Scientists say they can take the ocean’s temperature using waves of sound emanating from undersea earthquakes, and it could become an important new tool to track warming seas in the era of climate change, reports Paul Voosen for Science.
Keeping track of how quickly the oceans are heating up is vital to understanding the pace and severity of climate change. That’s because the oceans have absorbed roughly 90 percent of the warming caused by humanity’s rampant injection of greenhouse gases into the atmosphere, reports Stephanie Pappas for Scientific American.
According to Science, the technique of using sound to infer water temperature was initially proposed in 1979. In 1991, researchers tested it out by dunking massive, bass-heavy speakers into the Indian Ocean. The scientists were able to calculate how hot or cold the water was because temperature impacts the density of seawater. Sound travels more quickly though warm water because it is less dense than cold water. The method worked but was ultimately shelved due to concerns that subjecting sea life to unsolicited blasts of low-frequency sound was too high a price to pay for the data.
This new application, published last week in the journal Science, leverages the same properties of water and sound but uses the natural phenomenon of earthquakes occurring beneath the waves to supply the rumbly soundtrack.
Current methods for sensing ocean temperatures are sporadic, in the case of measurements taken from ships, or mostly probe the sea’s upper reaches, which are sampled by a fleet of thousands of floating sensors, explains Matt McGrath for BBC News. But these methods leave gaps in time and space that hamper our understanding of how the planet is responding to climate change.
These knowledge gaps manifest whenever and wherever oceanographic voyages aren’t running as well as in the swirling depths of the world’s oceans. And though a fleet of roughly 4,000 autonomous Argo floats spans much of the globe, their instruments can’t study waters deeper than around 6,500 feet. Moreover, many of the stretches of ocean climate researchers are most keen to monitor are sloshing beneath the ice in Earth’s polar regions, making those waters hard to access, reports Carolyn Gramling for Science News.
The researchers applied the new technique, called seismic ocean thermometry, using the sound waves produced by 2,047 pairs of so-called “repeaters,” earthquakes that occur in nearly the same size and location at different times, in the East Indian Ocean between 2005 and 2016, according to the paper.
Based on how long those sound waves took to traverse some 1,800 miles between Indonesia and a monitoring station on the island Diego Garcia, the researchers were able to figure out the average temperature of the entire stretch of water, according to a statement.
"It takes sound waves about half an hour to travel from Sumatra to Diego Garcia," Wenbo Wu, a geophysicist at the California Institute of Technology and the study’s lead author, tells BBC News. "The temperature change of the deep ocean between Sumatra and Diego Garcia causes this half-hour travel time to vary by a few tenths of a second. Because we can measure these variations very accurately, we can infer the small changes in the average temperature of the deep ocean, in this case about a tenth of a degree."
The results of the study demonstrate the promise of seismic ocean thermometry and suggest the eastern Indian Ocean may be warming slightly faster than researchers thought, per Scientific American. Temperature data collected by the bobbing fleet of Argo floats recorded a 0.047-degree Fahrenheit increase in the eastern Indian Ocean’s temperature over the last decade, while the earthquakes’ sound waves suggest it actually warmed by 0.08 of a degree.
Speaking with BBC News, Wu emphasizes that it’s too early to say whether this finding means our global sense of how quickly the seas are heating up might be an under or overestimate. “This is a result that applies to this particular region and this particular decade," he tells BBC News. He adds that the method will need to be applied in “many more regions and over different time frames to evaluate whether there is any systematic under or over-estimation of the deep-ocean trend globally.”
Frederik Simons, a geophysicist at Princeton University who was not involved in the research, tells Science News that the study’s authors have “really worked out a good way to tease out very subtle, slow temporal changes. It’s technically really savvy.”
Simons also tells Science News that seismic records for many locations extend farther back in time than the measurements taken by the Argo fleet, which start around 2000. This data could allow researchers to create fresh estimates for ocean temperatures further into the past. “The hunt will be on for high-quality archival records,” Simons says.