A global coral bleaching event that began last year has quickly grown to the largest on record, according to a US agency, with the impacted reef area continuing to grow.

From the beginning of 2023 through 10 October 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 on Friday.

He said the ongoing bleaching event -- the fourth since 1998 -- had surpassed the previous record of 65.7 percent in half the time, and "is still increasing in size."

Coral are marine invertebrates made up of individual animals called polyps.

They have a symbiotic relationship with the algae that live inside their tissue and provide their primary source of food, as well as their captivating colors.

When the 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. 

The last record had been set during the third global bleaching event, which lasted from 2014 to 2017 and followed previous events in 1998 and 2010.

NOAA's heat-stress monitoring is based on satellite measurements from 1985 to the present day.

Manzello said NOAA had confirmed reports of mass coral bleaching from 74 countries or territories since February 2023.

"This includes locations in the northern and southern hemisphere of the Atlantic, Pacific, and Indian Oceans," he told AFP by email.

Roughly 850 million people worldwide rely on coral reefs for food, jobs and to protect coastlines from storms and erosion, according to the nonprofit WWF.

The ecosystems provide a haven for ocean life, with over a quarter of marine species calling them home.