In 1991, scientists lowered large subwoofers into the water at Heard Island, a snowcapped volcanic island in the Indian Ocean.

A team of seismologists and oceanographers has shown that small earthquakes repeatedly emanating from the same spot beneath the ocean floor can take the place of the subwoofers.

The quakes generate reliable acoustic signals for measuring ocean temperatures, including at depths below 2000 meters, beyond the reach of other techniques.

If validated, the approach, published today in Science, could open an entirely new ocean observation system for understanding past and future climate change, says Frederik Simons, a geophysicist at Princeton University unaffiliated with the study.

The oceans absorb more than 90% of the energy trapped by global warming, and any change in the rate at which they soak up heat would have an outsize impact on how fast the atmosphere warms.

Two decades ago, robotic floats from the international Argo array began to monitor the warming of the ocean to a depth of about 2000 meters.

In 1979, Wunsch and Walter Munk, an oceanographer at the Scripps Institution of Oceanography who died last year, first proposed using sound waves to measure the ocean’s heat and structure.

At a depth of about 1000 meters, the speed of sound hits a minimum, forming a conductive channel between warm waters above and dense water below.

This waveguide enables sound waves to coast across entire ocean basins, says Bruce Cornuelle, a Scripps oceanographer who worked with Munk.

“That makes it much easier to extract the global warming signal,” says Jörn Callies, an oceanographer at the California Institute of Technology (Caltech) and co-author on 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).

That was until 1 year ago, when Wenbo Wu, a Caltech seismologist, realized that repeating earthquakes on slowly creeping faults below the sea floor could provide an alternative sound source.

When earthquakes shake the ocean floor, some of the energy is transformed into acoustic waves.

Their search went back to the Indian Ocean.

The resulting sound waves traveled through the ocean unfettered to Diego Garcia, a remote atoll south of India, where they hit land and turned back into seismic waves, picked up on the island’s seismometer.

Some 40% of their heat measurement came from water below 2000 meters, suggesting some warming is working its way deeper into the ocean, out of Argo’s current reach.

Even more alluring for Wijffels is the possibility of extending global warming trends back in time, before Argo, by detecting repeaters in old seismic records.

The newly bright prospects for ocean acoustic thermometry are also a validation for Munk, who was deeply saddened when his global acoustic dreams were muted, Cornuelle says.

*Correction, 17 September, 4:25 p.m.: A previous version of this story stated 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 cannot yet say where in the water column the warming occurred.