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New Insights on Glacier Ice Flow: A Shift in Understanding

Neal Iverson, a distinguished professor emeritus in the Department of the Earth, Atmosphere, and Climate at Iowa State University, highlighted two key concepts in ice physics while discussing a recent research paper published in the journal Science about glacier ice flow.

First, Iverson noted that glaciers contain varying types of ice. Certain sections exist at their pressure-melting temperature, making them soft and watery. He likened this temperate ice to an ice cube melting on a kitchen counter, where liquid water forms between the ice and the surface beneath it. This type of ice has historically posed challenges for researchers aiming to study and characterize its properties.

On the other hand, other glacier areas consist of cold, solid ice, much like an ice cube stored in a freezer. This colder ice has been the primary focus of glacier flow models and predictions in the past.

The research paper, titled “Linear-viscous flow of temperate ice,” specifically addresses the workings of temperate ice, a topic that Iverson, who is also a co-author of the study, finds crucial.

This paper reports on lab experiments that suggest the need to revise an existing foundational equation in glacier flow modeling called Glen’s flow law, named after the late British ice physicist John W. Glen. Iverson indicated that the new findings propose a revised value for how temperate ice responds to stress, observing that it would suggest smaller increases in flow velocity as a result of ice sheet shrinkage caused by climate change. This revision is significant because it may lead to projections of reduced glacier flow into oceans, thereby impacting forecasts of sea-level rise.

The Importance of Understanding Warm Glacier Ice

In Iverson’s laboratory, an impressive 9-foot-tall ring-shear device has been at work since 2009, simulating the forces and movements experienced by glaciers. This device was established with a $530,000 grant from the National Science Foundation (NSF), which also financially supported the current study.

The central feature of the device is a ring of ice approximately 3 feet in diameter and 7 inches thick, which is subjected to up to 100 tons of force to mimic the pressure from an 800-foot-thick glacier. To control the experiment’s conditions, the ice ring is surrounded by a circulating fluid that maintains the ice temperature to a high degree of precision. Electric motors enable the experimenters to rotate the ice at varying speeds ranging from 1 to 10,000 feet per year.

For the project’s innovative modifications, researchers introduced an additional gripper beneath the ice ring. This setup allowed for the shearing of the underlying ice as the top gripper rotated.

Collin Schohn, a former master’s student at Iowa State now working as a geologist, led a series of six experiments over six weeks to analyze the ice’s liquid water content—an aspect that had not been adequately measured in similar experiments since the 1970s.

Schohn described the experiments as akin to twisting a bagel from the top and bottom, effectively mixing the cream cheese in the middle. The data revealed that ice deformation occurred at a rate directly proportional to the applied stress, which challenges traditional expectations of ice behavior under pressure.

Re-evaluating the Implications

The study’s findings are pivotal, especially since temperate ice is most prevalent in the fast-flowing regions of ice sheets and mountain glaciers, which are critical contributors to sea level changes. “The urgency to accurately predict the flow of warm glacier ice is increasingly pressing,” the authors emphasized.

Adjustments to Glen’s Flow Law

The foundational equation known as Glen’s flow law is expressed as: ε ̇= Aτn.

In this equation, τ represents the stress on ice, while ε ̇ indicates its deformation rate, with A being a constant related to the ice temperature. The new findings propose a change in the value of the stress exponent n from the commonly used values of 3 or 4 to 1.0, emphasizing a fundamental shift in approach.

The authors pointed out that for decades, models have assumed the value of n to be 3 based on earlier experiments primarily involving cold ice. However, this assumption is being challenged due to the difficulties historically encountered in studying temperate ice at the pressure melting temperature, as noted by Lucas Zoet, a co-author and associate professor at the University of Wisconsin-Madison.

Analyses from the extensive shear-deformation experiments questioned the traditional value of n, revealing that temperate ice indeed behaves as linear-viscous (n = 1.0) under expected stress scenarios found at glacier beds and ice stream margins. The authors propose that this behavior results from melting and refreezing along the boundaries of miniaturized ice grain scales, whereby this process is proportionate to the stress applied.

The introduction of this new data will enable modelers to refine their ice sheet models with empirical relationships validated through laboratory work, as Zoet elaborated: “Enhancing our understanding directly translates to improving the accuracy of predictions.”

Reaching these conclusions required perseverance. Schohn remarked on the significant challenges faced throughout the project, while Iverson acknowledged that the journey spanned nearly a decade, emphasizing the importance of these findings for better predictions regarding temperate glacier ice and subsequent sea-level rise impacts.

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