AWI Forecasts Greenland Glacier Ice Loss with Simulation

As the chief of the Modeling Group within the Section of Glaciology on the Alfred Wegener Institute (AWI) Helmholtz Centre for Polar and Marine Research in Bremerhaven, Germany, Humbert works to extract broader classes from Nioghalvfjerdsbræ’s ongoing decline. Her analysis combines information from subject observations with viscoelastic modeling of ice sheet conduct. Through improved modeling of elastic results on glacial move, Humbert and her crew search to raised predict ice loss and the ensuing influence on international sea ranges.

She is acutely conscious that point is brief. “Nioghalvfjerdsbræ is one of the last three ‘floating tongue’ glaciers in Greenland,” explains Humbert. “Almost all of the other floating tongue formations have already disintegrated.”

One Glacier That Holds 1.1 Meter of Potential Global Sea Level Rise

The North Atlantic island of Greenland is roofed with the world’s second largest ice pack after that of Antarctica. (Fig. 1) Greenland’s sparsely populated panorama could seem unspoiled, however local weather change is definitely tearing away at its icy mantle.

The ongoing discharge of ice into the ocean is a “fundamental process in the ice sheet mass-balance,” based on a 2021 article in Communications Earth & Environment by Humbert and her colleagues. (Ref. 1) The article notes that the complete Northeast Greenland Ice Stream incorporates sufficient ice to lift international sea ranges by 1.1 meters. While the complete formation will not be anticipated to fade, Greenland’s total ice cowl has declined dramatically since 1990. This strategy of decay has not been linear or uniform throughout the island. Nioghalvfjerdsbræ, for instance, is now Greenland’s largest outlet glacier. The close by Petermann Glacier was once bigger, however has been shrinking much more shortly. (Ref. 2)

Existing Models Underestimate the Rate of Ice Loss

Greenland’s total lack of ice mass is distinct from “calving”, which is the breaking off of icebergs from glaciers’ floating tongues. While calving doesn’t straight increase sea ranges, the calving course of can quicken the motion of land-based ice towards the coast. Satellite imagery from the European Space Agency (Fig. 2) has captured a fast and dramatic calving occasion in motion. Between June 29 and July 24 of 2020, a 125 km2 floating portion of Nioghalvfjerdsbræ calved into many separate icebergs, which then drifted off to soften into the North Atlantic.

Direct observations of ice sheet conduct are precious, however inadequate for predicting the trajectory of Greenland’s ice loss. Glaciologists have been constructing and refining ice sheet fashions for many years, but, as Humbert says, “There is still a lot of uncertainty around this approach.” Starting in 2014, the crew at AWI joined 14 different analysis teams to match and refine their forecasts of potential ice loss via 2100. The venture additionally in contrast projections for previous years to ice losses that really occurred. Ominously, the consultants’ predictions have been “far below the actually observed losses” since 2015, as said by Martin Rückamp of AWI. (Ref. 3) He says, “The models for Greenland underestimate the current changes in the ice sheet due to climate change.”

Viscoelastic Modeling to Capture Fast-Acting Forces

Angelika Humbert has personally made quite a few journeys to Greenland and Antarctica to collect information and analysis samples, however she acknowledges the restrictions of the direct strategy to glaciology. “Field operations are very costly and time consuming, and there is only so much we can see,” she says. “What we want to learn is hidden inside a system, and much of that system is buried beneath many tons of ice! We need modeling to tell us what behaviors are driving ice loss, and also to show us where to look for those behaviors.”

Since the Eighties, researchers have relied on numerical fashions to explain and predict how ice sheets evolve. “They found that you could capture the effects of temperature changes with models built around a viscous power law function,” Humbert explains. “If you are modeling stable, long-term behavior, and you get your viscous deformation and sliding right, your model can do a decent job. But if you are trying to capture loads that are changing on a short time scale, then you need a different approach.”

To higher perceive the Northeast Greenland Ice Stream glacial system and its discharge of ice into the ocean, researchers on the Alfred Wegener Institute have developed an improved viscoelastic mannequin to seize how tides and subglacial topography contribute to glacial move.

What drives short-term adjustments within the hundreds that have an effect on ice sheet conduct? Humbert and the AWI crew give attention to two sources of those vital however poorly understood forces: oceanic tidal motion below floating ice tongues (such because the one proven in Fig. 2) and the ruggedly uneven panorama of Greenland itself. Both tidal motion and Greenland’s topography assist decide how quickly the island’s ice cowl is transferring towards the ocean.

To examine the elastic deformation attributable to these elements, Humbert and her crew constructed a viscoelastic mannequin of Nioghalvfjerdsbræ within the COMSOL Multiphysics software program. The glacier mannequin’s geometry is predicated on information from radar surveys. The mannequin solved underlying equations for a viscoelastic Maxwell materials throughout a 2D mannequin area consisting of a vertical cross part alongside the blue line proven in Fig. 3. The simulated outcomes have been then in comparison with precise subject measurements of glacier move obtained by 4 GPS stations, one among which is proven in Fig. 3.

How Cycling Tides Affect Glacier Movement

The tides round Greenland sometimes increase and decrease the coastal water line between 1 and 4 meters per cycle. This motion exerts great power on outlet glaciers’ floating tongues, and these forces are transmitted into the land-based components of the glacier as properly. AWI’s viscoelastic mannequin explores how these cyclical adjustments in stress distribution can have an effect on the glacier’s move towards the ocean.

The charts in Figure 4 current the measured tide-induced stresses performing on Nioghalvfjerdsbræ at three areas, superimposed on stresses predicted by viscous and viscoelastic simulations. Chart a reveals how displacements decline additional when they’re 14 kilometers inland from the grounding line (GL). Chart b reveals that cyclical tidal stresses reduce at GPS-hinge, positioned in a bending zone close to the grounding line between land and sea. Chart c reveals exercise on the location known as GPS-shelf, which is mounted on ice floating within the ocean. Accordingly, it reveals probably the most pronounced waveform of cyclical tidal stresses performing on the ice.

“The floating tongue is moving up and down, which produces elastic responses in the land-based portion of the glacier,” says Julia Christmann, a mathematician on the AWI crew who performs a key position in developing their simulation fashions. “There is also a subglacial hydrological system of liquid water between the inland ice and the ground. This basal water system is poorly known, though we can see evidence of its effects.” For instance, chart a reveals a spike in stresses under a lake sitting atop the glacier. “Lake water flows down through the ice, where it adds to the subglacial water layer and compounds its lubricating effect,” Christmann says.

The plotted pattern strains spotlight the higher accuracy of the crew’s new viscoelastic simulations, as in comparison with purely viscous fashions. As Christmann explains, “The viscous model does not capture the full extent of changes in stress, and it does not show the correct amplitude. (See chart c in Fig. 4.) In the bending zone, we can see a phase shift in these forces due to elastic response.” Christmann continues, “You can only get an accurate model if you account for viscoelastic ‘spring’ action.”

Modeling Elastic Strains from Uneven Landscapes

The crevasses in Greenland’s glaciers reveal the unevenness of the underlying panorama. Crevasses additionally present additional proof that glacial ice will not be a purely viscous materials. “You can watch a glacier over time and see that it creeps, as a viscous material would,” says Humbert. However, a purely viscous materials wouldn’t type persistent cracks the way in which that ice sheets do. “From the beginning of glaciology, we have had to accept the reality of these crevasses,” she says. The crew’s viscoelastic mannequin gives a novel strategy to discover how the land beneath Nioghalvfjerdsbræ facilitates the emergence of crevasses and impacts glacial sliding.

Figure 5. Aerial view of Nioghalvfjerdsbræ displaying the intensive patterns of the crevasses.

Julia Christmann/Alfred Wegener Institute

“When we did our simulations, we were surprised at the amount of elastic strain created by topography,” Christmann explains. “We saw these effects far inland, where they would have nothing to do with tidal changes.”

Figure 6 reveals how vertical deformation within the glacier corresponds to the underlying panorama and helps researchers perceive how localized elastic vertical movement impacts the complete sheet’s horizontal motion. Shaded areas point out velocity in that a part of the glacier in comparison with its basal velocity. Blue zones are transferring vertically at a slower charge than the sections which can be straight above the bottom, indicating that the ice is being compressed. Pink and purple zones are transferring sooner than ice on the base, displaying that ice is being vertically stretched.

These simulation outcomes recommend that the AWI crew’s improved mannequin might present extra correct forecasts of glacial actions. “This was a ‘wow’ effect for us,” says Humbert. “Just as the up and down of the tides creates elastic strain that affects glacier flow, now we can capture the elastic part of the up and down over bedrock as well.”

Scaling Up because the Clock Runs Down

The improved viscoelastic mannequin of Nioghalvfjerdsbræ is simply the newest instance of Humbert’s decades-long use of numerical simulation instruments for glaciological analysis. “COMSOL is very well suited to our work,” she says. “It is a fantastic tool for trying out new ideas. The software makes it relatively easy to adjust settings and conduct new simulation experiments without having to write custom code.” Humbert’s college college students regularly incorporate simulation into their analysis. Examples embrace Julia Christmann’s PhD work on the calving of ice cabinets, and one other diploma venture that modeled the evolution of the subglacial channels that carry meltwater from the floor to the ice base.

The AWI crew is happy with their investigative work, however they’re totally cognizant of simply how a lot details about the world’s ice cowl stays unknown — and that point is brief. “We cannot afford Maxwell material simulations of all of Greenland,” Humbert concedes. “We could burn years of computational time and still not cover everything. But perhaps we can parameterize the localized elastic response effects of our model, and then implement it at a larger scale,” she says.

This scale defines the challenges confronted by Twenty first-century glaciologists. The measurement of their analysis topics is staggering, and so is the worldwide significance of their work. Even as their information is rising, it’s crucial that they discover extra info, extra shortly. Angelika Humbert would welcome enter from folks in different fields who research viscoelastic supplies. “If other COMSOL users are dealing with fractures in Maxwell materials, they probably face some of the same difficulties that we have, even if their models have nothing to do with ice!” she says. “Maybe we can have an exchange and tackle these issues together.”

Perhaps, on this spirit, we who profit from the work of glaciologists will help shoulder among the huge and weighty challenges they bear.

References

1. J. Christmann, V. Helm, S.A. Khan, A. Humbert, et al. “Elastic Deformation Plays a Non-Negligible Role in Greenland’s Outlet Glacier Flow“, Communications Earth & Environment, vol. 2, no. 232, 2021.
2. European Space Agency, “Spalte Breaks Up“, September 2020.
3. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, “Model comparability: Experts calculate future ice loss and the extent to which Greenland and the Antarctic will contribute to sea-level rise“, September 2020.