Warmer Sounds

In just about every movie involving a submarine, there’s a dramatic attack scene. The sonar operator monitors the sound of the propellers of ships passing overhead, or sends out a “ping” and listens for its echo off the hull of another sub. He then gives the captain precise details on the location and speed of the enemy vessels.

That job is getting harder all the time. There’s more ocean traffic, which generates more background noise. Improvements in technology make submarines quieter than ever. And changes in Earth’s climate are creating subtle changes in the way sound travels underwater.

Sound travels about 3300 miles per hour in saltwater — more than four times faster than in the air. But that speed varies depending on the temperature, salinity, and pressure. Every degree of temperature change, for example, alters the speed of sound by five miles per hour. It also changes the way different layers of water bend sound waves.

And with ocean temperatures expected to climb by up to several degrees by the end of the century, that could have important consequences. Among other things, it could harm the ability of marine mammals to communicate, catch prey, or avoid predators. And it could make it harder for submarines to stay in contact with home base, to track vessels, and to watch out for whales and other marine mammals. So the navy is conducting experiments to see exactly how sound behaves at different ocean temperatures — perhaps helping future sonar operators win a tense battle at sea.