As more and more carbon dioxide enters the Earth's atmosphere, oceans are becoming more acidic. In fact, this acidification has been blamed for everything from killing off coral to aiding algae and even super-sizing fish ear bones. But the changing marine chemistry may also be altering the deep sea's acoustic environment, according to a new paper, published online Sunday in Nature Geoscience reports (Scientific American is part of Nature Publishing Group), making it much noisier for the animals that depend on sound to navigate the watery depths. 

"At present, the most debated negative effect of ocean acidification is reduced calcification rates," the researchers, led by Tatiana Ilyina of the School of Ocean and Earth Science and Technology at the University of Hawaii in Honolulu, wrote. "However, a less anticipated consequence of ocean acidification is its effect on underwater sound absorption."

As ocean water becomes more acidic—thanks in large part to human-generated carbon dioxide—concentrations of sound-absorbing chemicals (such as magnesium sulphate and boric acid) drop, which means noise, especially low-frequency rumbles (up to 5,000 hertz), travels farther.

Using models of carbon dioxide output and the world's oceans, the researchers found that sound absorption could fall by some 60 percent in high latitudes and deep waters within the next three centuries. Add to that more low-frequency noise from human ocean activity, such as construction, shipping and sonar, and you get a veritable cacophony for many deep-sea denizens.

"We predict that over the twenty-first century, chemical absorption of sound in this frequency range [100 to 10 hertz] will nearly halve in some regions that experience significant radiated noise from industrial activity," the authors conclude. 

Some low-frequency din is caused naturally by waves and rain on the ocean's surface—as well as by animals themselves. But, the authors noted, "high levels of low-frequency sound have a number of behavioral and biological effects on marine life, including tissue damage, mass stranding of cetaceans and temporary loss of hearing in dolphins."

Certainly, increased sound travel could actually aid some animals' aural acuity, sending whale communication farther than it travels now. There is evidence that, "marine species have adapted to varying levels of noise," the authors noted, but "the consequences of increasing long-range sound transmission in the frequencies that are important for many marine mammals are unknown."

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