A Forgotten Underwater Sound Experiment Almost Changed How We Measure Global Warming

Image: Walter Munk/National Academy of Sciences and Office of Naval Research

In 1991, scientists lowered 10 speakers into the water off of a remote Australian island in order to blast a monotonous 57 hertz tone from them for 10 days. Other researchers listened for the distant sound at 15 monitoring stations in ocean basins around the world. This was all being done in the name of science—specifically, to better understand the processes driving climate change.

Scientists have been taking the planet’s temperature for decades using thermometers, satellites, and other methods to track just how much carbon emissions have heated it up. But there’s a problem with those measurements: They only gather information about the surface. It’s like feeling your forehead when you’re sick rather than putting a thermometer under your tongue.


Understanding just how much the whole planetary system is heating up is a crucial piece of the scientific big picture, particularly if we want to know if climate change has any nasty surprises in store.

The 1991 sound experiment was cut off by a powerful storm that destroyed most of the speakers, but still proved wildly successful. “It was a seminal experiment,” Bruce Howe, an acoustic oceanographer at the University of Hawaii, told Earther. “It absolutely showed what it set out to do.”

Despite its success, the researchers’ dream of using acoustics to routinely measure global warming has yet to be realized.

The planet has been warming ever since humans started pouring carbon dioxide and other greenhouse gases into the atmosphere. Thanks to thermometers and ocean surface measurements around the world, we basically know how much: About one degree Celsius, as of earlier this week.


We also know this increase would be two or three times higher without the oceans. They take carbon dioxide out of the atmosphere, which helps reduce its warming impact, though it does make seas more acidic. The oceans also absorb a lot of the excess heat that works down through the water column.

That’s the global warming we never really see. But it’s still having a profound effect on the world, either through altering marine ecosystems or ocean currents. These changes can have knock on effects on storm systems, sea levels, and fisheries, all things that matter to us here on land.


There’s a network of floats around the world that dive below the ocean surface and take ocean temperature measurements to go with the thermometer measurements we have on the surface of our planet. But there’s no one clear metric to measure all the planetary heat.

It’s with all that in mind that Walter Munk, a renowned oceanographer at the Scripps Institution for Oceanography who celebrated his 100th birthday this year, conceived of his plan to blast the ocean with a low frequency sound. The idea is grounded in a few simple concepts.


The first is that as water warms, sound travels through it faster. Munk estimated that the speed of sound increases by five meters per second for every one degree Celsius of warming. Salinity can also play a role, but temperature has by far the biggest impact. If you’re regularly monitoring sound in the ocean, you can in theory monitor how much the water is warming.

The second is that sound propagates a long ways underwater. In 1960, scientists detonated 200-lb. and 300-lb. charges off the shores of Perth. The sound waves were detected on the other side of the world in Bermuda. That’s a wild story for another time but suffice to say, point proven. For Munk, that meant if you set up listening stations around the world, you could effectively measure ocean warming in different areas as well as the ocean as a whole.


Boom, planetary temperature taken.

If this sounds complicated, well, it is. There were also issues with “sound shadows” created by continents, and underwater topography that could make calculations more difficult, though they could be overcome if the speakers were put in the right place.


But considering that scientists currently use an array of thermometers around the world and satellites coupled with equations to help cover any gaps in coverage to come up with the global average temperature, it doesn’t seem too crazy.

In some ways, what Munk imagined was a much more elegant solution. The system would be self-contained and it would take the whole planetary temperature in one fell swoop.


There was just one problem: Nobody have ever tried to do it.

To start using acoustics to take the ocean’s temperature, Munk needed to prove you could measure the time it took to travel across the ocean to the hundredths of a second. And he needed to find a location to broadcast from that could be heard at multiple other locations.


He chose Heard Island, a remote volcano in the southern Indian Ocean about 2,500 miles southwest of Perth.

“We have chosen Heard Island as an ideal site for an unimpeded transmission to Bermuda,” Munk said in a 1990 lecture organized by the National Academy of Sciences and the Office of Naval Research. “As a bonus there is also an eastward path to San Francisco, and possibly a path through the Tasman Sea to Coos Bay, Ore.”


As the 1991 experiment grew closer, Munk and his colleagues identified 16 listening sites in total. There, researchers would use hydrophones—underwater listening devices—deployed from ships and moored to the bottom of the ocean to hear the 57-hertz sound Munk was planning to play from Heard Island.

One of the hydro-acoustic low frequency transmitters used in the experiment. Photo: The National Academy of Sciences and Office of Naval Research

In early January 1991, Munk pushed off from Perth with a team of scientists aboard the M/V Cory Chouest, a Navy research ship. In addition to the crew, it carried 10 hydro-acoustic low frequency transmitters for the experiment. They looked like oversized stopwatches, and would be responsible for making the sound heard around the world.

There was just one thing missing: permits. Late in the planning stages, the U.S. and Australian governments decided the researchers would need them to blast a 57-hertz sound at their planned 200 decibels due to the impact it could have on marine mammals.


“The Heard Island Feasibility Test raised the marine mammal question in
a big way, and we still must deal with it,” Brian Dushaw, an acoustics expert at the Nansen Environmental Research Center in Norway, told Earther.

To appease the marine wildlife community, Munk and his team scrambled to get a a second ship with observers to join them at Heard Island. They ended up commissioning the R/V Amy Chouest to push off with them and ferry nine marine mammal experts from the U.S. and Australia. They still didn’t have permits in hand when the left shore. If they didn’t come through, it would have made the experiment a no-go, but they were issued a week and a day before the test began from the U.S. and Australia respectively.


The researchers dropped their array of acoustic transmitters into the sea on January 21, and the countdown began to when the tests would commence.

Lowering a hydro-acoustic low frequency transmitter into the ocean near Heard Island. Image: One Man’s Noise

The plan was to begin the experiment on January 26, Australia Day, but the engineers on board wanted to test the transmitters ahead of time to make sure they were in working order. Munk gave them the go ahead. He recounts what happened next in the 1994 documentary One Man’s Noise about his work:

“I had lived that day for two years before it started, what would it be like, would we be heard at all? We really didn’t know if manmade acoustic instruments of this kind would be loud enough to received at such distances. Just before the zero hour, the people in the shop asked if they could test equipment...

I had gone to sleep when we received a very puzzled fax from Bermuda saying ‘we picked you up and what in the hell are you doing? You’re not supposed to start until tomorrow morning.’

And I thought, ‘Oh, that is just wonderful that that question has been answered.’”


The M.V Cory Chouest was able to broadcast 35 times over the next five days before rough seas put an abrupt halt to the experiment. The transmitters were all damaged or destroyed during a storm that churned up 30-foot waves and gale-force winds.

Researchers pull the broken hydro-acoustic low frequency transmitters out of the ocean following a powerful storm. Image: One Man’s Noise

But the 35 transmissions (out of a planned 48) were enough to prove beyond a doubt that sounds could be detected accurately across vast tracts of the ocean. Researchers from India to Bermuda to British Columbia were able to record the sound.

The results were so accurate, Munk was even able to track the path the sound took from Heard Island to distant points. Using data from a hydrograph on Whidbey Island in Puget Sound, he was able to chart a line from Heard Island through what he dubbed the “Polynesian window” south and east of New Zealand. In contrast, researchers in Japan heard nothing, suggesting that oceanic ridges near Tonga and Fiji may have blocked it. The results didn’t give researchers a whole Earth temperature, but they proved the proof of concept and spurred further research into the topic. A similar experiment took place in the Arctic Ocean in 1994, and the results were later confirmed by submarine measurements.


The Acoustic Thermometry of Ocean Climate (ATOC) experiment began broadcasting signals from Pioneer Seamount off central California in 1995 and Kauai in 1997, but both sites were shut down by 1999.

Noise from ships and oil exploration have been shown to have negative impacts on species from whales to shrimp. The observers at Heard Island saw no ill impacts of the experiment, though they were limited solely to ship observations.


Similarly, the ATOC findings show the experiment had “no significant biological impact,” but there’s still room for more research on what putting a permanent sound system in the ocean would mean. There’s also the matter of getting the public on board, which could be the biggest challenge.

“There are three sides to the marine mammal issue: the scientific or what is actually happening side, the regulatory side, and the public relations side,” Dushaw said. “Scientifically, we are in good shape as was demonstrated [by ATOC]. The public relations side lingers, but has died down quite a lot...even though the science and regulatory situation are mostly resolved.”


Scientists have continued to use passive listening to monitor ocean health, including monitoring the impacts of climate change on reefs.

Howe, the oceanographer at the University of Hawaii, thinks that the advances in passive observing since the Heard Island Feasibility Test could ultimately set up a resurgence in broadcasting again in a way that doesn’t impact marine life. And its value could extend well beyond climate change.


“Acoustic methods are one of the few ways to know and measure large scale temperatures. It’s very accurate,” he said. “I look at it as extending our terrestrial infrastructure into the ocean that is our last frontier in some sense.”

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