In this way, the researchers hoped to compile a global database of ocean temperatures ( SN: ).īut environmental groups lobbied against and ultimately halted the experiment, stating that the human-made signals might have adverse effects on marine mammals, as Wunsch notes in a commentary in the same issue of Science.įorty years later, scientists have determined that the ocean is in fact a very noisy place, and that the proposed human-made signals would have been faint compared with the rumbles of quakes, the belches of undersea volcanoes and the groans of colliding icebergs, says seismologist Emile Okal of Northwestern University in Evanston, Ill., who was not involved in the new study. In 1979, physical oceanographers Walter Munk, then at the Scripps Institution of Oceanography in La Jolla, Calif., and Carl Wunsch, now an emeritus professor at both MIT and Harvard University, came up with a plan to use these ocean properties to measure water temperatures from surface to seafloor, a technique they called “ ocean acoustic tomography.” They would transmit sound signals through the SOFAR Channel and measure the time that it took for the waves to arrive at receivers located 10,000 kilometers away. The waves bounce back and forth against the upper and lower boundaries of the channel, but can continue on their way, virtually unaltered, for tens of thousands of kilometers ( SN: 7/16/60). Formed by different salinity and temperature layers within the water, the SOFAR channel is a horizontal layer that acts as a wave guide, guiding sound waves in much the same way that optical fibers guide light waves, Wu says. But those changes are so small that, to be measurable, researchers need to track the waves over very long distances.įortunately, sound waves can travel great distances through the ocean, thanks to a curious phenomenon known as the SOFAR Channel, short for Sound Fixing and Ranging. As a result, the waves travel a bit faster when the water is warmer. Sound waves are carried through water by the vibration of water molecules, and at higher temperatures, those molecules vibrate more easily. That finding suggests that the technique, dubbed seismic ocean thermometry, holds promise for tracking the impact of climate change on less well-studied ocean regions, the researchers report in the Sept. In a new study, Wu’s team developed and tested a way to use earthquake-generated sound waves traveling across the East Indian Ocean to estimate temperature changes in those waters from 2005 to 2016.Ĭomparing that data with similar information from Argo floats and computer models showed that the new results matched well. So Wenbo Wu, a seismologist at Caltech, and colleagues are resurfacing a decades-old idea: using the speed of sound in seawater to estimate ocean temperatures. But that data collection is scanty in some regions, including deeper reaches of the ocean and areas under sea ice. To monitor the change, a global fleet of about 4,000 devices called Argo floats is collecting temperature data from the ocean’s upper 2,000 meters. Sound waves traveling thousands of kilometers through the ocean may help scientists monitor climate change.Īs greenhouse gas emissions warm the planet, the ocean is absorbing vast amounts of that heat.
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