Uncertainty Quantification of the Antarctic Ice-Sheet Retreat using a Multifidelity Quantile-Based Approach for Confidence Sets of Random Excursion Sets.

Abstract:

Assessing with quantified uncertainty the retreat of the grounded portion of the Antarctic ice sheet is of critical importance in understanding its vulnerability to future climate change, especially in marine sectors prone to instability mechanisms. In this presentation, we address this problem by developing and implementing a new approach within the probabilistic context of the random set theory.

In the first part of the presentation (Bulthuis et al., 2020), we present the proposed methodology on a model problem in glaciology. Our methology relies on the concept of confidence sets that either contain or are contained within the grounded portion of the Antarctic ice sheet with a specified level of probability. We seek such confidence sets in a parametric family of nested candidate sets defined as a parametric family of sublevel or superlevel sets of a membership function. We show that the problem of identifying a confidence set with a given probability level in such a parametric family is equivalent to a problem of estimating a quantile of a random variable obtained as a global extremum of the membership function over the complement of the excursion set. To construct such confidence sets, we propose a computationally efficient bifidelity method that exploits a spectral representation of this random variable to reduce the required number of evaluations of the computational model.

In the second part of the presentation (Bulthuis et al., 2019), we apply the proposed methodology to provide new probablistic projections for the retreat of the grounded portion of the Antarctic ice sheet. We carry out simulations over the next millenium under all four RCP scenarios using the f.ETISh ice-sheet model (Pattyn, 2017) and considering uncertainty in key physical processes that drive the Antarctic ice-sheet dynamics. We find that, irrespective of parametric uncertainty, the strongly mitigated RCP 2.6 scenario prevents the collapse of the West Antarctic ice sheet, that in both the RCP 4.5 and RCP 6.0 scenarios the occurrence of the marine ice-sheet instability in marine basins is more sensitive to parametric uncertainty, and that, almost irrespective of parametric uncertainty, RCP 8.5 triggers the collapse of theWest Antarctic ice sheet.