Swimming can be as peaceful as it is fun. Underwater, no one can tell you to do your homework or clean your room. Everything sounds muffled, quiet and peaceful.
To the ears of a whale or dolphin, though, the underwater world is getting less peaceful all the time. Noisy ships are more common than they used to be. Sound travels faster and farther underwater than it does in air. In some cases, a single sound can travel all the way across the deep ocean. Changes in the environment are making the oceans louder, too.
For ocean-dwelling animals, it’s like having new neighbors who play loud music all the time, even at night. The noise can hurt animals’ ears. It can make life more stressful for them. It can change their behavior, where they go and maybe even how they talk to each other.
“We know the ocean is getting louder,” says Mark McDonald, an ocean acoustician in Bellvue, Colo. An acoustician is a scientist who studies sound. “It can’t be good for anything that lives in the ocean, but it’s hard to know how bad it is.”
The sound of water
Like light, sound travels as waves that bounce of off some objects and get absorbed into others. Inside our ears, sound waves cause three little bones to vibrate. Tiny hairs turn those vibrations into messages that travel to the brain. That’s how we hear what’s happening — birds chirping, computers humming, kids playing.
Some people have voices that are high and shrill like a flute’s. Others have voices that are low and deep like a trombone’s. That quality is called pitch. Some sounds are loud like barking dogs. Others are quiet like droning dishwashers. The level of loudness is called volume. Scientists measure volume in decibels. A normal conversation measures about 60 decibels (dB). A lawnmower is 90 dB. A rock concert is 120 dB. And a gunshot is 140 dB. For people, any sound about 85 dB can damage hearing with enough exposure.
Both pitch and volume exist in water and in air. But sound travels five times faster in water — at a blazing speed of nearly a mile (1,500 meters) a second. Sound also travels farther underwater than it does in air, especially in deeper waters.
Down at about 2,600 feet (800 meters), sound travels through a deep sound channel for thousands of miles across the entire ocean. It would be like someone in New York calling to a friend in California — without the help of a phone.
Surfers and human swimmers don’t usually notice the nuances of noise in water because human ears aren’t designed to listen down there. But scientists have been using tools to monitor the clatter for decades.
At first, most information was kept secret by the military. They were working to detect enemy submarines. These deep-water ships send out sound pulses to navigate. The military’s constant listening picked up lots of other sounds besides submarines. But the military didn’t release the data, because it didn’t want to blow its cover.
In the early 1990s, scientists started releasing evidence that sound was changing underwater — and that it was changing a lot. Recent studies show that the sound level of the ocean has risen by 3 decibels every decade since at least the 1960s. In places with high amounts of shipping and other activity, noise levels have doubled each decade.
In the oceans, sonar pulses can top 200 dB. Large ship engines produce sounds that exceed 180 dB. The rise in ocean noise is due mostly to the rising number of ships in the sea. As engines become more powerful, they also make more noise.
With noisier oceans, scientists are most concerned about marine mammals, such as whales, dolphins and sea lions. These animals depend on sound to communicate with each other. Many also use a form of sonar to navigate.
Here’s how sonar works: An animal, much like a submarine, sends out pings of sound and then listens for echoes. By reading the time it takes for those sounds to bounce back to it, the animal can figure out the distance to objects that surround it. The technique allows the animal to avoid predators, catch prey and swim around obstacles.
Extra human-made noise, some researchers fear, might be interfering with ocean animals’ ability to hear each other or find their way.
“Sound is important to these animals,” says Ken Ramirez, a behaviorist and biologist at Shedd Aquarium in Chicago. “Recently, we’ve seen that when something unexpected happens, like the first time an ocean platform is set up or drilling begins, animals are surprised and their behavior patterns change. They may flee or relocate if the noise bothers them.”
There have also been an alarming number of strandings — when groups of dead animals wash ashore. Some studies point to loud noise as a cause.
Still, it’s not easy to draw conclusions from these observations, Ramirez says. Other scientists have found that dolphins and whales are really good at adapting to changes in their environment. Even though sea lions may move away from loud noises at first, for example, they eventually get used to those new sounds and stick around.
People are the same way. If you move from the country to the city, you’ll probably have trouble sleeping for a few weeks, bothered by the blaring sounds of traffic, sirens or trains. After a while, you probably won’t notice the noise anymore, and you’ll sleep just fine.
When it comes to ocean animals, there aren’t any perfect studies that clearly demonstrate whether rising levels of noise are good or bad.
“There is so much we still don’t know about how animals use their sound and what impacts various things have on them,” says Ramirez, who works with aquarium staff to make sure that animal tanks sound just like natural ocean environments. “Our learning about these animals is still in its infancy.”
Warmer, louder water
Sound isn’t just getting into the ocean more than it used to. It’s also traveling longer distances. And a gas called carbon dioxide is to blame.
Many human activities — including driving cars and heating homes — release carbon dioxide, or CO2, into the air. CO2 is a greenhouse gas. It lingers in the atmosphere and traps heat. It also gets into the oceans. And through some quirks of chemistry, CO2-rich water absorbs less sound. That allows noises to travel farther.
By 2100, the oceans could absorb as much as 70 percent less sound than they do today, found one recent study. That would make the underwater world even louder than just extra noise would. The study looked only at deep sounds with very low pitches. Boat engines produce those types of noises. So do whales.
One worry is that the pumped-up volume will make it harder for whales and other animals to hear each other. But the opposite could happen, too.
“If the ocean is becoming more transparent to sound, that means that if a whale produces sound, it can travel farther,” says study author Tatiana Ilyina, an oceanographer at the University of Hawaii at Manoa. “That means that his partners at the other end of the ocean can hear better. One impact could be that it would enable mammals to communicate over longer distances.”
“It is a very complicated topic,” she adds. “It’s not very clear at the moment what the implications for marine mammals will be.”
Among other mysteries, blue whales have been singing with deeper voices recently, McDonald has found. In some cases, pitch has dropped by more than 30 percent since the 1960s. Scientists have come up with a few hypotheses to explain the puzzling trend.
One idea is that whales are trying to distinguish their voices from the sounds of ship engines. If that were true, though, the whales should be singing with higher voices, not deeper ones.
“It’s surprising news,” McDonald says. “We haven’t found an answer we think is convincing.”
The mysteries of ocean noise just keep getting deeper and deeper. Glug, glug!