Will More Acidic Oceans Be Noisier?

In 2008, a group of marine chemists raised a red flag: As the ocean becomes more acidic over the next century, they said, noise from ships will be able to travel farther and possibly interfere with whales and other animals that rely on sound to navigate, communicate, and hunt. The press ran with it. Even the reputable magazine Scientific Americanposted a blog entry titled, “Could ocean acidification deafen dolphins?”

But when Tim Duda, who studies sound propagation in the ocean at Woods Hole Oceanographic Institution (WHOI), read the same reports, he thought, “Time out, it’s not that simple!” He and colleagues decided to apply their acoustical know-how to investigate this potential increase in ocean noise.

It’s true that the oceans are becoming more acidic as excess carbon dioxide spewed into the atmosphere by fossil-fuel burning dissolves into the sea. And as improbable as it might seem, there is a connection between ocean chemistry and sound propagation.

Several chemical reactions in the ocean influence and are influenced by sound waves, and at least one of them is also affected by acidity. Here’s how it works: Sound waves exert pressure as they pass through water. Pressure “squeezes” borate ions [B(OH)₄^–] into boric acid [B(OH)₃], a more compact molecule. In the process, the molecules absorb energy from passing sound waves.

“What that essentially means is that the water is squishy,” Duda said. “When you push on it [with a sound wave], it gives.”

It’s akin to bouncing a ball off a pillow. Just as the ball loses energy to the pillow, a sound wave loses energy to the borate-to-boric acid reaction. As a result, the sound won’t travel as far. Of particular concern are low-frequency sound waves, which travel farthest. They are the same frequency waves made by ships and used by some marine mammals.

Ordinarily, the boric acid is quickly changed back to borate, a reaction that does not absorb sound but that replenishes the supply of borate. Acidity comes into play because more acidic conditions tend to reduce the amount of borate in the water. That, in turn will mean less of a “pillow effect.” The chemists calculated that the increase in ocean acidity predicted in the near future could reduce the “pillow effect” by half, which would allow sounds such as shipping noise to travel farther and interfere with marine animals over greater expanses of the ocean.

All very well, said Duda, but that scenario does not take into account how sound actually behaves in the oceans. He got together with WHOI scientists Ilya Udovydchenkov, Scott Doney, and Ivan Lima, and colleagues at the Naval Postgraduate School in Monterey, Calif. They split into three groups. Each group designed a mathematical model of sound propagation in the oceans. Starting with hundreds of hypothetical ships generating noise at three frequencies (100, 500, and 1,000 Hertz) in the northern Pacific Ocean, they tested how far the noise would travel today and under ocean pH levels projected for 2050 and 2100.

Results of the three models varied slightly in their details, but all told the same tale: The maximum increase in noise level due to more acidic seawater was just 2 decibels by the year 2100—a barely perceptible change compared to noise from natural events such as passing storms and big waves.

Duda said the main factor controlling how far sound travels in the seas will be the same in 100 years as it is today: geometry. Most sound waves will hit the ocean bottom and be absorbed by sediments long before they could reach whales thousands of kilometers away.

The three teams published their results in three papers in the September 2010 issue of the Journal of the Acoustical Society of America.

“We did these studies because of the misinformation going around,” said Duda. “Some papers implied, ‘Oh my gosh, the sound absorption will be cut in half, therefore the sound energy will double, and the ocean will be really noisy.’ Well, no, it doesn’t work that way.”

This research was supported by the Office of Naval Research.