Modelling the advance and retreat of major Greenlandic tidewater glacier over the last 1000 years reveals high sensitivity to calving front forcing criteria

Iceberg calving from tidewater glaciers has contributed more than half of the total mass loss of the Greenland Ice Sheet over the past four decades, and observations have shown that episodes of increased iceberg discharge have coincided with rising air temperatures and/or the occurrence of warmer coastal waters into fjords within which they discharge. Despite significant advances in understanding over the last two decades, major uncertainties still remain in understanding how sensitive iceberg calving rates are to climate-induced exchanges of heat and freshwater around marine terminating ice sheet margins. This is partly because we do not know the long-term, multi-decadal to centuries historical context of the ice-ocean system that links our understanding of contemporary process with longer term glacier response to climate. 

In this study, we use the Ice-sheet and Sea-level System Model (ISSM) to simulate the advance and retreat of a fast-flowing tidewater glacier in southwest Greenland, Kangiata Nunata Sermia, over the last 1000 years to indentify the drivers of advance and retreat and evaluate calving-parameter choices against observed long-term ice-margin variability. While models have successfully reproduced observed recent retreat, their parameters are rarely tested against centennial- to millennial-scale records of advance. We explore the parameter space governing calving-front advance, focusing on submarine melt rates and von Mises calving-law stress thresholds for grounded and floating ice and validate model ensembles against a well-constrained millennial-scale record of advance and retreat. Using Latin Hypercube Sampling, we assess two criteria: whether the calving front advances at all, and whether it can reach the reconstructed Little Ice Age (LIA) position. 

We find that advance can occur across the full tested range of submarine melt rates, up to 1.5 m d⁻¹. However, successful advance to the LIA position is more tightly constrained by the von Mises stress thresholds. In several simulations, the calving front advances only as far as a widening in the fjord, unless the calving rate is reduced by setting a sufficiently high stress thresholds. Our results highlight a strong interaction between calving physics and fjord geometry in controlling long-term advance. This project contributes to improving confidence in multi-decadal to centennial projections of ice sheet behaviour through validating model performance over similar timescales including prolonged episodes of both glacier advance and retreat.