Glacial-cycle simulations of the Antarctic Ice Sheet with the Parallel Ice Sheet Model (PISM) – Part 1: Boundary conditions and climatic forcing

Abstract

Abstract. Simulations of the glacial–interglacial history of the Antarctic Ice Sheet provide insights into dynamic threshold behavior and estimates of the ice sheet's contributions to global sea-level changes for the past, present and future. However, boundary conditions are weakly constrained, in particular at the interface of the ice sheet and the bedrock. Also climatic forcing covering the last glacial cycles is uncertain, as it is based on sparse proxy data. We use the Parallel Ice Sheet Model (PISM) to investigate the dynamic effects of different choices of input data, e.g., for modern basal heat flux or reconstructions of past changes of sea level and surface temperature. As computational resources are limited, glacial-cycle simulations are performed using a comparably coarse model grid of 16 km and various parameterizations, e.g., for basal sliding, iceberg calving, or for past variations in precipitation and ocean temperatures. In this study we evaluate the model's transient sensitivity to corresponding parameter choices and to different boundary conditions over the last two glacial cycles and provide estimates of involved uncertainties. We also discuss isolated and combined effects of climate and sea-level forcing. Hence, this study serves as a “cookbook” for the growing community of PISM users and paleo-ice sheet modelers in general. For each of the different model uncertainties with regard to climatic forcing, ice and Earth dynamics, and basal processes, we select one representative model parameter that captures relevant uncertainties and motivates corresponding parameter ranges that bound the observed ice volume at present. The four selected parameters are systematically varied in a parameter ensemble analysis, which is described in a companion paper.

Glacial-cycle simulations of the Antarctic Ice Sheet with the Parallel Ice Sheet Model (PISM) – Part 2: Parameter ensemble analysis

“…This method yields reasonable results for an adequately resolved parameter space, as more advanced statistical techniques interpolate results between sparsely separated points in multi-dimensional parameter space. However, the fullfactorial simple averaging method strongly limits the number of varied parameters for available computer resources such that only the most relevant parameters to each class of climatic and boundary conditions were pre-selected (see companion paper; Albrecht et al, 2020) to cover a representative range of model responses.…”

“…PICO updates melt rates according to changes in ocean temperatures or the geometry of the ice shelves (while changes in salinity are neglected). A description of PISM for paleo-applications and sensitivity of the model to various uncertain parameter and boundary conditions are discussed in a companion paper (Albrecht et al, 2020).…”

Brief communication: On calculating the sea-level contribution in marine ice-sheet models

“…We have first applied a typical glacial-interglacial experiment (e.g. Golledge et al, 2014;Pollard et al, 2016;Albrecht et al, 2020) over the last 120 kyr (Fig. 3a) with the prescribed external sea-level change (based on sea-level reconstructions by Bintanja andvan de Wal, 2008, andLambeck et al, 2014) as a dominant forcing.…”

“…The sea-level contribution from an ice sheet can in principle be estimated through these different mass balance terms but is in practice typically based on changes in one prognostic variable, ice thickness, and considering corrections for the ice grounded below sea level (e.g. Bamber et al, 2013). However, complications arise, especially for longer timescales, when isostatic adjustment of the bedrock is considered.…”

“…We caution that it is very possible that the proposed solutions or equivalent approaches are already in use in several models, since the fundamental ideas have already been laid out (e.g. Bamber et al, 2013;de Boer et al, 2015) and are straightforward to implement. Our aim here is to provide concrete guidelines and a central reference of best practices for ice-sheet modellers.…”