Better predictions on rise of oceans on warming Earth

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, Ph.D. ’21, 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.”

Jerry X. Mitrovica, Harvard geophysicist

“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 at the Graduate School of Arts and Sciences 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.”

This work was partially supported by the Star-Friedman Challenge for Promising Scientific Research, the John D. and Catherine T. MacArthur Foundation, and the Los Alamos National Laboratory Director’s Postdoctoral Fellows Program.