Ocean’s hidden heat measured with earthquake sounds
In 1991,
scientists lowered large subwoofers into the water at Heard Island, a
snowcapped volcanic island in the Indian Ocean. The speakers emitted
low-frequency sounds that, like whale song, rumbled across entire oceans.
Picked up by receivers off the coasts of California and Bermuda, the signals
contained a crucial piece of information about the water they had
traversed: how
hot it was. It was a promising way to monitor Earth’s warming oceans,
but concerns about how the underwater noise might affect marine life soon
sidelined it, with only a few dedicated scientists keeping the technique alive.
Now, it is back—only this time, Earth itself is providing the noise.
A team of
seismologists and oceanographers has shown that small earthquakes repeatedly
emanating from the same spot beneath the ocean floor can take the place of the
subwoofers. The quakes generate reliable acoustic signals for measuring ocean
temperatures, including at depths below 2000 meters, beyond the reach of other
techniques. If validated, the approach, published today in Science,
could open an
entirely new ocean observation system for understanding past and
future climate change, says Frederik Simons, a geophysicist at Princeton
University unaffiliated with the study. “There’s a potential treasure trove of
data waiting to be analyzed.”
The
oceans absorb more than 90% of the energy trapped by global warming, and any
change in the rate at which they soak up heat would have an outsize impact on
how fast the atmosphere warms. Two decades ago, robotic floats from the
international Argo array began to monitor the warming of the ocean to a depth of about
2000 meters. But the float array, now 4000 strong, could not probe the
large volume of water at greater depths. “The inability to determine what is
going on in the deep water is a major obstacle to understanding the ocean and
climate, even today,” says Carl Wunsch, a retired oceanographer from the
Massachusetts Institute of Technology.
In 1979,
Wunsch and Walter Munk, an oceanographer at the Scripps Institution of
Oceanography who died last year, first proposed using sound waves to measure
the ocean’s heat and structure. Sound travels faster as water grows hotter or
denser, making its travel time a reliable gauge of temperature and density if
the sound source and receiver are at fixed locations.
The
technique did not require especially loud sources. At a depth of about 1000
meters, the speed of sound hits a minimum, forming a conductive channel between
warm waters above and dense water below. This waveguide enables sound waves to
coast across entire ocean basins, says Bruce Cornuelle, a Scripps oceanographer
who worked with Munk. “It’s like a 5-year-old grabbing a wrapping paper tube
and yelling in his brother’s ear.”
Besides
probing the entire width of an ocean, the sound waves—with vertical amplitudes
of thousands of meters—capture conditions from shallow waters all the way down
to the abyss. As a result, they average out smaller scale natural temperature
fluctuations, revealing basinwide changes of just a few thousandths of 1° per
year. “That makes it much easier to extract the global warming signal,” says
Jörn Callies, an oceanographer at the California Institute of Technology
(Caltech) and co-author on the new study.
After the
1991 demonstration at Heard Island, Munk won Department of Defense funding for
a follow-up experiment in the Pacific Ocean, called Acoustic Thermometry of
Ocean Climate (ATOC). But it became mired in controversy over its two
human-size speakers, placed off the coasts of Hawaii and California in prime
whale territory. “It became a political nightmare,” says Brian Dushaw, a
retired oceanographer who worked on ATOC. ATOC’s signals were no louder than
whale calls and ship traffic, but much of its $35 million budget went to
studies of the sound’s impact on marine mammals.
Military
secrecy also got in the way. To hear the signals, the project relied on
classified Navy hydrophones normally used to detect submarines. The scientists
couldn’t even publish the receivers’ locations, Wunsch says. “We didn’t tell
the Navy that if you published the signal, which we did, then you could figure
out where the receivers were,” Wunsch adds. The Hawaiian source, off Kauai, ran
until 2006, providing 10 years of warming data. But by then, oceanographers had
left acoustic thermometry behind and were relying on Argo, Dushaw says.
That was
until 1 year ago, when Wenbo Wu, a Caltech seismologist, realized that
repeating earthquakes on slowly creeping faults below the sea floor could
provide an alternative sound source. When earthquakes shake the ocean floor,
some of the energy is transformed into acoustic waves. Wu and his co-authors
just had to find the right source.
Their
search went back to the Indian Ocean. In earthquake records, they identified
more than 4000 earthquakes from faults in the ocean floor west of Sumatra
in Indonesia from 2004 to 2016, many of them between magnitude 3.5
and 5. Triangulating on the source, the team identified patches of fault less
than 100 meters apart that ruptured repeatedly, says Sidao Ni, a co-author and
seismologist at the Institute of Geodesy and Geophysics of the Chinese Academy
of Sciences. The resulting sound waves traveled through the ocean unfettered to
Diego Garcia, a remote atoll south of India, where they hit land and turned
back into seismic waves, picked up on the island’s seismometer.
Converting
those travel times to temperatures, Wu and his colleagues found that the
eastern Indian Ocean warmed 0.044°C over the decade. The annual fluctuations
matched up well with Argo measures from the same time, but the warming signal
was nearly double what the Argo floats detected. The disparity
suggests Argo is missing some heat, Callies says, at least for this basin
over this short span of time. Some 40% of their heat measurement came from
water below 2000 meters, suggesting some warming is working its way deeper into
the ocean, out of Argo’s current reach.
This work
is “quite extraordinary and very promising,” says Susan Wijffels, an Argo
leader at the Woods Hole Oceanographic Institution. If extended globally, it
could provide an independent check on Argo measurements, especially when
production of a new line of Argo floats that can descend 6000 meters, currently
deployed only in the dozens, ramps up. Even more alluring for Wijffels is the
possibility of extending global warming trends back in time, before Argo, by
detecting repeaters in old seismic records. “What a gift to the climate
community that’d be,” she says.
The team
thinks it can capture the earthquake-generated sounds more cleanly with
hydrophones than with land-based seismometers. That will let them use lower
power earthquakes, and by using the global network of hydrophones deployed as
part of the Comprehensive Nuclear Test Ban Treaty, they should be able to pick
up signals from repeaters throughout the world’s oceans.
Hydrophones
deployed under Arctic sea ice might gauge water temperatures in a place Argo
floats can’t reach. It might even be possible to use the crash of collapsing
ice in nearby Greenland—glacial earthquakes, as they’re known—as the sound source.
“It’s free data,” Dushaw says. “There’s no question someone will implement a
system to take advantage of this.”
The newly
bright prospects for ocean acoustic thermometry are also a validation for Munk,
who was deeply saddened when his global acoustic dreams were muted, Cornuelle
says. “I wish Walter had been around to see it. He’d be overjoyed.”
*Correction,
17 September, 4:25 p.m.: A previous version of this
story stated that 40% of the measured warming came from below 2000 meters.
Although 40% of the measured temperature came from water below 2000 meters, the
technique cannot yet say where in the water column the warming occurred.
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