Weighing the Antarctic ice sheet

Two grants will help scientists predict stability of the ice as climate warms

This is the second in a series of articles that describe how scholars at Washington University in St. Louis are bringing their varied skills to bear on the issue of climate change and global warming.

Edge of the Ross Ice Shelf. About 16 stories of ice are visible above the water line. Doug Wiens of Washington University in St. Louis is the principal investigator on a new grant to install seismographs on the shelf to learn more about its likely response to continued warming. (Credit: Courtesy of Lin Padgham)


If it melted entirely, global sea levels would rise nearly 200 feet. So understanding how the ice sheet is affected by global climate change is extremely important.

But it is also very difficult both because the ice’s response to warming is inherently complex and because there is little data to go on. Temperature records exist for only a few locations along the perimeter of the continent, and even those go back only 50 or 60 years.

To reach further back in time and provide a long-term record that can inform global climate models, scientists are turning to other means of measuring ice mass.

One of these is post-glacial rebound. At the end of the last glacial maximum, when ice sheets reached their maximum extent 20,000 to 25,000 years ago, the ice covering Antarctica was even thicker than it is today. As the cold eased and some of the ice melted, the land mass began to rebound, flexing slowly upward.

The rebound can be used to weigh the ice sheet and determine how much mass it has lost and is currently losing, said Doug Wiens, PhD, professor of earth and planetary sciences in Arts & Sciences at Washington University in St. Louis.


Calibrating the ice-mass scale

Terraces on the beach in Bathurst Inlet on the northern coast of Canada were formed as the Earth’s crust rebounded after the ice that covered it during the last glacial maximum melted. Antarctica is expected to respond differently than Canada, however, because of differences in the mantle beneath the crust. (Credit: Courtesy of Mike Beauregard)


During the last glacial maximum, Wiens explained, the weight of the ice bent the Earth’s crust, forcing the plastic rock in the upper layer of the Earth’s mantle to flow away from the loaded region.



The rate of rebound, however, depends on the viscosity of the mantle material, which in turn depends on temperature. It’s like the difference between honey stored in the refrigerator and in the cupboard. The cold honey is more viscous and flows more slowly than warm honey.

Listening in on West Antarctica




One of the NSF grants Wiens recently was awarded is a
continuation of POLENET that will fund the addition of 13 West Antarctic
seismic stations to the 32 already in the POLENET Antarctic network.


The stations are installed and maintained by WUSTL’s Patrick Shore, PhD, a computer specialist and lecturer in earth and planetary sciences; IRIS-PASSCAL, a consortium of research universities interested in seismology; together with Wiens and several graduate students.


“We see a lot of melting,” Wiens said, “although it is localized to certain areas.” East Antarctica, in contrast, is an elevated plateau, which seems to be gaining rather than losing ice mass, Wiens said.

“We know the mantle beneath West Antarctica is different from that under Canada or Sweden,” he said. “The ice sheets in the north melted completely about 8,000 years ago and it will take 20,000 years for the crust to rebound. So we know that the mantle there responds over a long period of time.



“We’re building a picture of the temperature structure that will constrain numerical models of how Antarctica responds to changes in ice mass,” Wiens said. This in turn will provide a means to answer critical questions about the ice sheet’s behavior in a warming world.


“We know it can happen, because it has happened in the geologic past at temperatures not much different from where we are now and certainly where we are headed,” he said.

Listening in on the Ross Ice Shelf

Larsen B breaking up in February 2002 as seen by Landsat satellite. (Credit: Image courtesy Landsat 7 Science Team and NASA GSFC )

In addition to West Antarctica, Antarctica’s ice shelves are a focus of scientific concern. The shelves are thick platforms of ice that ride on top of the water off some parts of the coastline.

There are many ice shelves but the biggest (about the size of France) is the Ross Ice Shelf, tucked into a deep embayment in West Antarctica. “This is the ice shelf that Scott, Amundsen and Shackleton crossed at the start of their journeys to the Pole,” Wiens said.




“Stations have been deployed on the land side of Erebus,” Wiens said, “and now we’re deploying them across the shelf on the ocean side.”

The second goal, however, is to learn more about the ice shelf itself, including its response to long-period ocean waves that lift and drop it. How the shelf responds to wave forcing will tell us how strong the ice is, Wiens said.



Again Wiens is careful not to create misconceptions. “I don’t think anyone thinks the Ross Ice Shelf is going to break up right away,” he said. “But other smaller ice shelves have broken up and they seem to break up suddenly, so I think there’s a long-term issue.”

In the meantime, he is just happy to be getting better data. “To see these questions slowly being answered is very exciting to me,” he said. “When we started POLENET six years ago, we had no idea what we would find.

“Now we’re beginning to fill in the details — and we’ve made some unexpected discoveries.” One of which is a new volcano discovered under the ice by Amanda Lough, a WUSTL doctoral student he advises.

NSF-funded research in this story: Douglas Wiens, Washington University, Award No. 1246712; Douglas Wiens, Washington University, Award No. 114518