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Until bleaching reared its head, many experts viewed rising sea levels as the chief peril of global warming for coral–and a relatively toothless one at that. “We thought reefs would respond by just growing higher,” says Strong. “Nobody was talking about changing sea chemistry.” Then researchers came to the creeping realization that rising ocean acidity is likely to throw a spanner in coral physiology.
The threat is glaringly simple. Currently, ocean pH hovers around 8.1. Carbon dioxide absorbed into the water column lowers the pH, and as it falls, fewer carbonate ions are available for shell-building critters to grab. Even in present conditions, corals are fighting an uphill battle: Erosion removes 80% of the calcium carbonate laid down. Acidification will accelerate that process as rising carbonic acid levels deplete carbonate. Eventually, corals, plankton, and other organisms will fail to form skeletons. And coral skeletons are to reefs what girders are to skyscrapers. “You have a potential world in which reefs and the limestone frameworks they have built are in net erosion,” says Hoegh-Guldberg.
IPCC scenarios of global emissions and ocean circulation indicate that by midcentury, atmospheric CO2 levels could reach more than 500 parts per million, and near the end of the century they could be above 800 ppm. The latter figure would decrease surface water pH by roughly 0.4 units, slashing carbonate ion concentration by half, paleocoral expert C. Mark Eakin, coordinator of NOAA’s Coral Reef Watch, testified last month at a hearing in the U.S. House of Representatives. Ocean pH would be “lower than it has been for more than 20 million years,” he said. And that does not factor in possible acidification from carbon-sequestration schemes now being considered.
Some coral species facing their acid test may become shape shifters to avoid extinction. New findings indicate that corals can survive acidic conditions in a sea anemone-like form and resume skeleton-building when returned to normal marine conditions (Science, 30 March, p. 1811). However, by pH 7.9, says Caldeira, “there would be a good chance reefs would be gone.”
The potential for an acid-induced coral cataclysm has cast a pall on the tight-knit community of reef specialists. “The reality of coral reefs is very dark, and it is very easy for people to judge coral reef scientists as pessimists,” says Mora. “We’re becoming alarmist,” adds Strong–for good reason, he insists. “How are reefs going to handle acidification? It’s not like sewage or runoff, where you may be able to just turn off the spigot.” Queensland’s Pandolfi, however, argues that it’s “too early to make really definitive doom-and-gloom statements.”
No one disputes that urgent action on greenhouse gas emissions is essential. “We could still have vibrant reefs in 50 years time,” Hughes says. But these will not be the reefs we know today. “They will be dominated by a different suite of species,” says Hughes, who notes that the shakedown is already under way.
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Stone R., 2007. A world without coral? Science 316: 678-681. Article.
