As the ocean chemistry changes with global warming, underwater noise is likely to become less muted.
Climate change sounds bad to a lot of people. It might sound even worse to underwater animals.
As ocean chemistry changes with global warming, a new study found, underwater noises could travel more than 70 percent further in the next century. The effect will be most extreme at high latitudes and in deep channels.
No one knows how the combination of more noise and farther-reaching noise will affect animals that live -- and listen -- under the sea.
"The ocean is becoming more transparent to sound," said Tatiana Ilyina, an oceanographer at the University of Hawaii in Manoa. "In other words, it's becoming louder."
As levels of carbon dioxide rise in the atmosphere, ocean waters are becoming more acidic. Besides decreased calcification in corals and other problems, ocean acidification causes changes in concentrations of salts in seawater. Some of those molecules, including boron ions, normally absorb sounds, particularly low-frequency sounds.
With acidification, numbers of boron ions drop, and low-frequency sounds travel farther.
Scientists have already noted a change in both ocean pH and in the way sound travels underwater. Ilyina and colleagues used climate models to predict what the future holds.
If we carry on with current levels of greenhouse gas emissions, they reported in the journal Nature Geoscience, pH would drop by 0.6 units, leading to a 60 percent decrease in sound absorption. In the worst-case scenario, sound absorption would drop by more than 70 percent by the year 2100.
The trend is limited to sounds with frequencies below about 1 kHz, with the largest effects at frequencies of about a few hundred Hz. Ships emit sounds in that range. So do at least 40 species of marine animals, including many dolphins, seals and whales.
The researchers pinpointed six acoustic hotspots, where heavy shipping traffic converges with large populations of animal life and a great likelihood of decreased sound-absorption. Hotspots included the North Atlantic, the North Pacific around Alaska, and the area around Japan. Deep channels of water at about 1,000 meters will also be affected.
It's worth watching those regions closely, Ilyina said, because no one knows exactly what's going to happen there. It's possible that chemical changes could help marine mammals, by allowing them to hear each other from farther away. Alternatively, the extra noise could disturb animals and change behaviors in unexpected ways.
Humpbacks and beluga whales have been known to flee from ships. Low-frequency sonar systems have been linked with hearing loss, tissue damage and mass strandings among marine mammals. The number of seafaring ships has doubled in the last 40 years.
"It is difficult to imagine a scenario where increasing low frequency background noise could be good for marine mammals," said Mark McDonald, an independent ocean acoustician in Bellvue, Colo.
"That said, it may be no worse than the impact on a human working in an office with a noisy air conditioner," he added. "We really don't know how bad or how minimal the impact may be."