In the Indian Ocean, the western Pacific around the Philippines and Indonesia, and the eastern Pacific around the Galapagos Islands, most of the corals have died, leading coral reef scientists announced.
All areas of the world except the central Pacific had been affected, they said, and in many places more than 90 per cent of the corals had been killed, by sea temperatures up to 2.4C hotter than normal.
"Reefs are living at the edge of being cooked," said Don McAllister, a Canadian coral scientist with the World Conservation Union, the international umbrella body for wildlife.
"In almost every region of the world, 1998 got too hot for corals to live normally," said Dr Thomas Goreau, an American scientist who heads the main international network reporting on coral reef conditions.
"All previous human-induced coral reef destruction pales beside what temperature has done this year."
It was an unprecedented natural disaster, he said. "The world's reefs will only be saved if global warming is stopped cold, now. Not in 10 years, but now." If it happens, the disappearance of coral reefs will be the first catastrophic global impact of climate change.
Coral reefs form the world's richest ecosystem. They also underpin tourism in more than 100 countries, thought to be worth $500bn (pounds 300bn) a year, providing their wildlife, coastal protection and all the soft white sand of tropical beaches.
The death of unprecedented quantities of coral was announced at the world conservation union in Buenos Aires, where ministers from 180 countries, including Britain, are trying to take forward last year's Kyoto Treaty on climate change.
The corals have been killed by bleaching - a process in which they turn white when high temperatures drive out the microscopic algae that provide food for the coral animals, and give them their colour.
Corals can survive only short periods of bleaching and in many areas of the world it had proved fatal, Dr Goreau said. Analyses of satellite measurements of sea-surface temperatures had shown that corals begin to die if temperatures rise just one degree celsius above normal for two months, or two degrees for one month, and this had been exceeded in huge areas of the world this year. The Indian Ocean was the worst hit, with most of its corals dying.
"We have lost reefs before, but never a whole ocean," Dr McAllister said.
The Red Sea and the Gulf had suffered badly, as had all of South-east Asia from Vietnam down through the Philippines to Indonesia, although Australia's Great Barrier Reef had escaped the worst effects.
Northern hemisphere coral reefs such as those in the Caribbean are being damaged at the moment. "Coral reefs are the most sensitive ecosystem of all to temperature increase. They are like the canary in the mine," Dr Goreau said. "They are the first ecosystem that will go and right now they can't take any - any - more warming."
This year is already certain to be the hottest in the official 150-year global temperature record. But British scientists, who, as revealed in The Independent two weeks ago, have reconstructed the temperature peaks of the past millennium, believe it will be the hottest for 1,000 years.
The US yesterday signed the Kyoto protocol. But the treaty will only become binding whenAmerica ratifies it in the Senate, which will not be before 2001, and not until the US sees action on greenhouse gas emissions from poorer nations as well as the industrialised world.
Leading article, Review, page 3
A World of Riches Beneath the Waves
Corals grow in the Indian Ocean, the western Pacific around the Philippines and Indonesia and the eastern Pacific around the Galpagos Islands
They form the sea's richest ecosystem, holding 25 per cent of the world's marine fish species in less than 0.3 per cent of the ocean's area. They are thought to provide more than 10 per cent of the world's annual fish catch of 89 million tonnes.
From 0.3 per cent of the ocean's surface they officially provide six million tonnes of the world's annual 89-million-tonne fish catch, and the true total could be over 10 million tonnes.
Corals are 80 times richer in fish than the rest of the sea and the total number of known species of fish, animals, plants and micro organisms they contain is 93,000. There may be a million species unknown to science.
The reefs give employment to between nine and ten million small-scale fishermen, but are also important for the tourist industry of more than 100 countries, estimated to be worth $500bn.
The main causes of death are sewage dumped in the sea, sediment caused by deforestation, or overheating. Corals "bleach" under heat stress - they turn white as the microscopic algae that provide their food and pink colouring are expelled. They are alive but starving. If the bleaching continues for any length of time, they die.
In many places, 90 per cent of corals have been killed by temperatures of up to 2.4C hotter than usual.