Benoit Urrity, TiPACCs Early Career Researcher, has successfully defended his dissertation and has been awarded his Ph.D. at the Institut des Géosciences de l’Environnement (IGE) in Grenoble, France on the 12 of January 2023.
The defence committee was comprised of:
Olivier Gagliardini and Fabien Gillet-Chauler (IGE, UGA, CNRS) – Directors
Hélène Seroussi (Dartmouth, Thayer School of Engineering) – Reviewer
Frank Pattyn (Université Libre de Bruxelles) – Reviewer
Emmanuel Chaljub (Université Grenoble Alpes) – President
Masa Kageyama (Laboratoire des sciences du climat et de l’environnement) – Examinator
This thesis is part of the H2020 European project TiPACCs (Tipping Points in Antarctic Climate Components) that will officially start the 1st of August 2019. The overall aim of the TiPACCs project is to assess the likelihood of large and abrupt near-future changes in the contribution of the Antarctic Ice Sheet to global sea level, caused by tipping points in the Antarctic continental shelf seas and the Antarctic Ice Sheet. Specifically, TiPACCs will study two closely-related tipping points of the Antarctic continental shelf seas and the Antarctic Ice Sheet and the physical processes that link them. The first tipping point involves an irreversible switch from ‘cold’ to ‘warm’ oceanic conditions below the large ice shelves of Antarctica. The second tipping point involves a switch from stable to unstable grounding-line configurations of the Antarctic Ice Sheet due to loss of ice-shelf buttressing. A particular focus of this study is the possibility that the first tipping point (i.e. a switch in ocean conditions from ‘cold’ to ‘warm’) may cause the second tipping point (i.e. onset of an unstable grounding line retreat) to be crossed. This possibility has not been explored before and the body of research leading to this insight is very recent. Within TiPACCs we will quantify the proximity of current climate to these tipping points and resulting implications for global sea level should they be crossed. The consortium of the TiPACCs project includes the leading groups in ice-flow and ocean circulation modelling in Europe (Uni Research As, NO; University of Northumbria, UK; Potsdam-Institut für Klimafolgenforschung, DE; Alfred-Wegener-Institut, De; Université Grenoble Alpes, FR; United Kingdom Research and Innovation, UK). The PhD student will work on the second tipping point (Ice tipping point, in WP2 of the project). The overall objective of the PhD is to determine the stability regime of the grounding lines of the Antarctic Ice Sheet and the existence of tipping points with respect to ice-shelf melt.
To this aim, the PhD student will conduct numerical simulations of the whole Antarctic Ice Sheet using the Elmer/Ice model. Before any perturbation experiments can be performed, a suitable reference state will be obtained following the recent model inter-comparison experiment focusing on ice-sheet initialisation for the Antarctic Ice Sheet (InitMIP-Antarctica). Surface-to-bed inverse initialisation methodology will be used to ensure that the model replicates current surface flow for current ice thicknesses. All data sets on ice thicknesses and surface velocities required for this purpose are already available. In a second step, the PhD student will conduct perturbations experiments with the aim of identifying the stability regime of the grounding lines of the Antarctic Ice Sheet in their current configurations. Stability will be tested by applying a step-wise melt perturbation bellow ice-shelves for periods sufficiently long to cause small, but numerically significant, shifts in the grounding-line positions. We expect periods of about one year to be adequate for this purpose. If individual grounding-lines migrate back to their original position as the perturbation is removed, one can conclude that they are (asymptotically) stable. If, on the other hand, the grounding-lines migrate away from the initial position with an increasing rate the configuration is unstable.