Just like the atmosphere, oceans are warming because of climate change. They have absorbed about 90% of the excess heat trapped by greenhouse gases. Now, researchers have found a new way to calculate the ocean’s rate of warming by studying undersea earthquakes.
Tracking the warming of the oceans has so far been challenging. Ship-based observations only capture snapshots in time of a small portion of the seas, whereas satellite observations can’t penetrate deeply below the surface. The most detailed picture of ocean heat was captured by a group of autonomous floats known as Argo that has been looking at the seas since the early 2000s -- but this isn't feasible at a global scale.
Now, a group of researchers at the California Institute of Technology and the Chinese Academy of Sciences have found a new way to measure ocean warming. In their study, they compared the speed of sound waves produced by undersea earthquakes. Sound travels faster in the water when it’s warmer so differences in speed can show changing temperatures.
“I am very impressed with the methods the study’s authors are using and the fact that they could pull this all off,” Frederik Simons, a Princeton University geophysicist, who was not involved in the research, told Scientific American. “They’re opening up a whole new area of study.”
The idea of measuring ocean heat with sound actually isn’t new. Back in 1979, oceanographers Carl Wunsch and the late Walter Munk proposed using sea-based acoustic emitters and land-based receivers to measure the speed of sound waves, calculating temperatures based on the results. But the idea never caught on.
Inspired by those earlier efforts, lead author Wenbo Wu realized that the seafloor actually produces its own regular sound waves in the form of earthquakes. These aren’t the seismic waves from the earthquakes but instead low-frequency acoustic waves that move through the water. This realization led Wu to explore their use to measure ocean warming.
To test the idea, the team focused on Indonesia’s island of Nias, where the Indo-Australian Plate is bumping under the Sunda Plate. They collected acoustic data from over 4,000 earthquakes of magnitude 3 or above from 2004 to 2016 and found 2,047 pairs of quakes that each shared the same point of origin.
They compared earthquakes that ruptured at the same spot over the years, which showed the ocean near Nias is warming by about 0.008-degree Fahrenheit per decade. This is more than the 0.0047-degree Fahrenheit of warming that had been previously suggested by Argo’s data.
The numbers are more accurate, the researchers believe, because the other information sources were limited. While less than one-degree Fahrenheit might not seem a lot, these temperature changes are happening over massive volumes of water in the eastern Indian Ocean. Nevertheless, it will take more work to confirm the results for other regions.
"It is important to emphasize that this is a result that applies to this particular region and this particular decade," said Dr. Wu to BBC News. We need to apply our method in many more regions and over different time frames to evaluate whether there is any systematic under- or over-estimation of the deep-ocean trend globally. It is much too early to draw any conclusions in this direction."
The study was published in the journal Science.