Greenland’s sprawling mile-thick ice sheet is shedding water at an alarming rate as the planet warms, causing sea levels to rise, but the how this glacial meltwater behaves has not been well understood.

New research conducted by an international team of geologists has developed a way to characterize how it moves through the ice and the timing of the meltwater’s exit into the ocean. The ice sheet temporarily holds a surprisingly large amount of meltwater during the Arctic summer before it reaches the sea, the researchers discovered.

The team’s novel methodology, based on satellite positioning data gathered by a network of geodetic stations along Greenland’s ice-free coast, will help improve the models scientists use to predict changes in sea level in response to climate change, said Tonie van Dam, a University of Utah geology professor who contributed to the research led by Denmark’s Delft University of Technology. Using this data, scientists may be able to better forecast sea levels as far out as a century into the future.

“We could start planning for that before we had this simple picture of what was happening. It was getting warmer, the snow was melting and going into the oceans, but then we learned that’s not quite how it is,”  said van Dam, who co-authored the study published in Nature along with U colleague Lin Liu, a visiting scholar from the Chinese University of Hong Kong.

“The surface is melting, but it’s also greasing the bottom of glaciers, so our glaciers are moving faster as well,” van Dam continued. “So every time we learn something like this, it helps us and fine-tune the simple model so that we can have better predictions about what sea level is going to do.”

And just as important is the team’s insights into interpreting data from the Greenland GPS Network (GNET) in new ways that illuminate the thawing of Greenland’s vast collection of glaciers.

“Understanding how water is stored and released within the ice sheet has always been a challenge,” said team leader Pavel Ditmar in a news release Delft University issued.

About 55% of global sea level rise since 2000 can be attributed to meltwater, and Greenland’s shrinking ice sheet is the largest contributor. It is melting faster than the larger ice sheet covering Antarctica.

Greenland’s ice sheet is shaped like an upside bowl, thickest at the middle and tapering around the sides.

“Most melting is not occurring at the top because it’s very cold and dry,” Liu said. “Most melting occurs near the edge of the ice.”

The ice sheet averages a mile in thickness, covering 660,000 square miles, or 80% of Greenland, the world’s largest island. It is currently losing ice at the fastest rate in at least 12,000 years. Today it holds 700,000 cubic miles of ice. Were it all to melt, sea levels would rise about 7 meters, rendering coastal areas around the world uninhabitable and reordering ecosystems on a massive scale.

Meanwhile, meltwater’s journey from the ice sheet to the North Atlantic and Arctic oceans is not straightforward. Much of it forms lakes on the ice then flows into layers of compressed snow, known as firn, glacial cavities or lingers at ice-bedrock interface. How much water is stored in these places and for how long has been an open question that should be answered to enable scientists to accurate predict how Greenland meltwater will affect global sea levels, according to van Dam.

The study describes an innovative way to track this water’s movement using positioning data generated by a network of geodetic stations that have been deployed along the Greenland coast for years.

Van Dam herself helped install some of the first stations on the island back in the 1990s, and the network has since grown to 58 GNET stations, now maintained by the Danish government after years of U.S. National Science Foundation support.

The equipment measures the minute movements of the bedrock, both vertically and horizontally, as water moves around the ice sheet and out to sea. In other words, the ground rises and falls ever so slightly in response to the loss and return of ice.

“This paper reported a residual of an observation after making many corrections and removing many signals. After doing that, we’re talking about five millimeters, which is very small, and that’s on the seasonal scale, from May to July,” Liu said. “We can show how much water has been temporarily retained or stored and how that is expressed in terms of the time evolution over a season from May to July to October, and how that retention varies across the peripheral region of Greenland.”

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The study was published on Oct. 30, 2024, in Nature under the title, Vertical bedrocks shifts reveal summer water storage in Greenland ice sheet. Other institutions involved with this research include the Southern University of Science and Technology in China; Utrecht University in the Netherlands; and Technical University of Denmark. Funding came from myriad sources, including  National Natural Science Foundation of China and Netherlands Earth System Science Centre.

• Brian Maffly Science writer, University of Utah Communications
  801-573-2382 brian.maffly@utah.edu