31 May 2022
31 May 2022
Status: this preprint is currently under review for the journal TC.

Compensating errors in inversions for subglacial bed roughness: same steady state, different dynamic response

Constantijn J. Berends1, Roderik S. W. van de Wal1,2, Tim van den Akker1, and William H. Lipscomb3 Constantijn J. Berends et al.
  • 1Institute for Marine and Atmospheric research Utrecht, Utrecht University, Utrecht, The Netherlands
  • 2Faculty of Geosciences, Department of Physical Geography, Utrecht University, Utrecht, The Netherlands
  • 3Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, USA

Abstract. Subglacial bed roughness is one of the main factors controlling the rate of future Antarctic ice-sheet retreat, and also one of the most uncertain. A common technique to constrain the bed roughness using ice-sheet models is basal inversion, tuning the roughness to reproduce the observed present-day ice-sheet geometry and/or surface velocity. However, many other factors affecting ice-sheet evolution, such as the englacial temperature and viscosity, the surface and basal mass balance, and the subglacial topography, also contain substantial uncertainties. Using a basal inversion technique intrinsically causes any errors in these other quantities, to lead to compensating errors in the inverted bed roughness. Using a set of idealised-geometry experiments, we quantify these compensating errors and investigate their effect on the dynamic response of the ice-sheet to a prescribed forcing. We find that relatively small errors in ice viscosity and subglacial topography require substantial compensating errors in the bed roughness in order to produce the same steady-state ice sheet, obscuring the realistic spatial variability in the bed roughness. When subjected to a retreat-inducing forcing, we find that these different parameter combinations, which per definition of the inversion procedure result in the same steady-state geometry, lead to a rate of ice volume loss that can differ by as much as a factor of two. This implies that ice-sheet models that use basal inversion to initialise their model state can still display a substantial model bias despite having an initial state which is close to the observations.

Constantijn J. Berends et al.

Status: open (until 26 Jul 2022)

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  • RC1: 'Comment on tc-2022-103', Anonymous Referee #1, 24 Jun 2022 reply

Constantijn J. Berends et al.

Constantijn J. Berends et al.


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Short summary
The rate at which the Antarctic ice sheet will melt because of man-made climate change is uncertain. Part of this uncertainty stems from processes occurring beneath the ice, such as the way the ice slides over the underlying bedrock. "Inversion methods" attempt to use observations of the ice-sheet surface to calculate how these sliding processes work. We show that such methods cannot fully solve this problem, so that a substantial uncertainty still remains in projections of sea-level rise.