Scientists unearth tremors to ‘hear’ the ocean warming

Shramidha Srivastava

20th September 2020

A graphic representation of the ‘Seismic Ocean Thermometry’. (Source: Caltech website)

Scientists at the California Institute of Technology (Caltech) and the Chinese Academy of Sciences have discovered a novel approach to detect ocean warming. In a paper published on September 18—titled ‘Seismic Ocean Thermometry’—in Science Magazine, it is illuminated that sound travels faster in the water when it is warmer. Thus, differences in speed can reveal changing temperatures, based on which, the researchers are comparing the speeds of sound produced by undersea earthquakes. 

The Intergovernmental Panel on Climate Change (IPCC) states that the oceans have taken up more than 90 per cent of the excess heat trapped by greenhouse gases. Tracking the warming levels, however, has not been a cakewalk. 

Scientists have been struggling to master this ‘hack’ of determining ocean heat with ship-based observations and satellite observations, however, they could only manage to capture minuscule portions of the sea. The most detailed picture of the ocean heat emerges from ‘Argo fleets’, a set of autonomous devices that have skimmed the seas since the early 2000s. Although these devices seep down to depths as low as 2,000 meters in order to measure heat and other parameters, they are limited in number and cannot sample any deeper. 

Speaking to BBC, a postdoctoral researcher at Caltech and lead author of the paper — Dr Wenbo Wu said was deeply inspired by the sounds of earthquakes produced by the seafloor — “I know these earthquakes are very powerful sources, so why not try to use the earthquakes for calculating ocean temperature?” 

The idea was tested near Indonesia’s island of Nias, where the Indo-Australian Plate jostles under the Sunda Plate. Wu and his colleagues gathered acoustic data from 4,272 earthquakes of magnitude 3–5 from 2004-2016 and found 2,047 pairs that shared the same point of origin. After comparing earthquakes that ruptured at the same spot over that span, they could pick out the mere fractions of a second that separated the acoustic wave speeds, says Sidao Ni, a co-author, and seismologist at the Chinese Academy of Sciences. In this process, they found that the ocean near Nias was warming by about 0.08-degree Fahrenheit per decade—more than the 0.047-degree Fahrenheit of warming per decade—propounded by Argo’s data, as reported by the Scientific American.  

Wenboo Wu claimed in his interview with the BBC the new findings are more accurate. Although the difference in numbers may not look enormous, these temperature changes are occurring over boundless volumes of water in the entire eastern Indian Ocean, which suggests—it takes considerable heat to warm that much water. 

The ground of this theory roots back to 1979, when oceanographers Carl Wunsch and Walter Munk proposed to calculate ocean temperatures using sea-based acoustic emitters. At that time the idea never really took off, partly because it was uneconomical, and partly out of concerns for marine animals, such as whales, that communicate through sound waves. 

As the research is in its initial stages, the paper has a few loopholes. According to Wu, the discovery is restricted to one region and decade and their methods need to be applied to more regions with different time frames to assess it efficiently.

Presently, the research team is working on data collected by a hydrophone network operated by the UN Comprehensive Nuclear Test Ban Treaty Organisation, according to BBC. 

This work is “quite extraordinary and very promising,” says Susan Wijffels, an Argo leader at the Woods Hole Oceanographic Institution speaking to Science Magazine. “If extended globally, it could provide an independent check on Argo measurements, what a gift to the climate community that’d be,” she further added. 

Sources: BBC, Scientific American, Science Magazine

Edited by Varun Paleli Vasudevan