The world's oceans are turning acidic due to the buildup of carbon dioxide (CO2) in the atmosphere, and scientists say the effects on marine life will be catastrophic.
In the next 50 to 100 years corrosive seawater will dissolve the shells of tiny marine snails and reduce coral reefs to rubble, the researchers say.
Four leading marine experts delivered this grim prognosis yesterday at the annual meeting of the American Association for the Advancement of Science in San Francisco, California.
The scientists stressed that increased ocean acidity is one of the gravest dangers posed by the buildup of atmospheric CO2.
"Ocean chemistry is changing to a state that has not occurred for hundreds of thousands of years," said Richard Feely of Seattle's Pacific Marine Environmental Laboratory.
"Shell-building by marine organisms will slow down or stop. Reef-building will decrease or reverse."
The organisms most directly affected are those that build hard shells or other mineral structures of calcium carbonate. These include numerous species of corals, marine snails, and crust-building algae.
As oceans absorb CO2 from the air, the gas reacts with water to produce carbonic acid. The acid in turn consumes the carbonate that sea creatures need to build their shells.
"This is a problem that no living corals have encountered in their past evolutionary history," said Charlie Veron, of the Australian Institute of Marine Science.
Over time, coral reefs have been able to adjust to changes in ocean temperature and sea level, Veron said. But acidification appears to be a problem that the ancestors of today's corals were unable to solve.
Mass extinctions of marine life in the distant past, he said, were probably caused by chemical changes similar to those happening today.
"It took coral reefs about four to ten million years to recover each time," he added.
In shallow waters where most corals presently grow, carbonate is not in short supply. But at greater depths carbonate concentration decreases until it reaches a point beyond which shell- and reef-production is no longer possible.
That critical threshold is rising closer to the surface as oceans grow increasingly acidic, limiting the depths at which corals and other organisms can live.
Feely said that this limit has already risen several hundred meters, particularly in the Indian and Pacific oceans. Recent measurements in the Gulf of Alaska found that carbonate was in short supply less than 325 feet (100 meters) below the surface.
James Orr, of the International Atomic Energy Agency's Marine Environmental Laboratory in Monaco, said that by the end of this century, shell- and reef-building sea creatures will be unable to live at any depth across a huge area of the world's oceans.
"Two-thirds of cold-water corals will be exposed to corrosive waters by 2100," Orr said.
Already, Feely said, ocean acidity has increased about 30 percent since industrialization began spurring harmful carbon emissions centuries ago. Unless emissions are reduced from current levels, an increase of 150 percent is predicted by 2100.
Such an increase would make the oceans more acidic than they've been at any time in the last 20 million years, he added.
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