The ocean may soon begin to sound more like New York City to dolphins and whales.

As carbon dioxide levels rise and make the world's oceans warmer and more acidic, the increasing amounts of the gas could also make the seas noisier, especially in the soprano range, a new study suggests.

The increased burning of fossils fuels such as coal and oil in the last few decades has injected more and more carbon dioxide into the Earth's atmosphere. The oceans absorb a large chunk of this extra carbon dioxide (about 40 percent).

The more carbon dioxide that water absorbs, the more acidic it becomes; this is why sodas, with their carbon dioxide bubbles, are acidic. This change in seawater chemistry alters the way sound moves through the ocean, allowing it to propagate farther, particularly for sounds two and a half octaves above "middle C," said researcher Keith Hester of the Monterey Bay Aquarium Research Institute in California.

Ocean acidity

Here is a sketch of how it works: The slow absorption of carbon dioxide by the ocean changes the water's pH.

The pH of a liquid is gauged on a scale of about -1 to 14, where 7 is neutral. Anything above that is basic (for example, household bleach has a pH of about 13), anything below is acidic (lemon juice has a pH of about 2). The standard pH of ocean water is about 8.3, or slightly basic/alkaline.

The Intergovernmental Panel on Climate Change (IPCC) estimates that the pH of ocean waters has decreased by about 0.02 units per decade over the last 20 years. The most conservative projections of the IPCC suggest that the pH of seawater could drop by a total of 0.3 units by 2050.

Not only does this increasing acidity threaten the ocean food chain by hampering the formation of shells and corals, it could also affect the communication of marine mammals by changing the way sound travels through the seawater.

Essentially, the more acidic seawater gets, the farther sound travels in it.

Sending sound farther

While the interactions causing this phenomenon aren't completely understood, chemists do know that seawater of different pHs absorbs specific frequencies differently. The more acidic that seawater is, the less low- and mid-frequency sound that it absorbs, letting those frequencies travel farther. With sounds traveling farther, the level of noise in the ocean will increase, Hester and his colleagues found in their study, detailed in the Oct. 1 issue of the journal Geophysical Research Letters.

Hester's calculations predict that the change in chemistry will have the greatest effect on sounds below about 3,000 cycles per second (or about two and a half octaves above "middle C" on the piano).

This range includes most of the frequencies that marine mammals, such as whales, use to communicate to find food and mates, as well as many of the underwater sounds generated by industrial activity and ships. (This human-generated noise has also increased in recent years, with evidence that it is affecting marine mammal communication.)

Hester and his team found that sound may already be traveling 10 percent farther than it did a few hundred years ago because of the carbon dioxide the ocean has already absorbed since the beginning of the industrial revolution. By 2050, they predict that sound could travel as much as 70 percent farther in some ocean areas (particularly the Atlantic Ocean).

The study was supported by grants from the David and Lucile Packard Foundation.