Committed retreat of Smith, Pope, and Kohler Glaciers over the next 30 years inferred by transient model calibration

D N Goldberg,P Heimbach,B Smith,I Joughin

doi:10.5194/tc-9-2429-2015

Abstract

Abstract. A glacial flow model of Smith, Pope and Kohler Glaciers is calibrated by means of control methods against time varying, annually resolved observations of ice height and velocities, covering the period 2002 to 2011. The inversion – termed "transient calibration" – produces an optimal set of time-mean, spatially varying parameters together with a time-evolving state that accounts for the transient nature of observations and the model dynamics. Serving as an optimal initial condition, the estimated state for 2011 is used, with no additional forcing, for predicting grounded ice volume loss and grounding line retreat over the ensuing 30 years. The transiently calibrated model predicts a near-steady loss of grounded ice volume of approximately 21 km3 a−1 over this period, as well as loss of 33 km2 a−1 grounded area. We contrast this prediction with one obtained following a commonly used "snapshot" or steady-state inversion, which does not consider time dependence and assumes all observations to be contemporaneous. Transient calibration is shown to achieve a better fit with observations of thinning and grounding line retreat histories, and yields a quantitatively different projection with respect to ice volume loss and ungrounding. Sensitivity studies suggest large near-future levels of unforced, i.e., committed sea level contribution from these ice streams under reasonable assumptions regarding uncertainties of the unknown parameters.

Highlights

Smith, Pope, and Kohler Glaciers, three narrow (∼ 10 km wide), interconnected West Antarctic ice streams, have exhibited substantial thinning and speedup in recent years
As these ice streams are smaller than neighboring Thwaites and Pine Island Glaciers – the contribution of Smith Glacier to total Amundsen Embayment grounding-line flux is ∼ 7– 8 times smaller than that of Pine Island or Thwaites (Shepherd et al, 2002) – focus is often placed upon these larger ice streams, with regard to both modeling and observations of the ice shelves and sub-shelf environments (e.g., Jenkins et al, 2010; Tinto and Bell, 2011; Favier et al, 2014; Joughin et al, 2014)
The degree to which this run agrees with the transient observations is demonstrated in Fig. 2a, c, and the top row of Fig. 3

Summary

Introduction

Pope, and Kohler Glaciers, three narrow (∼ 10 km wide), interconnected West Antarctic ice streams, have exhibited substantial thinning and speedup in recent years As these ice streams are smaller than neighboring Thwaites and Pine Island Glaciers – the contribution of Smith Glacier to total Amundsen Embayment grounding-line flux is ∼ 7– 8 times smaller than that of Pine Island or Thwaites (Shepherd et al, 2002) – focus is often placed upon these larger ice streams, with regard to both modeling and observations of the ice shelves and sub-shelf environments (e.g., Jenkins et al, 2010; Tinto and Bell, 2011; Favier et al, 2014; Joughin et al, 2014). A widely used methodology is one to which we will refer as “snapshot” calibration, first introduced by MacAyeal (1992), and which solves an inverse or optimal

Results

Discussion

Conclusion