Northern and southern branches of the sprawling Australian reef both suffered their most widespread coral bleaching, the Australian Institute of Marine Science (AIMS), external found.

Reefs  have been battered in recent months by tropical cyclones and outbreaks  of crown-of-thorns starfish that feast on coral, but heat stress driven  by climate change is the predominant reason, AIMS said.

AIMS  warns the habitat may reach a tipping point where coral cannot recover  fast enough between catastrophic events and faces a "volatile" future.

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AIMS surveyed the health of 124 coral reefs between August 2024 and May 2025. It has been performing surveys since 1986.

Often  dubbed the world's largest living structure, the Great Barrier Reef is a  2,300km (1,429-mile) expanse of tropical corals that houses a stunning  array of biodiversity. Repeated bleaching events are turning vast swaths  of once-vibrant coral white.

Australia's  second largest reef, Ningaloo – on Australia's western coast – has also  experienced repeated bleaching, and this year both major reefs  simultaneously turned white for the first time ever.

Coral  is vital to the planet. Nicknamed the sea's architect, it builds vast  structures that house an estimated 25% of all marine species.

Bleaching happens when coral gets stressed and turns white because the water it lives in is too hot.