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Before the industrial revolution, says Ken Caldeira of Stanford University, 98% of all corals lived in waters above the required calcium carbonate threshold. But the situation is changing, according to Caldeira, who has built a model to study how greenhouse gas emissions tinker with the chemistry of open water oceans.
Oceans absorb carbon dioxide from the atmosphere, which reacts with water to make carbonic acid which lowers the ocean's pH. As the pH drops, dissolved calcium carbonate decreases - impairing the corals' ability to rebuild themselves.
The United Nations' most optimistic plans to curb carbon dioxide emissions hope to stabilize the gas at a concentration of 450 parts per million by the year 2100. This scenario, which many economists consider to be too costly to achieve, would still lower the pH of the oceans by as much as 0.2, according to Caldeira's model. This would leave only 8 to 34% of the world's corals in waters containing healthy amounts of calcium carbonate.
More conservative goals to level carbon dioxide at 550ppm by 2150 would place half the southern oceans' coral in fatally under-saturated waters that could not support reef building even in protected laboratory settings. These reefs are likely to slowly dissolve away. Even at today’s level of 380 ppm, nearly half of the world's coral population lives in perilous waters low on calcium carbonate. One major storm or catastrophe could pose a serious threat to any one of these reefs, which may not have the necessary supply of minerals to repopulate. "We've already seen this in the Galapagos, where a single bleaching event wiped out a coral reef that had been around for thousands of years," Caldeira told New Scientist.
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http://environment.newscientist.com/article/dn14676-climate-change-could-stop-corals-fixing-themselves.html
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