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 charged molecules in seawater, and that these are affected by pH.
A lower, more acidic pH changes the structure of the molecules 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.
Already, they note, manmade carbon dioxide absorbed by the ocean has decreased sound absorption by 15 percent in parts of the north Atlantic, and commonly by 10 percent throughout the Atlantic and Pacific.
These values are probably underestimates, they note, because they only include CO2 absorbed from the atmosphere, and not additional sources of acidification.
Richard Zeebe of the University of Hawaii in Honolulu has studied both ocean acidification and seawater acoustics. "It slipped me to combine the two," he said. "That's the credit Peter Brewer should get."
Since seeing the new findings, Zeebe has been working with Brewer to create global maps of ocean sound transmission under future ocean acidification scenarios. "We can find where we're going to see the biggest changes," Zeebe said.
"Because of this noisier ambient level, marine mammals may have trouble communicating," he said. "At the same time, their sound may travel further in the ocean. We have to do a lot more work."
The most commonly discussed concern over ocean acidification has been that a more acidic ocean will harm organisms like corals and small diatoms that form calcium carbonate skeletons, which will dissolve at a lower pH. Other effects on sea life have been predicted, too, but "this is a consequence that is, for may people, very unexpected," Zeebe said.
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