In a report titled "Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers," scientists said that oceans worldwide absorbed approximately 118 billion metric tons of carbon dioxide between 1800 and 1994.

    Oceans are naturally alkaline, and they are expected to remain so, but the interaction with carbon dioxide is making them more acidic, according to scientists at the U.S. National Science Foundation, National Center for Atmospheric Research, and National Oceanic and Atmospheric Administration.

    The increased acidity lowers the concentration of carbonate ion, a building block of the calcium carbonate that many marine organisms use to grow their skeletons and create coral reef structures.

    Experimental studies have shown that coral calcification consistently decreases as the oceans become more acidic. This means that these organisms will grow more slowly, or their skeletons will become less dense, a process similar to osteoporosis in humans.

    As a result, reef structures are threatened because corals may be unable to build reefs as fast as erosion wears away the reefs. Many calcifying organisms, including marine plankton such as pteropods, are affected by the chemistry changes.

    Shelled pteropods are an important food source for salmon, mackerel, herring, and cod. If calcifying organisms such as pteropods are unable to sustain their populations, many other species may be affected, the scientists said.