Great Barrier Reef has suffered its third bleaching.
Experts have warned that huge patches of the reef have been affected.
It is the third time this has happened in the last five years, according to the scientist carrying out aerial surveys. They have looked at almost 500 reefs from the Torres Strait to Cairns
The worst outbreaks of mass bleaching occurred in 2016 and 2017. This results in a record killing of about half the shallow water corals on the world’s biggest reef system.
The scientists said that a fuller picture would be possible after more intensive surveying.
Corals bleach when they sit in waters that are unusually hot for too long. They can recover if temperatures fall, but are often killed when high temperatures are sustained.
Global heating caused by escalating atmospheric greenhouse gases is a major threat to the world’s coral reef ecosystems.