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Sound waves created by the rumble of underwater earthquakes provide a new way to study how climate change is warming the oceans, writes with reference to Science.

As greenhouse gas emissions heat the planet, the ocean absorbs a huge amount of this heat. To track changes, approximately 4,000 devices called Argo floats collect temperature data in the upper 2,000-meter layers of the ocean. But in some regions - including in deeper parts of the ocean and under sea ice-such data is scarce.

Wenbo Wu, a seismologist at the California Institute of technology, and his colleagues are reviving a long - standing idea: use the speed of sound in seawater to estimate ocean temperature. In the new study, Wu's team developed and tested a way to use sound waves generated by earthquakes propagating through the Eastern Indian ocean to estimate temperature changes in these waters from 2005 to 2016.

Comparing this data with similar information from Argo floats and computer models showed that the new results match up well. This discovery suggests that a method called seismic ocean thermometry can track the impact of climate change on less-studied areas of the ocean.

Sound waves are carried through water by the vibration of water molecules, and at higher temperatures these molecules vibrate more easily. As a result, waves travel a little faster when the water is warmer. But these changes are so small that in order to be measured, researchers need to track waves over very long distances.

Fortunately, sound waves can travel long distances across the ocean thanks to a curious phenomenon known as the SOFAR channel - short for Sound Fixing and Ranging (underwater sound channel). The SOFAR channel, formed by layers of varying salinity and temperature in water, is a horizontal layer that acts as a waveguide, directing sound waves in much the same way that optical fibers direct light waves, says Wu. Waves bounce off the upper and lower borders of the channel, but can continue their path almost unchanged for tens of thousands of kilometers.

In 1979, Oceanographic physicists Walter Munch, then at the SCRIPPS institution of Oceanography in La JOLLA, California, and Carl Wunsch, now a Professor Emeritus at the Massachusetts Institute of technology and Harvard University, developed a plan to use these properties of the ocean to measure water temperature from the surface to the sea floor - a method they called "ocean acoustic tomography". They were supposed to transmit audio signals through the SOFAR channel and measure the time it takes for waves to reach receivers located 10,000 kilometers away. In this way, the researchers hoped to create a global database of ocean temperatures.

But environmental groups lobbied to end the project and eventually stopped the experiment, saying that anthropogenic signals could have adverse effects on marine mammals, as Wunsch notes in a commentary for the same issue of the journal Science.

Forty years later, scientists determined that the ocean is actually a very noisy place, and that the supposed anthropogenic signals would be weak compared to the roar of earthquakes, the eruption of underwater volcanoes and the groan of colliding icebergs, said seismologist Emil Okal from northwestern University in Evanston (Illinois), who was not involved in the new study.

Wu and his colleagues have developed a workaround to avoid any environmental problems: instead of using human-made signals, they use earthquakes. When an underwater earthquake rumbles, it releases energy in the form of seismic waves known as P-waves and S-waves that vibrate through the sea floor. Some of this energy gets into the water, and when this happens, the seismic waves slow down, turning into T-waves.

These t teeth can also move through the SOFAR channel. So, to track changes in ocean temperature, Wu and his colleagues identified "repeaters" - earthquakes that the group believes originate from the same location but occur at different times. The Eastern Indian ocean, according to Wu, was chosen for this pilot study mainly because it is very seismically active, offering a large number of such earthquakes. After detecting more than 2,000 repeaters from 2005 to 2016, the team then measured the time difference between sound waves passing through the Eastern Indian ocean, which is about 3,000 kilometers long.

The data showed a slight warming trend in the waters of about 0.044 degrees Celsius over a decade. This trend is comparable to that indicated by real-time temperatures obtained using Argo buoys, but slightly speeds up the process. Wu says the team plans to test this technique next time with receivers further away, including off the West coast of Australia.

According to Ocala, this extra distance will be important to prove that the new method works. So far, the distances involved are very small, as are the estimated temperature changes. This means that any uncertainty in comparing the exact origin of two repeated earthquakes can lead to uncertainty in the passage time and, consequently, in temperature changes. But future research over long distances may help mitigate this problem, he says.

The new study "really opens up new horizons," says Frederick Simons, a geophysicist at Princeton University who was not involved in the study. "Scientists have really developed a good way to detect very subtle, slow temporal changes. It's really technically sound."

And, Simons adds, in many places, seismic records are several decades older than the temperature data collected by Argo buoys. This means that scientists will be able to use seismic ocean thermometry to get new estimates of past ocean temperatures. "The hunt for high-quality archival records will continue."