How tiny defects in ice affect glacier movement and sea level rise

Source: MIT. In between two rocky hills, an icy blue glacier flows down and meets the water. Credit: Meghana Ranganathan.

As glaciers melt and break off into the sea, they are raising global sea levels at an alarming rate. To better predict and prepare for future sea-level rise, scientists need to understand how glaciers melt and what influences their flow.

A new study by scientists at MIT offers fresh insights into glacier flow based on microscopic defects in the ice.

This study, published in the journal Proceedings of the National Academy of Sciences, shows that a glacier’s flow is strongly influenced by how tiny defects move through the ice.

By understanding these microscopic deformations, researchers developed a new model to predict how glaciers flow.

Using this model, they mapped ice flow in various locations across the Antarctic Ice Sheet.

The traditional view of ice sheets as uniform blocks is challenged by this study. Instead, the ice sheet is shown to be more varied in how it flows in response to warming.

The study’s findings significantly change our understanding of the conditions under which marine ice sheets may become unstable and cause rapid sea-level rise.

“This study shows the impact of small-scale processes on large-scale behaviors,” says Meghana Ranganathan, Ph.D., who led the study as a graduate student at MIT and is now a postdoc at Georgia Tech. “These tiny mechanisms at the molecular level can affect the stability of the West Antarctic Ice Sheet.”

“Glaciers are speeding up, and there are many variations,” adds co-author and MIT Associate Professor Brent Minchew. “This is the first study to bridge the gap from laboratory experiments to real ice sheets, helping us understand the likelihood of sudden sea-level rise.”

Microscopic ice flow

Glacier flow describes the movement of ice from the peak of a glacier or the center of an ice sheet down to the edges, where the ice eventually breaks off and melts into the ocean. This process contributes to rising sea levels.

In recent years, the oceans have risen at unprecedented rates due to global warming and accelerated glacier melting. While polar ice loss is a significant contributor to sea-level rise, it remains the biggest uncertainty in making predictions.

“Part of the challenge is scale,” Ranganathan explains. “The fundamental mechanisms that cause ice to flow happen at a very small scale that we can’t see. We aimed to pinpoint the microphysical processes governing ice flow, which haven’t been represented in sea-level change models.”

The new study builds on experiments from the early 2000s by geologists at the University of Minnesota, who studied how small ice chips deform under stress. They identified two microscopic mechanisms of ice flow: “dislocation creep,” where tiny cracks move through the ice, and “grain boundary sliding,” where individual ice crystals slide against each other.

These geologists found that ice’s sensitivity to stress depends on which mechanism is dominant. Ice is more sensitive to stress when defects occur via dislocation creep rather than grain boundary sliding.

Ranganathan and Minchew realized these microscopic findings could redefine how ice flows at larger scales.

“Current models for sea-level rise assume a single value for ice sensitivity to stress and hold this constant across an entire ice sheet,” Ranganathan explains. “But these experiments showed significant variability in ice sensitivity, depending on which mechanism is at play.”

Creating a new map

For their new study, the MIT team developed a model to estimate an icy region’s sensitivity to stress, directly relating to how likely the ice is to flow.

The model uses data such as ambient temperature, average ice crystal size, and estimated ice mass to calculate how much the ice deforms by dislocation creep versus grain boundary sliding. Based on this, the model estimates the region’s sensitivity to stress.

The scientists fed real-world data from various locations across the Antarctic Ice Sheet into the model. They then generated a map of ice sensitivity to stress across the ice sheet. Comparing this map to satellite and field measurements, they found a close match, suggesting the model could accurately predict future glacier and ice sheet flow.

“As climate change thins glaciers, it could affect ice sensitivity to stress,” Ranganathan says. “The instabilities we expect in Antarctica could be very different, and this model helps us capture those differences.”

This new understanding of glacier flow and ice sheet stability is crucial for predicting future sea-level rise and preparing for its impacts.