By Paul Woosen
In 1991, scientists submerged large subwoofers on Heard Island, a snowcapped volcanic island in the Indian Ocean. The speakers emitted low-frequency sounds that, like a whale song, thunder across the oceans. The signals, taken by receivers off the coasts of California and Bermuda, contained a crucial piece of information about the water they passed through: how hot it was. It was a promising way to observe the Earth’s warmer oceans, but concerns about how underwater noise could affect marine life were soon dispelled, with only a few dedicated scientists keeping the technology alive. Now, it’s back – only this time, the earth itself will provide sound.
A team of seismologists and oceanographers have shown that frequent small earthquakes from the same spot below the ocean floor can replace subwoofers. Earthquakes produce reliable acoustic signals for measuring ocean temperatures, including depths below 2000 meters beyond the reach of other technologies. If valid, the approach, published today Science, Could open up a whole new ocean observation system to understand past and future climate change, says Frederick Simmons, a geophysicist at Princeton University, who is not involved in the study. “Waiting for data analysis is a potential treasure.”
The oceans absorb more than 90% of the 90 degrees trapped by global warming, and any change in the temperature at which they heat up will affect how fast the atmosphere heats up. Two decades ago, robotic floats from the International Argo Array began monitoring ocean temperatures to a depth of about 2000 meters. But the float array, now 4000 strong, could not detect large amounts of water at large volume depths. “The inability to determine what is going on in deep water is a major obstacle to understanding the oceans and the climate,” says Carl Wunsch, a retired oceanographer at the Massachusetts Institute of Technology.
In 1979, Wunsch and Verter Monke, an oceanographer at the Scripps Institute of Oceanography who died last year, proposed the first use of sound waves to measure ocean heat and structure. Sound travels faster when the water is hotter or hotter, making a reliable gauge of temperature and density for its travel time if the sound source and receiver are in fixed locations.
The technology does not require particularly loud resources. At a depth of about 1000 meters, the speed of sound is broken to a minimum, which carries between the hot water above and the ga water below. Scripps oceanographer Bruce Cornell, who worked with Munk, says the waveguide enables sound waves along the shores of ocean basins. “It’s like a 5-year-old like King who grabs a wrapping paper tube and booms it into his brother’s ear.”
In addition to examining the full width of the ocean, sound waves, which have an amplitude of thousands of meters, travel from shallow water to the abyss under dull conditions. As a result, they average small-sized natural temperature fluctuations, showing fundamental changes in a few thousand of 1 of each year. “This makes it easier to predict global warming,” says Jern Kies Liz, an oceanographer at the California Institute of Technology and co-author of the new study.
After a 1991 demonstration on Hard Island, Munke won the Department of Defense funding for a follow-up experiment called Acoustic Thermometry of Ocean Climate (ATOC) in the Pacific Ocean. But he got into controversy by taking his two human-sized speakers, placing them in the main whale area off the coast of Hawaii and California. “It became a political nightmare,” says Brian Dusho, a retired oceanographer working at ATOC. The ATOC’s signals were no louder than whale calls and ship traffic, but most of its 35 35 million budget went to study the impact of noise on marine mammals.
Military secrecy was also found in this way. To hear the signals, the project was based on commonly classified Navy hydrophones to find submarines. Vince says even scientists have not been able to publish the locations of the receivers. “We didn’t tell the Navy that if you released the signal, which we did, you can report where the receivers were,” Vance added. The air source, away from Kauai, lasted until 2006, providing 10 years of warming data. By then, Dusho says, oceanographers had left behind sound thermometry and relied on argo.
That was 1 year ago, when Caltech seismologist, Wenbo Wu realized that repeating an earthquake on a slowly dissolving fault at the bottom of the ocean could provide an alternative sound source. When an earthquake shakes the ocean floor, some of the energy is converted into radiant acoustic waves. Wu and his co-authors just had to find the right source.
Their search went back to the Indian Ocean. In seismic records, they identified more than 4,000 earthquakes from 2004 to 2016 in seabed west of Sumatra in Indonesia, many of them between 3.5.itude and 5. magnitude, triangular at the source, the team identified fault patches less than 100 meters. Sido Ni, co-author and seismologist at the Institute of Geophysics and Geophysics at the Chinese Academy of Sciences, says that unless it erupts frequently. The resulting sound waves protruded from the ocean to a remote atoll in southern India, Diego Garcia, where they collided with the ground and turned back into seismic waves, taking off on the island’s seismometer.
Turning that travel time into a temperature, Wu and his colleagues observed that the eastern Indian Ocean was 0.044 0.0 C in a decade. The annual fluctuations matched Argo’s measures well from the same time, but the warming signal was almost double what Argo floats had discovered. This difference suggests that the argo loses some heat, says Call Liz, at least for this basin in the short term. About 40% of their heat measurements came from water below 2000 meters, indicating that some temperatures outside the current reach of Argo are working deep in the ocean.
Susan Wijfils, Argo leader of the Woods Hall Oceanography Institute, says the work is “absolutely extraordinary and very promising.” If expanded globally, it could provide an independent check on the measurement of argo, especially when the production of a new line of argo floats descending go000 meters, currently only arranged in dozens, ramps. Even more compelling for Wijfills is the possibility of a timely reversal of the global warming trend, before Argo, by finding repeaters in old seismic records. He says, “What a gift the climate community has.”
The team believes it can capture seismic noise more accurately with hydrophones than with land-based seismometers. This will enable them to use low-power earthquakes, and using a global network of hydrophones deployed as part of the Comprehensive Nuclear Test Ban Treaty, they should be able to take signals from repetitions in the world’s oceans.
Hydrophones deployed under Arctic sea ice can measure water temperature in a location that cannot reach argo floats. Nearby Greenland – Glacier earthquakes, as they are known, are also possible to use damaged ice crashes as a sound source. “It’s free data,” says Dusho. “There is no question that someone will implement a system to take advantage of it.”
Cornuel said the new bright prospects of sea acoustic thermometry are also a recognition for Munk, who was deeply saddened when global sound nightmares were silenced. “I wish Walter had been around to see him. It will be fun. ”
* Update, September 17, 4:25 pm: A previous version of the story stated that 40% of the measured warming fell below 2000 meters. Although 40% of the measured temperature came from water below 2000 meters, the technology has not yet been able to tell where the warming occurred in the water column.
