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Oceans to get noisier as they become more acidic

30.09.2008
As mounting levels of human-generated carbon dioxide make the oceans warmer and more acidic, the seas will transform in yet another, unexpected way -- sounds will travel farther underwater, a new study concludes. A corresponding increase in background noise in the oceans could affect the behavior of marine mammals, the researchers say.

Conservative projections by the United Nations Intergovernmental Panel on Climate Change (IPCC) suggest that the chemistry of seawater could change by 0.3 pH units by 2050. If so, this intensification of ocean acidity would allow sounds to travel up to 70 percent farther underwater than in today's oceans.

The projected impact on ocean sound emerges from calculations by Keith Hester and his colleagues at the Monterey Bay Aquarium Research Institute (MBARI) in Moss Landing, Calif. The researchers will publish their findings on Wednesday, 1 October 2008, in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

Ocean chemists have known for decades that the absorption of sound in seawater changes with the chemistry of the water itself. As sound moves through seawater, it causes groups of atoms to vibrate, absorbing sounds at specific frequencies. This involves a variety of chemical interactions that are not completely understood. However, the overall effect is strongly controlled by the acidity of the seawater.

The bottom line is this: the more acidic the seawater, the less low- and mid-frequency sound it absorbs.

Thus, as the oceans become more acidic, sounds will travel farther underwater so the level of underwater sound will rise. 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 of sounds 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.

The MBARI researchers say that sound already may be traveling 10 percent farther in the oceans than it did a few hundred years ago.

However, they predict that by 2050, under conservative projections of ocean acidification, sounds could travel as much as 70 percent 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.

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

Hester's research shows how human activities are affecting the Earth in far-reaching and unexpected ways. As the researchers put it in their paper, "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, and the effects are being felt not only in biological impacts but also on basic geophysical properties, including ocean acoustics."

This research was supported by grants from the David and Lucile Packard Foundation.

Title:
"A noisier ocean at lower pH"
Authors:
Keith C. Hester, Edward T. Peltzer, William J. Kirkwood and Peter G.
Brewer: Monterey Bay Aquarium Research Institute, Moss Landing, California, USA.
Citation:
Hester, K. C., E. T. Peltzer, W. J. Kirkwood, and P. G. Brewer (2008), Unanticipated consequences of ocean acidification: A noisier ocean at lower pH, Geophys. Res. Lett., 35, L19601, doi:10.1029/2008GL034913.
Contact information for coauthors:
Keith Hester, Postdoctoral Fellow, phone: +1 (831) 775-2072, email:
khester@mbari.org

Peter Brewer, Senior Scientist, phone: +1 (831) 775-1706, email:
brpe@mbari.org

Peter Weiss | American Geophysical Union
Further information:
http://www.agu.org

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