More carbon dioxide may create a racket in the seas

Here is another consequence of rising carbon dioxide emissions: the oceans are getting louder.

It has long been known that chemical compounds in seawater, including boric acid, absorb sound, as energy from sound waves stimulates certain reactions. As the oceans grow more acidic, a result of increasing absorption of atmospheric CO2, the seawater chemistry changes, resulting in fewer reactions and less acoustic energy used. That means sounds will travel farther and be louder at a given distance from a sound source.

Tatiana Ilyina and Richard E. Zeebe of the University of Hawaii and Peter G. Brewer of the Monterey Bay Aquarium Research Institute looked at the future impact of this phenomenon. Using a global ocean model and projections of CO2 emissions, they predicted regional changes in acidity, and thus sound absorption.

Writing in Nature Geoscience, they report that in high latitudes and deepwater formations (where acidification is expected to be worse), sound absorption could fall 60 percent by 2100.

So the oceans will not be as quiet — what’s wrong with that? Plenty, potentially.

Most of the chemical absorption of sound occurs at relatively low frequencies, from about 1,000 to 5,000 hertz. Propeller noise and other ship sounds fall in the same range, as does some military and research sonar. So this “background” noise, especially prevalent near shipping lanes, will be louder. That may be bad news for marine mammals, which use sounds in the same range for communication and echolocation while foraging.

“We’re not saying that during the next 100 years all dolphins will be deafened,” Dr. Zeebe said. “But the background noise could essentially override or mask the sounds that they’re depending on.”

Then again, he said, because sounds will travel farther, the animals may be able to communicate over longer distances. The researchers are continuing their studies using more sophisticated models and more precise sound sources.

Henry Fountain, The New York Times, 28 December 2009. Article (subscription required).

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