MIAMI - IN less than 50 years, oceans may be too acidic for coral reefs to grow because of carbon emissions from the burning of fossil fuels by humans, according to research released on Thursday.

And unless still rising carbon dioxide emissions fall in the near future, existing reefs could all be dying by 2100, scientists said.

Australia's Great Barrier Reef, the world's largest coral expanse, and Caribbean reefs will be among the first casualties, according to the scientists who worked on a major coral project worldwide.

'We need rapid reductions in carbon dioxide levels,' said marine science professor Ove Hoegh-Guldberg of Australia's University of Queensland and a lead author of the study.

'The impact of climate change on coral reefs is much closer than we appreciated,' he said in a telephone interview from Australia. 'It's just around the corner.'

The study found emissions of carbon dioxide, the main 'greenhouse' gas contributing to global warming, are boosting acidity so much that sea water covering 98 per cent of all coral reefs may be too acidic by 2050 for some corals to live, and while others may survive they would be unable to build reefs.

'Unless we take action soon there is a real possibility that coral reefs, and everything that depends on them, will not survive this century,' researcher Ken Caldeira said.

Coral reefs, delicate undersea structures resembling rocky gardens that are made by tiny animals called coral polyps, are important nurseries and shelters for fish and other sea life.

They are also considered valuable protection for coastlines from high seas.

Reefs are a critical source of food for millions of people and are important for tourism from Australia to the islands of the Caribbean and the Florida Keys. -- REUTERS