The Island of Doubt

To the growing list of consequences of global warming add underwater noise pollution, which may make life difficult for the whales and dolphins who are already facing increased background noises from shipping. It may sound like a stretch, but it's actually pretty straightforward science.

The connection is laid out in a new paper published Wednesday in Geophysical Research Letters. "Unanticipated consequences of ocean acidification: A noisier ocean at lower pH" by Keith C. Hester and colleagues at the Monterey Bay Aquarium start off by describing the now well-established trend of declining pH levels in the world's oceans, and fact that sound absorption rates in the ocean are determined in part by the chemistry of the water. So we know that anthropogenic noise will grow as we the oceans continue to get more acidic.

Figuring out just how far a sound will travel before it attenuates to nearly nothing is a relatively simple matter. Underwater sounds first spread, and therefore diminish in intensity, spherically and then in two dimensions, like a flat cylinder. Added to that little formula is a factor known as α (alpha), which describes the absorption of sound according to what's in the water, including pH levels.

There's nothing that we or any other mortal can do about the attenuation due to spreading — that's simply a mathematical certainty. But absorption due to chemistry is another matter. And that little α can have quite an effect, depending on the frequency. At high frequencies, above 10 kHz, the effect of α is insignificant. Down in the hundreds of hertz range, though it can be dramatic. And that just happens to be the range most marine mammals use.

Hester et al write that

A very realistic pH change of −0.3 accompanied with warming will lead to sound absorption below 1 kHz being reduced by almost half of current values.

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).

It all depends on the frequency. Hester et al write that anthropogenic noise pollution "is largely due to shipping from 0.01 to 1.0 kHz. However, other sources such as military and industrial activities create sound waves across the entire low-frequency spectra." But it's clear that not every part of the spectra will be plagued by the same degree of noise. My guess is the ultra-low frequencies around 20 Hz used by blue whales (Balaenoptera musculus (photo at right) and fin whales (B. physalus) are probably not as noisy as the higher frequencies typically associated with beaked whales and dolphins.

If that's so, then, in theory, the biggest whales should actually benefit from reduced sound absorption, as their signals will go further before fading, without much in the way of background noise to overcome. The news may not be so good for the smaller mammals, the effect on which are harder to predict.

I called up Hester and asked him for this thoughts on this line of thinking. He explained that he and his co-authors are geochemists, not marine biologists, and it's up to marine mammal experts to take the research to the next step. "It could be a bad thing, it could be a good thing," he said. "We just don't know."

On the other hand, as Hester pointed out, "It's a matter of large physical changes in the oceans," and that almost certainly won't be a good thing. For one thing, falling pH levels are going to be catastrophic for many of the smallest species in the ocean, including zooplankton who will find it impossible to build shells in acidic oceans, the corals that are extremely vulnerable to any change in pH. And if the little guys at the base of the food web go, so goes entire ocean ecosystems, including the great whales.
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Keith C. Hester, Edward T. Peltzer, William J. Kirkwood, Peter G. Brewer (2008). Unanticipated consequences of ocean acidification: A noisier ocean at lower pH Geophysical Research Letters, 35 (19) DOI: 10.1029/2008GL034913