Recent mass bleaching, two cyclones and flooding have left parts of the Great Barrier Reef with “significant coral mortality”, according to new data from the Australian Institute of Marine Science (AIMS).

The in-water surveys of 19 reefs between Lizard Island and Cardwell from August and October found up to 72 percent coral mortality on 12 reefs, the research showed on Tuesday.

In one northern section of the reef, across the Cooktown-Lizard Island sector, more than a third of hard coral cover was lost, the “largest annual decline” in 39 years of government monitoring, the AIMS said.

The scientists said the development was a result of the Austral summer, Tropical Cyclones Jasper and Kirrily, as well as a freshwater inundation between December 2023 and March 2024.

The agency’s scientists are currently collecting data on reefs in the southern region of the Great Barrier Reef.

“These initial results show the vulnerability of the Reef to bleaching events, which are increasing in frequency, footprint and intensity under climate change. Its resilience is being severely tested,” said Manuel Gonzalez Rivero, AIMS acting research programme director.

The reef, a living organism, stretches some 2,300km (1,429 miles) off the northeastern coast of Australia and is considered one of the most species-rich habitats on earth. It is home to hundreds of corals, more than 1,600 species of fish, 133 species of sharks and rays, and 30 species of whales and dolphins, among other creatures.

But repeated mass bleaching events have threatened to rob the tourist draw of its wonder, turning banks of once-vibrant corals into a sickly shade of white.

Bleaching occurs when water temperatures rise and the coral expels microscopic algae, known as zooxanthellae, to survive. If high temperatures persist, the coral can eventually turn white and die.

This year had already been confirmed as the fifth mass bleaching on the reef in the past eight years. But the latest survey also found a rapidly growing type of coral – known as Acropora – had suffered the highest rate of death. This coral is quick to grow, but one of the first to bleach.

Head researcher Mike Emslie told public broadcaster ABC the past summer was “one of the most severe events” across the Great Barrier Reef, with heat stress levels surpassing previous events.

“These are serious impacts. These are serious losses,” he said.

‘Approaching a tipping point’

Richard Leck, the head of oceans for the World Wide Fund for Nature (WWF)-Australia, said the initial surveys confirmed his “worst fears”.

“The Great Barrier Reef can bounce back but there are limits to its resilience,” he said. “It can’t get repeatedly hammered like this. We are fast approaching a tipping point.”

Leck added the area surveyed was “relatively small” and feared that when the full report was released next year “similar levels of mortality” would be observed.

He said it reinforced Australia’s need to commit to stronger emission reduction targets of at least 90 percent below 2005 levels by 2035 and move away from fossil fuels.

The country is one of the world’s largest gas and coal exporters and has only recently set targets to become carbon neutral.