Laboratory research aimed at altering ice at its pressure-melting point resembled the action of twisting a bagel at both ends to mix the cream cheese inside, according to fresh findings. This research has the potential to create more precise models of temperate glacier ice and enhance forecasts related to glacier movement and rising sea levels.
Neal Iverson shared two foundational insights about ice physics when discussing a recently published research paper concerning glacier ice flow from the journal Science.
Initially, the eminent professor emeritus from Iowa State University’s Department of the Earth, Atmosphere, and Climate explained that glaciers contain various types of ice. Some glacier regions exist at their pressure-melting temperature, resulting in soft, watery ice.
This temperate ice is comparable to an ice cube placed on a kitchen counter, where water collects between the ice and the surface, he noted. Studying and understanding temperate ice has been challenging.
In contrast, other regions of glaciers contain cold, hard ice, similar to ice cubes that remain frozen. This is the type of ice that is primarily analyzed and served as the foundation for glacier flow models and projections.
The new research, titled “Linear-viscous flow of temperate ice,” focuses on the former, as mentioned by Iverson, who co-authored the paper and oversaw the project.
The paper outlines laboratory experiments and findings suggesting that a standard value in the “empirical foundation of glacier flow modeling” — an equation called Glen’s flow law, named after late British ice physicist John W. Glen — needs adaptation for temperate ice.
When updated, this value in the flow law “will predict smaller increases in flow velocity in response to higher stresses from ice sheet shrinkage as the climate warms,” Iverson explained. This indicates that models may reflect reduced glacier flow into oceans and foresee less sea-level rise.
A pressing need to understand warm glacier ice
Inside Iverson’s campus lab, a 9-foot-tall ring-shear device simulates glacial forces and movement, operational since 2009, funded by a $530,000 grant from the National Science Foundation. The present study also received support from NSF grants.
The core of this device comprises a 3-foot-wide, 7-inch-thick ice ring. Beneath it lies a hydraulic press capable of applying up to 100 tons of force to mimic the weight of an 800-foot-thick glacier. Surrounding the ice ring is a bath of circulating fluid that maintains the ice temperature to a hundredth of a degree. A plate with grippers driven by electric motors above the ice ring can spin the ice at speeds from 1 to 10,000 feet annually.
For this research, the team enhanced the device by introducing another gripper beneath the ice ring, enabling rotation of the upper gripper to shear the ice below.
Collin Schohn, a former master’s student at Iowa State and now a geologist at BBJ Group in Chicago, took the lead as the first author of the current research paper. He conducted a series of six experiments with the modified device, each lasting roughly six weeks. These experiments included measuring the ice’s liquid water content, an approach last used in the 1970s for similar studies.
“The experiments focused on deforming the ice at its melting temperatures while varying the stresses,” Schohn remarked.
Iverson compared these experiments to the act of gripping a bagel at both ends and twisting to spread the cream cheese inside.
The experimental results indicated that ice deformed at a speed directly proportional to the stress exerted, Iverson noted. Conventional thought would suggest that with increasing stress, the ice would soften, leading to progressively larger increases in speed.
Why is this significant?
Temperate ice is prevalent near the bases and edges of the fastest-moving sections of ice sheets and in rapidly flowing mountain glaciers, both of which contribute ice to oceans and affect sea level. “Thus, accurate modeling and forecasting of warm glacier ice flow is increasingly crucial,” stated the authors.
Updating n to 1.0
Glen’s flow law is expressed as: ε ̇= AÏ„n.
This equation connects the stress on ice, τ, with its deformation rate, ε ̇, where A denotes a constant based on a specific ice temperature. The findings from the new experiments suggest that the stress exponent value, n, should be revised to 1.0, in contrast to the often-assumed values of 3 or 4.
The authors noted, “For generations, the stress exponent value n has been considered to be 3.0 based on Glen’s original experiments and numerous subsequent studies primarily involving cold ice (-2 degrees C and lower).” (They also observed that other research related to “cold ice in ice sheets” has indicated even higher values for n, reaching 4.0.)
The reason for this was largely attributed to the challenges posed by conducting experiments with ice at the pressure melting temperature, mentioned co-author Lucas Zoet, a former postdoctoral research associate at Iowa State and now the Dean L. Morgridge Associate Professor of geoscience at the University of Wisconsin-Madison. Zoet, who also co-supervised the project, has constructed a smaller version of the ring-shear device with transparent walls for his laboratory.
However, the data from the extensive shear-deformation experiments conducted in Iverson’s lab prompted a reevaluation of the previously assigned n value. The team determined that temperate ice behaves in a linear-viscous manner (n = 1.0) “under typical liquid water content and stress conditions commonly found near glacier beds and ice stream margins,” they asserted.
The authors proposed that melting and refreezing occurring along the boundaries of individual millimeter-to-centimeter sized ice grains likely causes this effect, which should manifest at rates that are linearly proportional to the applied stress.
This new data provides modelers with the opportunity “to ground their ice sheet models in physical relationships verified in the laboratory,” Zoet remarked. “Enhancing this knowledge leads to better accuracy in predictions.”
It took a significant amount of dedication to gather the data supporting the new n value.
“We’ve discussed this project for several years,” Schohn expressed. “Getting it to function properly was quite difficult.”
In conclusion, Iverson stated, “taking into account all the setbacks and developments, this endeavor spanned about a decade.”
This lengthy venture, according to the researchers, will be vital for creating more accurate models of temperate glacier ice and improving forecasts of glacier flow and sea-level rise.
