A global episode of heat-related coral bleaching has grown to the largest on record, US authorities said Friday, sparking worry for the health of key marine ecosystems.

From the beginning of 2023 through October 10, 2024, “roughly 77 percent of the world’s reef area has experienced bleaching-level heat stress,” Derek Manzello of the National Oceanic and Atmospheric Administration (NOAA) told AFP.

When ocean water is too warm—such as during heat waves which have hit areas from Florida to Australia in the past year—coral expel their algae and turn white, an effect called “bleaching” that leaves them exposed to disease and at risk of dying off.