Knowledge of the mechanisms for ice flow in glaciers is
incomplete but new research is adding more details to that knowledge. The new
knowledge involves mechanisms of brittle deformation produced by “ice quakes,”
small seismicity events within the ice. Implications of the new research
findings include better predictions of future sea level changes. Currently,
there is a contrast between modeled and observed sea level changes that the new
research may be able to bridge by adjusting the models. The new research
justifies changing those models. Specifically, it leads to lower projections
for future sea level rise that should be included in future climate modeling.
A January 2025 paper by
Schohn et al. in the journal Science addresses shear deformation in ice. The
authors note that the standard equation for ice flow, Glen’s Flow Law, needs to
be revised to account for temperate ice, which is described below.
Specifically, the new research predicts that actual ice flow velocities will be
less than those predicted by Glen’s Flow Law. This paper was based on
sophisticated laboratory experiments.
A February 2025 paper by
Fichtner et al. in the journal Science describes the new mechanisms that were
discovered within the Greenland Ice sheet with borehole fiber optics.
New research reported in two
different papers in Science addresses what is called ‘temperate ice’ in
contrast to ‘cold ice’ which is colder and less vulnerable to slippage. The
temperate ice has some liquid water at grain boundaries that allows it to flow.
Increased pressure at depth causes the slight melting at these boundaries. The
presence of small grains of sulfates from nearby volcanoes sets the conditions
for the ice to flow.
Thus, we have two different
papers, one based on laboratory experiments and the other based on borehole
fiber optics, that address and attempt to quantify total ice flow by taking
into account temperate ice flow rather than just cold ice flow.
Below is the abstract to the
Schohn et al. paper.
Abstract
Accurately modeling the deformation of temperate glacier
ice, which is at its pressure-melting temperature and contains liquid water at
grain boundaries, is essential for predicting ice sheet discharge to the ocean
and associated sea-level rise. Central to such modeling is Glen’s flow law, in
which strain rate depends on stress raised to a power of n = 3 to 4. In sharp
contrast to this nonlinearity, we found by conducting large-scale,
shear-deformation experiments that 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. This linearity is likely caused by diffusive
pressure melting and refreezing at grain boundaries and could help to stabilize
modeled responses of ice sheets to shrinkage-induced stress increases.
The abstract to the Fichter
et al. paper is shown below.
Abstract
Ice streams are major regulators of sea level change.
However, standard viscous flow simulations of their evolution have limited
predictive power owing to incomplete understanding of involved processes. On
the Greenland ice sheet, borehole fiber-optic observations revealed a brittle
deformation mode that is incompatible with viscous flow, over length scales
similar to the resolution of modern ice sheet models: englacial ice quake
cascades that are unobservable at the surface. Nucleating near volcanism-related
impurities that promote grain boundary cracking, the ice quake cascades appear
as a macroscopic form of crystal-scale wild plasticity. A conservative estimate
indicates that seismic cascades are likely to produce strain rates that are
comparable in amplitude with those measured geodetically, providing a plausible
missing link between current ice sheet models and observations.
According to Discover Magazine’s Avery Hurt:
"Inside the ice stream are thin layers of sulfates
left over from volcanoes," explains Fichtner. "These impurities make
the ice in these areas a little weaker than the surrounding ice, and the
stresses localize near these weak layers, causing them to crack. This is what
produces the ice quakes."
This discovery shows that ice streams move with what
Fichtner calls a "stick-slip" motion rather than always flowing
smoothly like viscous honey.
Glacier ice flows by means of many different mechanisms,
explains Kristin Poinar, a University at Buffalo scientist who studies the
Greenland Ice Sheet.
"It can ooze slowly and viscously; it can move rapidly
and elastically," she says. "What we haven't appreciated before is
that these micro-slip events might add up to be fairly significant to the
overall amount of flow."
Fichtner plans to study
alpine glaciers next to determine if they also follow the newly discovered flow
mechanism. It makes me wonder if there will be enough volcanic ash or fallen
aerosols to make grain layers that can flow similarly to the glaciers in
Greenland.
References:
Ice
Quakes Cause Glacial Ice to Flow Toward the Ocean. Avery Hurt. Discover Magazine.
March 26, 2025. Ice
Quakes Cause Glacial Ice to Flow Toward the Ocean
Hidden
cascades of seismic ice stream deformation. Andreas Fichtner, Coen Hofstede,
Brian L. N. Kennett, Anders Svensson, Julien Westhoff, Fabian Walter, Jean-Paul
Ampuero, Eliza Cook, Dimitri Zigone, [...] , and Olaf Eisen. Science. February
6, 2025. Vol 387, Issue 6736. pp. 858-864. DOI: 10.1126/science.adp8094. Hidden cascades of
seismic ice stream deformation | Science
Linear-viscous
flow of temperate ice. Collin M. Schohn, Neal R. Iverson, Lucas K. Zoet, Jacob
R. Fowler, and Natasha Morgan-Witts. Science. January 9, 2025. Vol 387, Issue
6730. pp. 182-185. DOI: 10.1126/science.adp7708. Linear-viscous flow
of temperate ice | Science
Glacier Experts Uncover Critical Flaw in Sea-Level Rise Predictions. Iowa State University. SciTech Daily. January 14, 2025. Glacier Experts Uncover Critical Flaw in Sea-Level Rise Predictions
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