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Acid ocean = noisy ocean
POSTED 13 NOVEMBER 2008
sonar sound button
(sonar noise)

Sonar, ships and geologic exploration are making more noise in the ocean. Sound travels thousands of times farther than light in water, and many marine animals rely on sound for navigating, hunting, mating and keeping the family together.

coast guard boat underwater noise button
(underwater outboard noise)
Change in the movement of sound could cause havoc among animals that have evolved for millions of years in a stable sound environment, and so we were disturbed to read that ocean noise is likely to intensify over the next few decades. The cause? A rise in acidity, due to greater absorption of the greenhouse gas carbon dioxide.

A study published last month suggested that sound will travel 70 percent further by year 2050.

Scientists are already worried that ocean acidification will eat away the shells of plankton and shellfish, but no comprehensive studies have documented any changes in sound transmission through the ocean. Instead, Keith Hester, a post-doctoral fellow at the Monterey Bay Aquarium Research Institute, rested his forecast on the basic physics of sound in salt water.

Three researchers investigate a beached whale's body
Photo: NOAA
The use of naval sonar is associated with increases in the number of beached whales in the sonar's range. Researchers theorize that in the panic ensuing from the painful noises, dolphins and whales abandon the deep ocean and sometimes run aground in shallow areas. A 70 percent increase in the range that sounds travel in the ocean due to increased acidity would vastly increase the size of these sonic danger zones, as well as the overlap of the zones with marine mammals' feeding areas and migratory routes.

Noise goes to work

The salt molecules in ocean water break into ions and float around in little groups, says Hester. If a sound wave encounters magnesium sulfate, for example, it can cause a hydroxide ion to "pop out" and then jump back into place, removing a bit of energy from the sound wave.

Raising the acidity, Hester says, alters the distribution of ions and molecules, allowing the sound waves to retain more energy, and thus travel further.

The ocean remains somewhat alkaline, with a pH of 8.1 (neutral pH is 7.0). However, the Intergovernmental Panel on Climate Change predicts that its pH will decline by 0.3 units from its pre-industrial level of pH 8.2 by 2050. Each hour, the IPCC estimates, the ocean absorbs 1 million tons of carbon dioxide.

White smoke billows from two smoke stacks in winter scene
Generating plant in Sheboygan, Wis.. © David Tenenbaum
Fossil fuels make greenhouse gases that warm the Earth. When carbon dioxide dissolves in the ocean, the water becomes more acidic.

The growing acidity is not just a result of carbon dioxide, however. Hester and Peter Brewer, his advisor at Monterey, say other changes will likely accelerate acidification. First, more organic matter will decompose in the acidic water, releasing another dollop of carbon dioxide. Second, ships, power plants and vehicles that burn fossil fuel also release compounds of nitrogen and sulfur, which can cause acid rain to fall in the ocean.

"All three effects will change the ocean and make it more acidic," Hester says.

Two multi-colored graphs with depth and latitude axis
Courtesy Keith Hester
Calculated percent change in sound absorption at 440 hertz (concert A), from the change in acidity due to fossil fuel in the Pacific (top) and Atlantic oceans (bottom). The 1990s are compared to pre-industrial conditions.

So what?

Ocean acidification may seem like a sideshow, next to the planetary effects of greenhouse gases and global warming -- until you remember that the oceans cover about two-thirds of the Earth's surface. More noise may make life harder for marine mammals.

Many environmentalists maintain that intense blasts of sonar, used to detect submarines, disorient, injure, and even kill marine mammals. As evidence, they point to dying whales found on beaches after sonar tests, sometimes with blood oozing from their ears.

icture of two whale's echolocation range in different PH
Courtesy © 2008 MBARI; (base image courtesy David Fierstein).
Increasing carbon dioxide in the atmosphere leads to an increase in the acidity of seawater, which in turn allows sounds (such as whale calls or ship noise) to travel farther.

If sound waves are traveling further in the ocean, the damage from loud noise will spread further, giving whales and dolphins less place to hide.

The issue is heating up. In August, 2008, the U.S. Navy and environmental groups agreed to some restrictions on the use of sonar. However, on Nov. 12, the U.S. Supreme Court ruled in favor of the Navy in a second legal dispute about military sonar. "Forcing the Navy to deploy an inadequately trained antisubmarine force jeopardizes the safety of the fleet,” wrote Chief Justice John Roberts.

humpback whale noise button
(humpback noise)

Hester, who refused to take a position on the issue of marine mammals, nonetheless says "we are having a really significant impact on the world oceans. It will be of interest to the military [which is concerned with hiding friendly submarines and detecting enemy ones], and to biologists, to see what effect it is going to have on marine mammals, if any... 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."

- David Tenenbaum

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Bibliography

• Unanticipated consequences of ocean acidification: A noisier ocean at lower pH, Keith Hester et al, Geophysical Research Letters, Vol. 35, L19601, doi:10.1029/2008GL034913, 2008.


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