What can stall a glacier?

Much talk surrounds the deterioration of glaciers and ice sheets—particularly, how they are thinning and losing mass with global warming; but the mechanisms are complicated and scientists want to know more about their flow. The Antarctic ice sheet, in particular, piques interest because it contains enough fresh water to raise sea levels 60 meters (200 feet), if it were to melt completely. So how does the ground beneath the ice influence mobility? When and how does the ice sheet stall? When does it accelerate? And how does this ebb and flow contribute to global sea level rise?

Glacial dynamics

Aircraft flying over Pine Island Glacier

During a flight over the Pine Island Glacier ice shelf, the DC-8 aircraft banks over the Amundsen Sea and the clean edge of the ice shelf front. The Pine Island Glacier moves at about 1.5 miles per year. Credit: Jefferson Beck/NASA, 2011

To date, humans have never seen what lies beneath the massive Antarctic ice sheet. Now ground-penetrating radar from airplane flyovers has pierced the ice, two-miles thick at its densest in Antarctica, and through mathematical modeling, a dynamic landscape has materialized. Narrow ridges of dirt and rock under glaciers in western Antarctic form enough friction to slow down ice into the sea.

Since ice is quite heavy, ice flows under its own weight. “You can think of a huge dollop of honey or ketchup that creates a sort of cap-shape as it drips on a plate,” said Olga Sergienko, a researcher at Princeton University. “That’s pretty much what big ice sheets do.” With lots of snow, the ice sheet begins to compact under its own weight, forcing a flow from the ice sheet summits to the margins out into the sea.

Location of the two glaciers

This is a satellite image from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) of the Amundsen Sea Embayment where both the Pine Island and Thwaites glaciers are located. Blue lines outline areas where computations for basal (beneath the ice sheet) conditions were performed. White lines show the location where glaciers leave the Antarctic ground and float out into the sea. Credit: NSIDC

These semi-circular ridges, or ribs, beneath the Pine Island and Thwaites glaciers, which are losing mass at an accelerating rate, may work as a buffer against climate change. Just like a car achieves faster speeds on a highway than a dirt road, so do glaciers move differently depending on the topography beneath. “Understanding what’s going on at the base of these glaciers can help scientists figure out how these glaciers may react to current and future environmental conditions,” Sergienko said. These tiger stripes, named after Princeton’s tiger mascot, act like anti-slip strips on staircases. Friction prevents slippage.

Cracking the ice

How do scientists know these ridges exist? Scientists created a model to hypothesize what lies beneath. “We first took available observations—specifically, data collected from satellites,” Sergienko said. “And then plugged in the data into mathematical models to tell us what the most likely spatial pattern should be.” The model computes how a glacier should flow.

A hypothesized image of the narrow ribs beneath the glaciers.

The above satellite image from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) depicts an inferred basal (beneath the ice sheet) condition for Pine Island (PIG) and Thwaites Glaciers. Dark red colors indicate high basal traction, while blue colors indicate low basal traction. On both glaciers, the areas with high basal traction are organized in confined narrow ribs that are embedded into much larger areas with low basal traction. Credit: NSIDC

“Then we compared the computed velocity with the observed velocity and tried to best match the two,” Sergienko said. For the match, she needed to know what the topography under the ice was doing. Basal resistance is the force underneath that slows down the ice above. It is the missing variable in the model, and since all the other factors are determined from observed data, the model can infer basal resistance. Once it is calculated, scientists have a clearer image of the type of landscape pattern beneath. In other words, the model helps the scientists see through the ice.

The art of water

So how do these tiger stripes form? These ridges form and subside in response to natural processes over roughly 50 to 100 years. Specifically, water, which melts out of the ice, slips into the space between the ice sheet and bedrock. Subglacial water pools and interacts with subglacial sediment to form regular patterns. Ice pushes on sediment, while subglacial water accumulates between bumps. Overtime, the pattern deepens and sets into ridges.

But since water keeps carving at the bedrock beneath the ice sheet, the landscape is vulnerable to shifts. “This study really brings forward the importance of the water that gets under the bed,” Sergienko said. It means that if the ribs were to flatten at some point, glaciers and ice sheets would flow faster and shed more ice mass into the ocean. Sea levels would rise. But, on the other hand, if more ridges formed elsewhere, than that would slow down the glacier flow and sea level rise would decrease. “This shows that ice sheet dynamics are very complicated processes that are determined internally,” Sergienko added. “They play together with external forces, like warming climate and oceans.” But they can also change, and knowing about these shifts may help scientists better calculate their eventual contribution to sea level rise.

Reference

Sergienko, Olga V. and Richard C. A. Hindmarsh. 2013. Regular Patterns in Frictional Resistance of Ice-Stream Beds Seen by Surface Data Inversion. Science 342, 1086-1089, doi:10.1126/science.1243903.

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