AS OCEANS WARM, SOUNDS TRAVEL FARTHER

Ocean chemists have been aware for years that the absorption of sound in seawater changes with the chemistry of the water itself. A new study by researchers at the Monterey Bay Aquarium Research Institute (MBARI) shows that as the oceans become warmer and more acidic in a warmer world, sounds will travel farther under- water, potentially changing the behavior of sea mammals.

The MBARI researchers, led by Keith Hester, say that sound already may be traveling 10% 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% farther in some ocean areas (particularly in the Atlantic Ocean). This could dramatically improve the ability of marine mammals to communicate over long distances. It could also increase the amount of background noise that they have to live with.

"The waters in the upper ocean are now undergoing an extraordinary transition in their fundamental chemical state at a rate not seen on Earth for millions of years," says Hester. "And the effects are being felt not only in biological impacts but also on basic geophysical properties, including ocean acoustics."

Hester explains that as sound moves through seawater, it causes groups of atoms to vibrate, absorbing sounds at specific frequencies. Although the chemical interactions are not completely understood, the overall effect is strongly controlled by the acidity of the seawater. According to Hester's calculations, such a change in chemistry will have the greatest effect on sounds below about 3,000 cycles per second (two-and-one-half octaves above middle C on a piano).

This range includes most of the low-frequency sounds used by marine mammals in finding food and mates. It also includes many of the underwater sounds generated by industrial and military activity, as well as by boats and ships. Such human-generated underwater noise has increased dramatically over the last 50 years, as human activities in the ocean have increased.

There are no long-term records of sound absorption over large ocean areas. However, the researchers cite a study off the coast of California that showed an increase in ocean noise between 1960 and 2000 that was not directly attributable to known factors such as ocean winds or ships.

The research was published in an October issue of Geophysical Research Letters. (SOURCE: American Geophysical Union)