By Ian Sample on Guardian.co.uk
The world's coral reefs will begin to disintegrate before the end of the century as rising carbon dioxide levels in the atmosphere make the oceans more acidic, scientists warn.
The research points to a looming transition in the health of coral ecosystems during which the ability of reefs to grow is overwhelmed by the rate at which they are dissolving.
More than 9,000 coral reefs around the world are predicted to disintegrate when atmospheric carbon dioxide levels reach 560 parts per million.
The amount of carbon dioxide in the atmosphere today stands at around 388ppm, but is expected to reach 560ppm by the end of this century.
Coral reefs are at the heart of some of the most biodiverse marine ecosystems in the world. They are home to more than 4,000 species of fish and provide spawning, refuge and feeding areas for marine life such as crabs, starfish and sea turtles.
"These ecosystems which harbour the highest diversity of marine life in the oceans may be severely reduced within less than 100 years," said Dr Jacob Silverman of the Carnegie Institution in Stanford University, California.
Coral reefs grow their structural skeletons by depositing aragonite, a form of calcium carbonate, from calcium ions in sea water. As oceans absorb atmospheric carbon dioxide, they become so acidic the calcium carbonate dissolves.
Silverman's team studied a coral reef in the northern Red Sea and calculated its response to increasingly acidic waters. The research showed that the ability of the coral to build new structures depended strongly on water acidity and to a lesser extent temperature.
From these data the researchers created a global map of more than 9,000 coral reefs, which showed that all are threatened with disintegration when carbon dioxide levels in the atmosphere reach 560ppm. Silverman was speaking at the American Association for the Advancement of Science meeting in San Diego.
In a separate study, Simon Donner, an environmental scientist at the University of British Columbia in Canada, warned that carbon dioxide in the atmosphere is already at a high enough level to cause devastating coral bleaching.
Corals have a symbiotic relationship with microscopic algae that live on them. The algae give coral reefs their vibrant colours, but are also an important food source for the habitat's marine life. When sea temperatures rise, the corals expel the algae and turn white. Once this happens the coral is deprived of energy and dies.
"Even if we froze emissions today, the planet still has some warming left in it. That's enough to make bleaching dangerously frequent in reefs worldwide," said Donner.
Bleaching had become increasingly widespread in recent years, Donner said. In 2006, severe bleaching struck the southern part of Australia's Great Barrier Reef, the largest coral reef system in the world. Last year scientists reported that a "lucky combination" of circumstances had allowed the coral to recover from the disaster.