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New Work Reveals First Hard Evidence Of Elusive Sea Level Fingerprint

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When ice sheets melt, something unusual and totally unexpected happens to sea levels.

It behaves much like a seesaw. Ocean levels drop in the vicinity of where these glacial ice masses melt. But they do rise thousands of miles away. It happens mostly because the ice sheet loses its gravitational pull, which makes the water spread out.

The patterns are called “sea level fingerprints” because each glacier or ice sheet that melts has a different effect on sea level.

Modern sea level science has been built around elements of the concept, which is at the core of the understanding that global sea levels don’t rise uniformly. It has been around for more than a century.

But the theory that most people believe has had a problem for years. Researchers have never been able to unequivocally identify a sea level fingerprint in their studies.

A group of scientists led by Sophie Coulson, a Harvard graduate, and including Jerry X. Mitrovica, a Harvard geophysicist, think they have found the first.

The findings were reported in a new study that was just released today in the journal Science. The finding confirms almost a decade of sea level science and helps consolidate trust in models projecting future sea level rise.

“Ocean level projections, urban and coastal planning — all of it — has been built on the idea of fingerprints,” explains 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.”

Changing tides, currents, and winds make it difficult to detect sea-level fingerprints. What makes it such a puzzle is that researchers are trying to identify millimeter-level motions of the water and link them to melting glaciers thousands of kilometers away.

Mitrovica linked the hunt 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 adds. “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 report uses newly published satellite data from a European marine monitoring agency that gathers over 30 years of observations in the region of the Greenland Ice Sheet and much of the ocean near the centre of Greenland to catch the seesaw in ocean levels from the fingerprint.

Mitrovica and his Scripps Institute of Oceanography colleague David Sandwell were intrigued by the satellite data.

In the past, satellite records from this region had only extended as far as the southernmost point of Greenland. However, in this latest release, the data went ten degrees higher in latitude, which enabled them to get a better look at a possible clue of the seesaw generated by the fingerprint.

Mitrovica instantly contacted Coulson, a former Ph.D. student in Mitrovica’s lab who is now a postdoctoral fellow at Los Alamos National Laboratory, in order to confirm whether or not this was in fact the fingerprint signal sea level scientists had been searching 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,” according to Mitrovica.

Coulson was in the U.K. to see his family. He is an expert at modeling changes in sea level and crustal deformation caused by melting ice sheets and glaciers. as soon as the datasets were delivered to her email, she instantly recognized the potential, she claimed.

Coulson quickly put together the best observations she could find from the past 30 years about how the height of the ice in the Greenland Ice Sheet had changed, as well as reconstructions of how the height of glaciers had changed in the Canadian Arctic and Iceland.

She combined these various datasets to produce predictions of the region’s sea level change from 1993 to 2019, which she then contrasted with the fresh satellite data.

It fit perfectly. A one-to-one match showed with more than 99.9% certainty that the pattern of sea level change shown by 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 says. “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,” adds Mitrovica, “is one of the highlights of my career, a book end to all the theoretical and computational work we’ve built with a community of international colleagues.”

The scientific study generally takes years to generate the results and then get drafted into a report, but here the researchers were able to act fast. 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 research had already been completed. Since Mitrovica and his team released their work on sea level fingerprints roughly 20 years ago, a lot of the theory, technology, and methodologies have all been well established and progressed. These calculations were widely accepted and have been incorporated into practically all models projecting sea level rise.

“This was high risk, high reward science and no one expected a detection this quickly. We benefited an incredible amount from the groups supporting us, notably the Star-Friedman Challenge,” Mitrovica remarks.

The question with the biggest global repercussions is where this all leads now that the first sea-level fingerprint has been found.

“More detections will come,” Mitrovica adds. “Soon the full power of fingerprint physics will be available to project sea level changes into the next decade, century, and beyond.”

Source: 10.1126/science.abo0926

Image Credit: Getty

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