Mass bleaching has killed more than a third of the coral in the northern and central parts of Australia's Great Barrier Reef, scientists say.
The ARC Centre of Excellence for Coral Reef Studies released the results of its survey of the 1,430-mile reef off Australia's east coast on Monday.
The scientists found that about 35% of the coral in the northern and central sections of the reef are dead or dying. Some parts of the reef had lost more than half of the coral to bleaching, though corals to the south have escaped with little damage.
Terry Hughes, director of the reef studies centre at James Cook University in Queensland, says the extent of the damage has serious implications.
Older corals take longer to bounce back from bleaching, and probably will not have a chance to recover before the next bleaching event occurs, he said.
And dying coral affects much more than the coral itself, harming other creatures that rely on coral for food and shelter.
The damage is part of a massive bleaching event that has been damaging reefs around the world for the past two years.
Experts say the bleaching has been triggered by global warming and El Nino, a warming of parts of the Pacific Ocean that changes weather worldwide. Hot water puts stress on coral, causing it to turn white and become vulnerable to disease.
This is the third mass bleaching event in 18 years to strike the Great Barrier Reef, and in each case, the areas that suffered the worst bleaching were those where the water was hottest for the longest period of time, Mr Hughes said.
This time, the southern half of the reef was spared largely due to a lucky break that arrived in the form of a tropical cyclone.
The remnants of the storm which had lashed the South Pacific brought cloud cover and heavy rains to the region, cooling the ocean enough to stop bleaching that had just begun in the south. About 95% of the coral in the southern portion of the reef has survived.
Last year, the United Nations' heritage body expressed concern about the state of the Great Barrier Reef and urged Australia to boost its conservation efforts.
"Is it surprising? Not any more. Is it significant? Absolutely," said Mark Eakin, the coral reef watch co-ordinator for the US National Oceanic and Atmospheric Administration.
"We're talking about losing 35% of the population of coral in some of these reefs - that's huge."
Experimental approaches to the bleaching dilemma have included attempts to lower water temperatures by using shades to cover corals, he said.
But such efforts require massive amounts of preparation and can only be done in small areas. Other solutions may lie in finding ways to minimise additional stressors to the already fragile reef.
"Anything you can do to reduce the level of injury and stress coming from other sources, the better the chance that the corals are going to survive," Mr Eakin said.
"Those reefs that have recovered after events like this are the ones that are the most protected, least visited and least disturbed."