New research shows temperate glacier ice flows more steadily than previously thought, leading to lower projections of sea-level rise. [emphasis, links added]
Neal Iverson started with two lessons in ice physics when asked to describe a research paper about glacier ice flow that has just been published by the journal Science.
First, said the distinguished professor emeritus of Iowa State University’s Department of the Earth, Atmosphere, and Climate, there are different types of ice within glaciers.
Parts of glaciers are at their pressure-melting temperature and are soft and watery.
That temperate ice is like an ice cube left on a kitchen counter, with meltwater pooling between the ice and the countertop, he said. Temperate ice has been difficult to study and characterize.
Second, other parts of glaciers have cold, hard ice, like an ice cube still in the freezer. This is the kind of ice that has typically been studied and used as the basis of glacier flow models and forecasts.
The new research paper deals with the former, said Iverson, a paper co-author and project supervisor.
The paper describes lab experiments and the resulting data that suggest a standard value within the “empirical foundation of glacier flow modeling” – an equation known as Glen’s flow law, named after the late John W. Glen, a British ice physicist – should be changed for temperate ice.
The new value when used in the flow law “will tend to predict increases in flow velocity that are much smaller in response to increased stresses caused by ice sheet shrinkage as the climate warms,” Iverson said.
That would mean models will show less glacier flow into oceans and project less sea-level rise. …snip…
Resetting n to 1.0
Glen’s flow law is written as: ε ̇ = Aτn.
The equation relates the stress on ice, τ, to its rate of deformation, ε ̇, where A is a constant for a particular ice temperature.
Results of the new experiments show that the value of the stress exponent, n, is 1.0 rather than the usually assigned value of 3 or 4.
The authors wrote, “For generations, based on Glen’s original experiments and many subsequent experiments mostly on cold ice (-2 degrees C and colder), the value of the stress exponent n in models has been taken to be 3.0.” (They also wrote that other studies of the “cold ice of ice sheets” have placed n higher yet, at 4.0.)
That was, in part, “because experiments with ice at the pressure melting temperature are a challenge,” said Lucas Zoet, a paper co-author, a former postdoctoral research associate at Iowa State and the Dean L. Morgridge Associate Professor of geoscience at the University of Wisconsin-Madison.
Zoet, a co-supervisor of the project, has built a slightly smaller version of the ring-shear device with transparent walls for his laboratory.
However, data from the large-scale, shear-deformation experiments in Iverson’s lab has raised questions about the assigned value for n.
Temperate ice is linear-viscous (n = 1.0) “over common ranges of liquid water content and stress expected near glacier beds and in ice stream margins,” the authors wrote.
They proposed that the cause is melting and refreezing along the boundaries of individual, millimeter-to-centimeter scale grains of ice, which should occur at rates linearly proportional to the stress.
These new data allow modelers “to base their ice sheet models on physical relationships demonstrated in the laboratory,” Zoet said. “Improving that understanding improves the accuracy of predictions.”
It took some perseverance to get the data supporting the new value of n.
“We had been batting this project around for years,” Schohn said. “It was really hard to get this to work.”
In the end, Iverson said, “Considering all the failures and development, this was about a 10-year process.”
A long process, the researchers said, that’s essential for more accurate models of temperate glacier ice and better predictions of glacier flow and sea-level rise.
Top photo by Vince Gx on Unsplash
Read full article at SciTechDaily
