Climate change has affected many things about the ocean – the temperature; sea level; which creatures live where. But it’s also changed something surprising: how the ocean sounds.
Two years ago, acoustic engineer Lee Freitag, from the Woods Hole Oceanographic Institution, discovered a stark change in the Arctic Ocean’s Beaufort Sea: Sounds, he found, could now travel about four times farther than they could a decade ago.
The U.S. Navy wanted to know more. Their submarines communicate through underwater sound-wave transmissions, making it vital to their operations to understand exactly where their signals are being carried.
The initial discovery that something was awry with the ocean’s ability to carry sound was gleaned from a network of sensors – there are currently 12 – spread throughout the ocean. But those measurements only give snapshots of what’s going on at those exact 12 locations: To know more they needed to get mobile.
So last March, the Navy invited Henrik Schmidt’s ocean acoustics team from the Massachusetts Institute of Technology up to their ICEX camp, perched on an ice sheet above the Beaufort Sea. There, the scientists dropped a 3.5m, 400kg drone through a hole in the ice.
As the drone sank, it traveled through the three key layers of water. The first, a slightly warmer layer about 50–80m depth, has long existed in summer months. But thanks to climate change, this layer is now permanent and warmer than before.
“The ice has been melting more in the summer, the waters are being exposed to the atmosphere and the solar heating warms them up,” explains John M. Toole, an oceanographer at Woods Hole Oceanographic Institution.
The second was a cold layer at 100–200m. The third was another warm layer centered around 200m deep. This layer was also created by climate change, and is the result of heat flowing into the Arctic Ocean from the Pacific through the Bering Strait (among other factors), says Mary-Louise Timmermans, an oceanographer at Yale University.
As ongoing research has shown, these three layers have created a channel in the ocean – sometimes called the Beaufort Lens – through which sound waves travel particularly well. The sound waves travel in the coldest middle layer, and the surrounding warmer layers keep the sound waves contained. This happens because of a particular quirk of how sound waves interact with water of different temperatures.
At low water temperatures, such as those in the Arctic, sound waves propagate most slowly through the very coldest water. Sound waves also bend toward the places where they travel slowest. So when a sound wave reaches the buffering warmer layers, it bends back toward the colder layer in the middle. The result is that sounds that used to peter out after 100km can now be detected 400km away.
The Navy is learning how the Beaufort Lens works acoustically, but they still don’t know exactly where it extends geographically – a map that is probably changing every year because of ocean warming. Unfortunately, Schmidt and his drone weren’t able to give them all the information they need.
Piloting and recovering a drone from a hole in the middle of an ice sheet proved much trickier than fetching it out of the comparative spaciousness of the open ocean or off the edge of an ice sheet. What’s more, Schmidt was using a Navy tracking system normally used for 100m-long submarines, which wasn’t accurate enough for their 4m vehicle. One day, they lost the drone underwater for more than 24 hours before eventually recovering it.
The scientists were able to measure the properties of the lens in some new places, but not nearly as many as they’d hoped. They hope to be back at the Navy’s next ICEX camp in 2018.