"The geologic record tells us the chemical effects of ocean acidification would last tens of thousands of years," said Ken Caldeira, of the Carnegie Institution in Washington, D.C. "But biological recovery could take millions of years. Ocean acidification has the potential to cause extinction of many marine species."

Caldeira presented his findings at the American Geophysical Union/American Society of Limnology and Oceanography meeting on Monday.

He explained that when CO2 from the burning of coal, oil and natural gas dissolves in the ocean, some of it becomes carbonic acid, which over time makes ocean water more acidic. When carbonic acid input is modest, sediments from the ocean floor can buffer the increases in acidity.

At the current rate of input, however - nearly 50 times the natural background from volcanoes and other sources - it overwhelms the buffering mechanism, Caldeira said, adding that previous estimates suggest ocean pH values could drop by as much as half a unit in less than 100 years, from the natural level of 8.2 to about 7.7. On the pH scale, lower numbers mean a more acidic liquid, while higher numbers are more alkaline.

A significant drop in ocean pH would be particularly damaging to the corals, which use calcium carbonate to make their shells. Under normal conditions, ocean water is supersaturated with this mineral, making it easy for such creatures to grow, but a more acidic ocean would more easily dissolve calcium carbonate, putting the coral and related species at particular risk.

Caldeira said the last time the oceans endured such a drastic change in chemistry was 65 million years ago, at about the same time the dinosaurs went extinct. Though researchers do not yet know exactly what caused the ancient acidification, they do know it was connected to the cataclysm that wiped out the dinosaurs.

In addition, he said, the overall pattern of extinction in the ocean is consistent with ocean acidification. The fossil record reveals a precipitous drop in the number of species with calcium carbonate shells that live in the upper ocean - particularly corals and plankton. During the same period, species with shells made from resistant silicate minerals were more likely to survive.

"Ultimately, if we are not careful, our energy system could make the oceans corrosive to coral reefs and many other marine organisms," Caldeira said. "These results should help motivate the search for new energy sources, such as wind and solar, that can fuel economic growth without releasing dangerous carbon dioxide into the environment."

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