Scientists Use Rumble of Deep-Sea Quakes to Take Ocean Temperature

(CN) — Researchers have successfully demonstrated a new technique to help measure Earth’s warming oceans that was once thought to be too dangerous and potentially harmful to marine life: monitoring the distant sounds from deep-sea earthquakes.

For decades scientists and researchers have sought to explore the most potentially effective means of measuring the temperatures of Earth’s oceans, all the more imperative as ocean temperatures have risen due to climate change. Roughly 90% of the carbon emissions humans pump into the atmosphere are absorbed by Earth’s oceans, which boosts their temperature even as they mitigate even more serious global warming.

But the process wreaks untold havoc on undersea life and vulnerable ecosystems, so scientists have spent years figuring out how to better gauge the extent of the oceanic warming. This is no easy task, however, as researchers have found that the increases are hard to accurately assess on such a massive scale, with scientists often being limited by single-point measurements across an area that represents only a small fraction of the ocean’s vast depths.

But researchers now believe they have devised a new way to better measure global ocean temperature increases, one that refines an older measuring technique that was written off decades ago.

In a study published Thursday in the journal Science, Wenbo Wu and research colleagues report they have fully demonstrated they can use the noise from undersea earthquakes as a way of measuring ocean temperatures on a sizeable scale — a technique known as seismic ocean thermometry.

In this technique, researchers rely on the low-frequency soundwaves produced at the bottom of the ocean by seafloor earthquakes. Because the speed of sound underwater is dependent on the temperature of the water the sound travels through, researchers used fluctuations in the travel time of soundwaves to calculate the surrounding ocean temperatures.

The principles behind seismic ocean thermometry were first explored around four decades ago, and while it held theoretical promise at the time, researchers found the technique was both outrageously expensive and harmful to harm sensitive marine life. Efforts to further explore the method were scrapped.

Researchers in Thursday’s study say they have successfully resurrected this once-dismissed technique, making it more cost-effective and less harmful to marine life by utilizing passive, natural sounds — in this case, the sound of seaquakes — instead of manmade ones. They say the benefits of the new method could be significant.

“Applying our method across the world ocean can provide better constraints on the ocean temperature change due to natural fluctuations and human-caused global warming,” Wu and fellow author of the study Joern Callies said in an email. “The resulting data will provide a very useful complement to existing measurements collected from ships and with profiling autonomous floats.”

Scientists tested this new approach to seismic ocean thermometry using real-world data taken from the East Indian Ocean. This resulted in researchers pouring over and analyzing data from just over 2,000 pairs of repeating earthquakes that took place sometime between December 2004 and June 2016 in Earth’s third largest ocean.

Using this massive collection of earthquake data and the way in which the resulting sounds traveled through the water, researchers were able to create a marine basin-wide temperature profile in the East Indian Ocean that spanned nearly 2,000 miles — a profile that has already supplied researchers with a dire warning.

The research uncovered an alarming, decade-long warming trend for that section of ocean that greatly exceeds what scientists had previously estimated. 

In a related perspective, Carl Wunsch said the new application of seismic ocean thermometry could be the key to unlocking an entirely new way to measure changes in ocean temperature.

“Wu et al. demonstrate how an intriguing combination of physical oceanography and classical seismological techniques potentially opens the way for an entirely new and globally capable observation system,” Wunsch said.

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