Millstein Joanna D.

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Millstein
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Joanna D.
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  • Article
    Ice viscosity is more sensitive to stress than commonly assumed
    (Nature Research, 2022-03-10) Millstein, Joanna D. ; Minchew, Brent M. ; Pegler, Samuel S.
    Accurate representation of the viscous flow of ice is fundamental to understanding glacier dynamics and projecting sea-level rise. Ice viscosity is often described by a simple but largely untested and uncalibrated constitutive relation, Glen’s Flow Law, wherein the rate of deformation is proportional to stress raised to the power n. The value n = 3 is commonly prescribed in ice-flow models, though observations and experiments support a range of values across stresses and temperatures found on Earth. Here, we leverage recent remotely-sensed observations of Antarctic ice shelves to show that Glen’s Flow Law approximates the viscous flow of ice with n = 4.1 ± 0.4 in fast-flowing areas. The viscosity and flow rate of ice are therefore more sensitive to changes in stress than most ice-flow models allow. By calibrating the governing equation of ice deformation, our result is a pathway towards improving projections of future glacier change.
  • Thesis
    The flow and fracture of Antarctic Ice Shelves
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2024-02) Millstein, Joanna D. ; Minchew, Brent M.
    This thesis explores the deformation of glacier ice, with an aim of deriving insights into its flow and fracture through satellite remote sensing methods. Glaciers deform through the driving force of their own weight and display dramatic responses to both external forcing, such as changes in climate, and internal forcing like variations in stress. Here, we focus on Antarctic ice shelves, the fast-flowing extensions of the ice sheet that impart stabilizing resistive stresses onto the grounded area. Processes contributing to dynamic change through melting, calving, the flow and fracture ice, can be explored and quantified with expirical relationships derived from mechanical properties. To understand the stability and future projections of glaciers and ice sheets in a changing climate, it is critical that we quantify and calibrate these processes. In Chapter 2, we leverage modern satellite remote sensing products to gain new insights into the flow of glacier ice. We validate and calibrate the constitutive relation for glacier ice across Antarctic ice shelves using a simple relationship between ice thickness and surface strain rate data. We find that the constitutive relation should employ an exponent n = 4, in contrast to the commonly used n = 3. This finding implies that ice shelves are more sensitive to changes in the stress state than typically assumed. Next, in Chapter 3 we derive a spatiotemporally dense dataset of surface strain rate fields across the Brunt Ice Shelf, the site of active full-thickness fractures, known as rifts. This dataset provides a mechanical framework with which we can analyze dynamic change, allowing us to quantify surface deformation with radar remote sensing. Lastly, Chapter 4 presents a fatigue-crack growth model for active rifts. This empirical framework sets bounds on rift propagation rates over periods of weeks and months and, in doing so, presents a simple parameterization of rift growth rates that can be implemented using observational data. This work provides a promising method to resolve fracture evolution over the period of weeks to years. Ultimately, this thesis uses observational data to validate theoretical models of glacier change, advancing our grasp on the dynamics of Antarctic ice shelves.
  • Article
    Stability of ice shelves and ice cliffs in a changing climate
    (Annual Reviews, 2024-01-11) Bassis, Jeremy N. ; Crawford, Anna ; Kachuck, Samuel B. ; Benn, Douglas I. ; Walker, Catherine C. ; Millstein, Joanna D. ; Duddu, Ravindra ; Åström, Jan ; Fricker, Helen A. ; Luckman, Adrian
    The largest uncertainty in future sea-level rise is loss of ice from the Greenland and Antarctic Ice Sheets. Ice shelves, freely floating platforms of ice that fringe the ice sheets, play a crucial role in restraining discharge of grounded ice into the ocean through buttressing. However, since the 1990s, several ice shelves have thinned, retreated, and collapsed. If this pattern continues, it could expose thick cliffs that become structurally unstable and collapse in a process called marine ice cliff instability (MICI). However, the feedbacks between calving, retreat, and other forcings are not well understood. Here we review observed modes of calving from ice shelves and marine-terminating glaciers, and their relation to environmental forces. We show that the primary driver of calving is long-term internal glaciological stress, but as ice shelves thin they may become more vulnerable to environmental forcing. This vulnerability—and the potential for MICI—comes from a combination of the distribution of preexisting flaws within the ice and regions where the stress is large enough to initiate fracture. Although significant progress has been made modeling these processes, theories must now be tested against a wide range of environmental and glaciological conditions in both modern and paleo conditions.