internationally recognised indicator of reef health — but declines were sharpest in the south.

Coverage fell by nearly a third in the southern region, from 38.9 per cent to 26.9 per cent.

North of Cooktown, coverage fell by roughly a quarter.

In the central region, hard corals shrunk nearly 14 per cent.

Even with those sharp declines, when coming from such a high base, overall coverage is now hovering around long-run averages.

AIMS chief executive officer Selina Stead said ocean warming caused by climate change was clearly impacting coral reefs.

“The future of the world’s coral reefs relies on strong greenhouse gas emissions reduction, management of local and regional pressures, and development of approaches to help reefs adapt to and recover from the impacts of climate.”