Reply on RC2

Abstract

Numerical simulations of the Greenland Ice Sheet (GrIS) over geologic timescales can greatly improve our knowledge of the critical factors driving GrIS demise during climatically warm periods, which has clear relevance for better predicting GrIS behavior over the upcoming centuries. To assess the fidelity of these modeling efforts, however, observational constraints of past ice-sheet change are needed. Across southwestern Greenland, geologic records detail Holocene ice retreat across both terrestrial-based and marine terminating environments, providing an ideal opportunity to rigorously benchmark model simulations against geologic reconstructions of ice-sheet change. Here, we present regional ice sheet modeling results using the Ice-sheet and Sea-level System Model (ISSM) of Holocene ice sheet history across an extensive fjord region in southwestern Greenland covering the landscape around the Kangiata Nunaata Sermia (KNS) glacier and extending outward along the 200 km Godthåbsfjord. Our simulations, forced by novel reconstructions of Holocene climate and recently implemented calving laws, assess the sensitivity of ice retreat across the KNS region to atmospheric and oceanic forcing. Our simulations reveal that the geologically reconstructed ice retreat across the bedrock landscape above sea-level in the study area was likely driven by fluctuations in surface mass balance in response to early Holocene warming – and likely not influenced significantly by the response of adjacent outlet glaciers to calving and ocean-induced melting. The impact of ice calving within fjords, however, plays a significant role by enhancing ice discharge at the terminus, leading to interior thinning up to the ice divide that is consistent with reconstructed magnitudes of early Holocene ice thinning. Our results, benchmarked against geologic constraints of past ice margin change, suggest that while calving did not strongly influence Holocene ice margin migration across terrestrial portions of the KNS forefield, it strongly impacted regional mass loss. While these results may provide an analog to how similar fjord-dominated regions of the GrIS may respond to future warming, they also illustrate that implementation and resolution of ice calving in paleo ice flow modeling is important towards making more robust estimations of past ice mass change.