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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 new research.
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.
The study, published in the latest issue of the journal Science, should serve as a warning to delegates to a UN climate conference in Bali, Indonesia, this week, the researchers said.
"We need rapid reductions in carbon dioxide levels," said Ove Hoegh-Guldberg, a marine science professor at 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.
They produce $476 billion a year in economic value worldwide, according to The Nature Conservancy environmental group, and are considered a storehouse of potential 21st century medicines for cancer and other diseases.
The polyps secrete calcium carbonate to build the stony base of the reef.
Corals grow slowly, as little as one centimetre per year and the fragile structures they create are easily damaged by ship groundings, storms and other threats.
The researchers, who based their work on computer simulations of ocean chemistry, said about one-third of carbon dioxide, or CO2, put into the atmosphere is absorbed by the ocean, slowing global warming but polluting the sea.
The CO2 produces carbonic acid, the substance that gives soft drinks their fizz. The acid reduces concentrations of carbonate-ions, which are critical to reef building.
Current levels of CO2 in the atmosphere are 380 parts per million, researchers said, but rising quickly as humans increase their emissions by burning fossil fuels.
If trends hold, the concentration could rise to 880 ppm by 2100. But even if atmospheric CO2 stabilised at 550 ppm, which would take a concerted international effort, no existing coral reef could survive, the researchers said.
"We have the world at stake here. It's a global emergency," said Hoegh-Guldberg. "We've got to have (CO2) levels falling by 2015."
Australian and Caribbean reefs are at the greatest risk because they already have lower carbonate-ion concentrations and therefore would "reach critical levels sooner," he said.
The research should serve as a warning to those who look after reefs to ramp up the fight against other threats to them, which include overfishing, pollution from nearby land and a host of diseases, the researchers said.
"We need to think of this as the straw that broke the camel's back," said Peter Sale of the United Nations University.
© 2007 Reuters