A physics-based study of ice stream dynamics

Project Information

PHAST

Grant agreement ID: 101076793

DOI

10.3030/101076793

EC signature date 25 April 2023

Start date 1 June 2023

End date 31 May 2028

Funded under

• European Research Council (ERC)

Total cost

€ 1 826 073,00

EU contribution

€ 1 826 073,00

Coordinated by

LUDWIG-MAXIMILIANS-UNIVERSITAET MUENCHEN
Germany

Periodic Reporting for period 1 - PHAST (A physics-based study of ice stream dynamics)

Reporting period: 2023-06-01 to 2025-11-30

Summary of the context and overall objectives of the project

Ice streams are river-like corridors of fast flowing ice that account for the vast majority of ice discharge to the ocean in continental ice sheets. Their most outstanding feature is that they can appear spontaneously within a slowly moving ice sheet, self-organize in evenly spaced patterns, and switch on and off over time. Yet, a full explanation of ice stream formation and evolution is one of the longest standing open problems in glaciology. This knowledge gap has precluded fundamental investigations on the role of ice streams in driving ice sheet change, and also casts doubt on the ability of state-of-the-art ice sheet simulation codes to project future sea levels.

Recent work identified an entirely novel class of feedbacks related to the onset of basal sliding at frozen-thawed thermal transitions (so-called `subtemperate sliding') which could be the missing ingredient needed to fully explain observed ice stream dynamic behaviors. Through a unique combination of theory, numerical, and observational work, PHAST aims to revolutionize
the state-of-the-art understanding of ice stream dynamics, and by doing so to lead the way towards answering fundamental questions about the role of ice streams dynamics in driving ice sheet change over timescales ranging from decades to millennia.

Specifically, PHAST will tackle the following outstanding questions:

1) What are the core physical ingredients responsible for spontaneous ice stream formation and activation/ stagnation cycles? Do sliding onset physics play a role? Do the dominant processes change depending on external forcing (e.g. climate)?

2) How does basal sliding first start? Does the laboratory-derived understanding of subtemperate sliding hold in natural settings? How should sliding onset be parameterized in ice flow models?

3) How can sliding onset physics be faithfully described in large-scale ice sheet simulation codes used for sea level rise projections? What about all other processes relevant to ice stream dynamics? Ultimately, what is the impact of ice stream dynamics on projections of ice sheet mass loss and sea level rise for
the next century?

By developing new modeling tools and generating process-level understanding across scales ranging from few tens of meters to thousands of kilometers, PHAST will lead the way towards unraveling the complex interplay between internal ice sheet dynamics and climate over timescales ranging from decades to millennia.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

WP1: We implemented a temperature-dependent friction law in the full-Stokes ice flow simulation code Elmer/Ice. We have set up and run first-ever transient simulations of an ice sheet flow line experiencing a frozen/temperate basal transition via a region of sub-temperate sliding. Initial results show that previously detected small-scale temporal instabilities associated with subtemperate sliding give rise to macroscopic (ice sheet scale) surge-type behaviour with a travelling wave pattern, originating as an instability of the subtemperate region.

WP2: The geophysical characterization of the Grenzgletscher is at an advanced stage. We have performed extensive ground-based ice penetrating radar work, as well as active and passive seismic work to characterize the thermal and physical properties of the ice/bed interface. Given difficulties (shadowing) with remote sensing techniques, we have performed two terrestrial radar interferometry surveys and deployed a 16-sensor GNSS network on the glacier to characterize the glacier surface velocity. The analysis of the data collected is still ongoing, but it is already clear that the drilling area (region experiencing subtemperate sliding) will be in the accumulation zone, certainly above 3600 m. A technical solution for the borehole instrumentation is now defined, with current work focusing on the specifics of cable-to-sensor node connectors to ensure the highest possible reliability. Drilling and installation of the borehole instruments is foreseen for spring/summer 2026.

WP3: Work performed so far is concerned with defining the optimal vertical spacing of the borehole tiltmeters that will be deployed at the Grenzgletscher in the 2026 season, as informed by in-situ geophysical observations collected in the past two seasons. A second ongoing line of work concerns the set-up of a forward/inverse ice flow model of the area of interest at our field site, which will support the interpretation of the borehole observations.

WP4: Starting in fall 2025. A recent collaborative publication (Hank, Tarasov, Mantelli; GMD, 2023) confirmed extreme sensitivity of ice sheet mass balance to the choice of subtemperate sliding parameterization in ice flow models of reduced mechanical complexity, thus laying the groundwork for our planned work.

WP5: recruitment of the project team is now complete.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

To date, ice streams account for over 80% of Antarctic ice mass loss to the ocean. Yet, the current lack of understanding of the processes enabling ice streams to emerge spontaneously out of a slow-moving ice sheet and to switch on and off over time means that we do not presently know whether ice stream dynamics matter at all for ice sheet mass balance over centennial or millennial timescales. What this means in practice is that should any of the currently active ice streams switch off, or should new ice streams switch on over the projection period, projections could easily be off by 10s of centimeters of potential sea level rise by 2100, with clear societal consequences. Through theory, fieldwork and numerical modelling PHAST sets out to tackle this challenge. By doing so, PHAST takes the first step towards the goal to understand whether ice stream dynamics can control ice sheet mass balance on par with climate, or instead whether climate is the only driver of ice sheet change.

Fieldwork on the Grenzgletscher in April 2025. Credit Loïc Perez.