The measurements have been ongoing for 40 years. Coral bleaching occurs as a result of increased temperature in the sea and means that the microorganisms that live in symbiosis with the coral animals remove themselves from the corals. The corals then become white and die.
The Australian Institute of Marine Science has monitored the health of 124 coral reefs within the Great Barrier Reef between August 2024 and May 2025, when record-high sea temperatures last year led to "unprecedented levels of heat stress".
The report now shows that the northern and southern parts of the reef have been affected by "the largest annual decrease in coral cover" ever measured. The reef has been ravaged by tropical storms and attacked by sea urchins that eat corals.
But the main reason for the spread of coral bleaching is climate change, according to Mike Emslie, who led the study.
There is no doubt about it, he says.
The bleaching now noted in the Great Barrier Reef is the sixth in nine years. But the coral growth that has occurred at the same time will compensate for the loss, the researchers believe.
For the Great Barrier Reef to recover, several years of coral growth and minimal environmental impact are required, according to the researchers.
