ALBERT

Description: Theoretical and numerical work has firmly established that grounding lines of marine-type ice sheets can enter phases of irreversible advance and retreat driven by the marine ice sheet instability (MISI). Instances of such irreversible retreat have been found in several simulations of the past and future evolution of the Antarctic Ice Sheet. However, hitherto the stability regime of Antarctic Ice Sheet grounding lines in their current position has not been assessed. Here we conduct a systematic numerical stability analysis of all the grounding lines of the Antarctic Ice Sheet to determine if they are currently undergoing irreversible retreat through MISI. To do this, we initialise three state-of-the-art ice-flow models, Úa, Elmer/Ice, and PISM, to replicate the current geometry of the Antarctic Ice Sheet, and then apply small, but numerically significant, perturbations in ocean-induced ice-shelf melt. We find that the grounding lines around Antarctica migrate slightly away from their initial position while the perturbation is applied, and then revert to the initial state once the perturbation is removed. There is no indication of irreversible or self-sustaining retreat. This suggests that present-day grounding-line retreat is driven by external climate forcing alone. Hence, if the currently observed mass imbalance were to be removed, the grounding-line retreat would likely stop. However, under present-day climate forcing, further grounding-line retreat is expected, and our accompanying paper (Part B, Reese et al., 2022) shows that this could eventually lead to a collapse of some marine regions of West Antarctica.

Description: Observations of ocean-driven grounding line retreat in the Amundsen Sea Embayment in Antarctica give rise to the question of a collapse of the West Antarctic Ice Sheet. Here we analyse the committed evolution of Antarctic grounding lines under present-day climate conditions to locate the underlying steady states that they are attracted to and understand the reversibility of large-scale changes. To this aim, we first calibrate the sub-shelf melt module PICO with observed and modelled melt sensitivities to ocean temperature changes. Using the new calibration, we run an ensemble of historical simulations from 1850 to 2015 with the Parallel Ice Sheet Model to create model instances of possible present-day ice sheet configurations. Then, we extend a subset of simulations best representing the present-day ice sheet for another 10,000 years to investigate their evolution under constant present-day climate forcing. We test for reversibility of grounding line movement if large-scale retreat occurs. While we find parameter combinations for which no retreat happens in the Amundsen Sea Embayment sector, we also find admissible model parameters for which an irreversible retreat takes place. Hence, it cannot be ruled out that the grounding lines – which are not engaged in an irreversible retreat at the moment as shown in our companion paper (Part A, Urruty et al., subm.) – will evolve towards such a retreat under current climate conditions. Importantly, an irreversible collapse in the Amundsen Sea Embayment sector evolves on millennial timescales and is not inevitable yet, but could become so if forcing on the climate system is not reduced in the future. In contrast, we find that allowing ice shelves to regrow to their present geometry means that large-scale grounding line retreat into marine basins upstream of Filchner-Ronne and Ross ice shelves is reversible. Other grounding lines remain close to their current positions in all configurations under present-day climate.

Description: Drastic changes in our planet’s frozen landscapes have occurred over recent decades, from Arctic sea ice decline and thawing of permafrost soils to polar amplification, the retreat of glaciers and ice loss from the ice sheets. In this chapter, we assess multiple lines of evidence for tipping points in the cryosphere – encompassing the ice sheets on Greenland and Antarctica, sea ice, mountain glaciers and permafrost – based on recent observations, palaeorecords, numerical modelling and theoretical understanding. With about 1.2°C of global warming compared to pre-industrial levels, we are getting dangerously close to the temperature thresholds of some major tipping points for the ice sheets of Greenland and West Antarctica. Crossing these would lock in unavoidable long-term global sea level rise of up to 10 metres. There is evidence for localised and regional tipping points for glaciers and permafrost and, while evidence for global-scale tipping dynamics in sea ice, glaciers and permafrost is limited, their decline will continue with unabated global warming. Because of the long response times of these systems, some impacts of crossing potential tipping points will unfold over centuries to millennia. However, with the current trajectory of greenhouse gas (GHG) emissions and subsequent anthropogenic climate change, such largely irreversible changes might already have been triggered. These will cause far-reaching impacts for ecosystems and humans alike, threatening the livelihoods of millions of people, and will become more severe the further global warming progresses.

Description: Observations of ocean-driven grounding line retreat in the Amundsen Sea Embayment in Antarctica raise the question of an imminent collapse of the West Antarctic Ice Sheet. Here we analyse (1) whether the Antarctic grounding lines are undergoing a Marine Ice Sheet Instability in their current position and (2) the committed evolution of Antarctic grounding lines under the present-day ocean and atmospheric conditions. To this aim, we first calibrate a sub-shelf melt parameterisation, that is derived from an ocean box model, with observed and modelled melt sensitivities to ocean temperature changes, making it suitable for present-day simulations and future sea-level projections. Using the new calibration, we conduct a systematic numerical stability analysis of all the grounding lines of the Antarctic Ice Sheet to determine if they are currently undergoing irreversible retreat. In a second step, we run an ensemble of historical simulations from 1850 to 2015 to create model instances of possible present-day ice sheet configurations. Then, we investigate the long-term ice sheet evolution and reversibility under constant bathymetry and present-day climate and ocean forcing.