Ocean Acidification Turns Up the Volume

Sept. 30, 2008 -- Beyond the damage it is expected to inflict upon sensitive marine ecosystems, ocean acidification may also allow sound to travel farther underwater, creating a louder deep sea din.

The change may have implications for marine mammal communication, and for military, commercial and scientific applications.

Peter Brewer and colleagues at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., discuss the phenomenon in Geophysical Research Letters this week.

"There's no doubt about the effect we describe. The physics of both ocean acidification and sound absorption is impeccable," Brewer said. "When I went through the calculation, I realized the effect was probably larger than most people thought."

Researchers have known for years that sound waves in the range of human hearing are absorbed by are absorbed by certain groups of charged molecules that stick together in seawater, and that these are affected by pH.

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A lower, more acidic pH changes the structure of the molecule clusters so that the sound absorption property is lost.

The Intergovernmental Panel on Climate Change has predicted that by mid-century, the surface ocean water pH could be 0.3 units lower than pre-industrial levels thanks to carbon dioxide emissions. Under this scenario, Brewer and colleagues calculate that sound would travel up to 70 percent further.