In 1991, scientists lowered large subwoofers into the water on Heard Island, a snow-capped volcanic island in the Indian Ocean. The speakers emitted low-frequency sounds that, like whale song, echoed across entire oceans. Received from receivers off the coast of California and Bermuda, the signals contained crucial information about the water they had crossed: how hot it was. It was a promising way to monitor the warming of Earth̵
A team of seismologists and oceanographers has shown that small earthquakes emanating repeatedly from the same spot under the ocean floor can take the place of subwoofers. Earthquakes generate reliable acoustic signals to measure ocean temperature, even at depths below 2,000 meters, beyond the reach of other techniques. If validated, the approach, published today in Science, could open up a whole new ocean observation system for understanding past and future climate change, says Frederik Simons, a geophysicist at Princeton University not affiliated with the study. “There is a potential treasure trove of data waiting to be analyzed.”
Oceans absorb more than 90% of the energy trapped by global warming, and any change in the rate at which they absorb heat would have a huge impact on the rate of warming of the atmosphere. Two decades ago, robotic floats from the Argo International Array began monitoring ocean warming down to a depth of about 2,000 meters. But the floating matrix, now 4000 strong, could not probe the large volume of water at greater depths. “The inability to determine what is happening in deep water is a major barrier to understanding the ocean and climate, even today,” says Carl Wunsch, retired oceanographer at the Massachusetts Institute of Technology.
In 1979, Wunsch and Walter Munk, an oceanographer from the Scripps Institution of Oceanography who died last year, first proposed using sound waves to measure ocean heat and structure. Sound travels faster as the water gets warmer or denser, making travel time a reliable indicator of temperature and density if the sound source and receiver are in fixed positions.
The technique did not require particularly noisy sources. At a depth of about 1000 meters, the speed of sound reaches a minimum, forming a conductive channel between the warm water above and the dense water below. This waveguide allows sound waves to traverse entire ocean basins, says Bruce Cornuelle, Scripps oceanographer who worked with Munk. “It’s like a 5-year-old who grabs a tube of wrapping paper and yells into his brother’s ear.”
In addition to probing the entire width of an ocean, sound waves, with vertical amplitudes of thousands of meters, capture conditions from shallow water to the abyss. As a result, they average fluctuations in natural temperature on a smaller scale, revealing basin-level variations of just a few thousandths of 1 ° per year. “This makes it much easier to extract the global warming signal,” says Jörn Callies, oceanographer at the California Institute of Technology (Caltech) and co-author of the new study.
After the 1991 demonstration at Heard Island, Munk won Department of Defense funding for a follow-up experiment in the Pacific Ocean, called Acoustic Thermometry of Ocean Climate (ATOC). But he got mired in controversy over his two human-sized speakers, located off the coast of Hawaii and California in privileged whale territory. “It became a political nightmare,” says Brian Dushaw, a retired oceanographer who worked at ATOC. The ATOC’s signals were no stronger than whale calls and ship traffic, but much of its $ 35 million budget went to studies on the impact of sound on marine mammals.
Military secrecy also got in the way. To listen for signals, the project relied on Navy classified hydrophones normally used to detect submarines. Scientists couldn’t even publish the locations of the receivers, Wunsch says. “We didn’t tell the Navy that if you released the signal, which we did, you could figure out where the receivers were,” adds Wunsch. The Hawaiian source, off the coast of Kauai, operated until 2006, providing 10 years of warming data. But by that point, oceanographers had abandoned acoustic thermometry and relied on Argo, Dushaw says.
This was until 1 year ago, when Wenbo Wu, a Caltech seismologist, realized that the recurrence of earthquakes on creeping faults under the sea floor could provide an alternative sound source. When earthquakes shake the ocean floor, some of the energy is transformed into acoustic waves. Wu and his co-authors just had to find the right source.
Their research traced back to the Indian Ocean. In earthquake records, they identified more than 4,000 faulty earthquakes in the ocean floor west of Sumatra in Indonesia from 2004 to 2016, many of them between magnitudes 3.5 and 5. By triangulating on the source, the team identified lower fault zones. 100 meters apart that it broke repeatedly, says Sidao Ni, a co-author and seismologist at the Chinese Academy of Sciences’ Institute of Geodesy and Geophysics. The resulting sound waves traveled unrestricted across the ocean to Diego Garcia, a remote atoll south of India, where they hit the ground and turned into seismic waves, detected by the island’s seismometer.
By converting those travel times into temperatures, Wu and his colleagues found that the eastern Indian Ocean warmed by 0.044 ° C over the decade. The annual fluctuations matched well with Argo measurements of the same period, but the warming signal was nearly double what the Argo floats detected. The disparity suggests that Argo is lacking some heat, Callies says, at least for this basin in this short amount of time. About 40% of their heat measurement came from water below 2,000 meters, suggesting that some warming is making its way deeper into the ocean, out of Argo’s current range.
This work is “truly extraordinary and very promising,” says Susan Wijffels, leader of Argo at the Woods Hole Oceanographic Institution. If extended globally, it could provide independent control over Argo measurements, especially as production of a new line of Argo floats capable of descending 6,000 meters, currently deployed in only dozens, increases. Even more tempting to Wijffels is the possibility of extending global warming trends back in time, before Argo, by detecting repeaters in old seismic records. “What a gift that would be for the climate community,” he says.
The team thinks they can capture the sounds generated by the earthquake more cleanly with hydrophones than with terrestrial seismometers. This will allow them to use lower-powered earthquakes and, using the global network of distributed hydrophones as part of the Nuclear Test Ban Treaty, they should be able to collect signals from repeaters in all oceans of the world.
Hydrophones deployed under Arctic sea ice could measure the water temperature at a point that Argo floats cannot reach. It may also be possible to use the crash of the collapsing ice in neighboring Greenland – glacial earthquakes, as they are called – as a sound source. “It’s free data,” Dushaw says. “There is no doubt that someone will implement a system to take advantage of it.”
The bright new prospects for ocean acoustic thermometry are also a confirmation for Munk, who was deeply saddened when his global acoustic dreams were turned off, says Cornuelle. “I wish Walter had been around to see him. He would be delighted. “
* Correction, September 17, 4:25 pm: An earlier version of this story claimed that 40% of the measured warming came from below 2000 meters. Although 40% of the measured temperature came from water below 2000 meters, the technique still cannot tell where the heating in the water column occurred.