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Good News—and Bad—for Coral Reefs

Reports show that the ocean's unique ecosystems are adapting to fluctuations in water temperatures likely caused by global warming, but increasing acidic levels may prove fatal for the world's coral reefs.
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A school of fish pass over a coral reef at Hanauma Bay on January 15th, 2005, in Honolulu, Hawaii.

A school of fish pass over a coral reef at Hanauma Bay on January 15th, 2005, in Honolulu, Hawaii.

One of the most troubling warning signs of environmental degradation in general, and climate change in particular, is the gradual destruction of the world's coral reefs. A 2004 World Wildlife Fund report found that 20 percent of the world's coral reefs have been effectively destroyed, 24 percent are at imminent threat of collapse, and 26 percent face a long-term threat of destruction. Coral bleaching—which essentially turns these icons of biodiversity into lifeless skeletons—has been linked to elevated sea-surface temperatures, a likely result of global warming.

Two new studies offer hope for these unique ecosystems, while a third warns of the "very real possibility" they will not survive the century.

According to a report in the journal Ecological Monographs, coral reefs in areas where the temperature varies significantly with the season are more likely to survive as the ocean warms. In essence, they have already adapted to fluctuations in temperature, so as the water heats up, they are better able to cope than their counterparts who are accustomed to stable year-round temperatures.

These findings, the result of an eight-year study of reefs off the coast of East Africa, offer "a ray of hope" for the reefs, according to Tim McClanahan, senior scientist working for Wildlife Conservation Society's Coral Reefs Program and lead author of the study. They also have practical ramifications. Now that we know which coral reefs have the best chance of survival, we should make conserving them "our highest priority," he said in a release.

Meanwhile, a new study conducted by scientists from Queen's University in Ontario, Canada, and the Australian Institute for Marine Science suggests that coral reefs may have the ability to develop greater resistance to bleaching. The report in the journal The American Naturalist looks at the complex relationship between algae and their coral hosts and concludes "there is no necessary impediment to them evolving greater bleaching resistance," said Troy Day of Queen's University, a mathematical biologist specializing in evolutionary ecology, in a release. "However, predicting the precise rates at which this may occur, or whether the rate will be sufficient to outpace projected sea temperature rise as a result of climate change, will require much more information."

But warming water temperatures is only part of the problem, according to chemical oceanographers Ken Caldeira and Lon Cao of the Carnegie Institution for Science. In a paper published in the December 14th issue of Science magazine, the pair, along with 17 colleagues, warn that the increasing acidity of the oceans—which, like global warming, is a byproduct of increased carbon dioxide in the atmosphere—could be fatal to coral reefs.

"About one-third of the carbon dioxide put into the atmosphere is absorbed by the oceans," Caldeira notes. While this helps slow global warming, it also means the acidity of the oceans is increasing dramatically, since the absorbed carbon dioxide produces carbonic acid (the same acid that gives soft drinks their fizz).

Aragonite, the mineral used by corals (and many other marine organisms) to grow their skeletons, dissolves more readily as the water grows more acidic. If current trends continue, 98 percent of present-day reef habitats will be bathed in water too acidic to permit reef growth by the middle of the century, according to Calderia and Cao.

"These changes come at a time when reefs are already stressed by climate change, overfishing, and other sorts of pollution," Caldeira says. "So unless we take action soon, there is a very real possibility that coral reefs—and everything that depends on them—will not survive the century."