Noisy oceans a surprising result of rising CO2: scientists

An unanticipated consequence of rising atmospheric carbon dioxide has some marine chemists worried about how ocean creatures will manage in an increasingly noisy underwater world.

Scientists with California’s Monterey Bay Aquarium Research Institute said Tuesday they expect sound to travel farther underwater as carbon dioxide from the air dissolves into the water and the oceans become less alkaline.



In a paper to be published in Wednesday’s Geophysical Research Letters, they predict changes in ocean chemistry will have the greatest effect on sounds below about 3,000 cycles per second (2½ octaves above middle C on a piano).

While sounds used by marine mammals in finding food and mates fall into this range, so too do many of the underwater sounds generated by industry and the military, as well as by boats and ships. Couple that with dramatic increases in human-generated underwater noise over the last 50 years, and it adds up to a substantial increase in volume.

Ocean chemists have known for decades that the way seawater absorbs sound is affected by the chemistry of the water. As sound moves through seawater, it causes groups of atoms to vibrate, absorbing sounds at specific frequencies.

While scientists say they don’t completely understand the process, the overall effect is strongly controlled by the acidity of the seawater. The bottom line is the lower the pH of the seawater, the less low- and mid-frequency sound it absorbs.

The aquarium’s researchers say that sound already may be travelling 10 per cent farther in the oceans than it did a few hundred years ago.

They predict that by 2050, under conservative projections of ocean acidification, sounds could travel as much as 70 per cent farther in some ocean areas, particularly in the Atlantic Ocean.

CBC News, 30 September 2008. Article.

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