When glacial ice sheets
melt, something counterintuitive happens to sea levels. Logic might suggest
that nearby levels would rise, but instead they fall. Thousands of miles away,
however, they do go up in a kind of seesaw effect. Why? The answer is that water
disperses away owing to the loss of gravitational pull toward the ice sheet.
The patterns of how that happens are
called sea level fingerprints, since each incidence is unique. Elements of the
concept — which lies at the heart of the understanding that global sea levels
don’t rise uniformly — have been around for more than a century, and modern sea
level science has been built around it. But there’s long been a knowledge gap
in the widely accepted theory. A sea level fingerprint has never definitively been
detected by researchers.
A team of scientists — led by Harvard
alumna Sophie Coulson and including Harvard geophysicist Jerry X. Mitrovica — believe they have detected the first. The findings
are described in a new study published Thursday in Science. The work validates almost a century of sea level
science and helps solidify confidence in models predicting future changes,
projections that have grown more important on the warming planet.
“In sea level physics, almost everyone
assumed that the fingerprints existed, but they had never been detected at a
comparable level of confidence.”
“Ocean level projections, urban and
coastal planning — all of it — has been built on the idea of fingerprints,”
said Mitrovica, the Frank B. Baird Jr. Professor of Science in the Department
of Earth and Planetary Sciences. “That’s why fingerprints are so important.
They allow you to estimate what the geometry of the sea level changes is going
to be like … so we now have much more confidence in how sea level changes are
going to evolve. … If fingerprint physics wasn’t correct, then we’d have to
rethink all modern sea level research.”
Sea level fingerprints have been
notoriously difficult to track because of the major fluctuations in ocean
levels brought on by changing tides, currents, and winds. That presents
researchers with the challenge of trying to detect millimeter level motions of
the water and link them to melting glaciers thousands of miles away.
Mitrovica compared the search to the one
for the subatomic particle the Higgs Boson.
“Almost all physicists thought that the Higgs existed, but it was nevertheless a transformative accomplishment when it was firmly detected,” Mitrovica said. “In sea level physics, almost everyone assumed that the fingerprints existed, but they had never been detected at a comparable level of confidence.”
The new study uses newly
released satellite data from a European marine-monitoring agency that captured
more than 30 years of observations around the Greenland Ice Sheet and much of
the ocean close to Greenland to capture rising and falling ocean levels from
the fingerprint.
The satellite data caught the eye of
Mitrovica and colleague David Sandwell of the Scripps Institution of Oceanography.
Typically, records from this region had only extended up to the southern tip of
Greenland, but in this new release the data reached 10 degrees higher in
latitude, allowing the scientists to eyeball a potential hint of the seesaw
caused by the fingerprint.
Mitrovica quickly turned to Coulson, a
former doctoral student in his lab and now a postdoctoral fellow at Los Alamos
National Laboratory, to verify whether this was truly the fingerprint signal
sea level scientists had been chasing for decades.
“She was the best person to … accurately
model what the fingerprint would look like given our understanding of how the
Greenland Ice Sheet has been losing mass, and she could establish whether that
prediction matched the satellite observation,” Mitrovica said.
Coulson, an expert in modeling sea level
change and crustal deformation associated with melting ice sheets and glaciers,
was visiting family in the U.K. when the data sets hit her inbox. She
immediately recognized the potential, she said.
Coulson quickly collected three decades
worth of the best observations she could find on ice height change within the
Greenland Ice Sheet as well as reconstructions of glacier height change across
the Canadian Arctic and Iceland. She combined these different data sets to
create predictions for the region from 1993 to 2019, which she then compared
with the new satellite data. The fit was perfect: a one-to-one match that
showed with more than 99.9 percent confidence that the pattern of sea level
change revealed by the satellites is a fingerprint of the melting ice sheet.
“I was completely amazed. There it was —
a sea level fingerprint, proof of their existence,” Coulson said. “This was a
really, really exciting moment for all of us. There are very few moments in
science which provide such simple, remarkable clarity on complex Earth
processes.”
“This work, led so remarkably by Sophie,
is one of the highlights of my career, a bookend to all the theoretical and
computational work we’ve built with a community of international colleagues,”
added Mitrovica, whose group was the first to present models and predictions of
what sea level fingerprints should look like.
In scientific research it usually takes
years to develop results and then draft a paper, but here the researchers were
able to act quickly. In total, the process took only a few months from when
they saw the satellite data to when they submitted the piece.
That’s because much of the theory,
technology, and methods had already been well developed and refined since
Mitrovica and his team presented their work on sea level fingerprints about 20
years ago — computations that were widely accepted and have been factored into
almost all models predicting sea level rise.
Now that the first sea level fingerprint
has been detected, the question with the biggest global implications is where
it all leads.
“More detections will come,” Mitrovica said. “Soon the full power of fingerprint physics will be available to project sea level changes into the next decade, century, and beyond.”
Journal article: https://www.science.org/doi/10.1126/science.abo0926
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