The phenomenon, known as coral bleaching because the reefs turn bone white when the colourful algae that give the coral its colour and food is lost, has been reported throughout south east Asia, the Indian Ocean and the Pacific.
Divers and scientists have described huge areas of previously pristine reef being turned into barren white undersea landscapes off the coast of Thailand and Indonesia.
The popular island tourist destination the Maldives have also suffered severe bleaching. Reefs in the Caribbean could also be under threat.
High ocean temperatures this year are being blamed for the bleaching, which experts fear could be worse than a similar event in 1998 which saw an estimated 16 per cent of the world’s reefs being destroyed.
Dr Mark Eakin, coordinator of the National Oceanic and Atmospheric Administration’s Coral Reef Watch, said: “The bleaching is very strong throughout south east Asia and the central Indian Ocean.
“The reports are that it is the worst since 1997/1998. This is a really huge event and we are going to see a lot of corals dying.”
Coral reefs provide refuge and food to nearly a quarter of all marine species, making them among the most biologically diverse habitats on the planet. Bleaching can also rob fish and other species of important shelter and food sources.
Although reefs can often recover from bleaching, it leaves the coral vulnerable to damage from storms, infections and other environmental stress, increasing the risk of deaths.
Coral reef monitoring teams have reported mass bleaching of coral reefs off the coast of Thailand, Malaysia, Vietnam, Cambodia and Indonesia while the Maldives, Sri Lanka and reefs off the coast of east Africa have also been hit.
With ocean temperatures reaching record levels and combined with the end of an El Nino episode, scientists fear there could be even more damage to corals as the year continues.
Scientists in Thailand have reported reefs suffering 90% of their corals being bleached and up to 20% of the corals dead.
Olivia Durkin, who is leading the bleaching monitoring at the Centre for Biodiversity in Peninsular Thailand, said: “This year’s severe coral bleaching has the potential to be the worst on record.
“Extensive bleaching, death and disease are reported not only in corals, but giant clams, sea anemones and soft corals are also losing their symbiotic algae.”
Corals are a delicate combination of animal, algae and rock that form intricate undersea structures, providing shelter for thousands of brightly coloured fish and also acting as nurseries for the young of many larger open sea fish.
Coral colonies are made up of polyps, which secrete a stony skeleton that forms the intricate and delicate looking structures. A microscopic algae known as zooxanthellae live within the coral where they convert energy from the sun into food for the coral animals.
Bleaching usually occurs when ocean temperatures exceed a threshold that is around one degree higher than the average seen during the warmest summer months.
Although scientists do not fully understand why it happens, bleaching is thought to occur when these prolonged periods of these high temperatures combine with excessive sunlight levels.
This causes the symbiotic algae in the coral to become over active, causing it to poison the coral host and leading to the coral expelling the algae into the surrounding water to defend itself.
Without the algae to provide food and nutrition, the corals grow weak and leaves them vulnerable to disease and damage from storms.
In many cases the coral dies, leaving an undersea wasteland that quickly becomes infested with weedlike algae which covers every surface.