Basal environment uncertainty and the triggering of Antarctic Ice Sheet tipping points

Recent studies show that critical thresholds, tipping points, likely exist for the Antarctic Ice Sheet, and potentially individual drainage basins within Antarctica. Surpassing these critical thresholds for key glacier drainage basins can have significant impacts on the long-term sea-level contribution of the ice sheet, which may be irreversible. The marine ice sheet instability (MISI) is considered to be one of the key feedback mechanisms capable of triggering tipping dynamics in ice sheets, wherein grounding line retreat past a stable position along the bed can trigger further rapid and extensive retreat due to the internal dynamics of ice flow. Model simulations that exhibit this instability tend to project much higher degrees of sea-level rise than those where MISI is not initiated. As such, it is important to understand what factors determine if MISI takes effect and the timing of its onset. The sliding velocity of a glacier is a fundamental variable in calculating overall ice flow, and how the sliding velocity evolves in time is a critical factor in determining if MISI is triggered or not during model simulations projecting future glacier dynamics. While it is well understood that the sliding velocity is highly dependent upon the basal environment, much remains unknown regarding its specific characteristics including where sediment accumulates and how much is present, how meltwater flows through the basal environment, and how to best represent basal friction felt by the glacier. Uncertainties associated with both the physical processes governing ice sheet responses to climate warming, and the parameter choices associated with those physical processes make the analysis of potential ice sheet tipping points particularly difficult. Here, we aim to better understand the influence that the basal environment has on the tipping behavior of the Antarctic Ice Sheet. We use the Parallel Ice Sheet Model (PISM) to identify critical temperature thresholds that will lead to irreversible ice sheet mass loss and quantify the associated long-term sea-level commitment from Antarctica. Climate forcings will be dictated according to the TIPMIP protocols, following stylized warming scenarios and exploring the ice sheet equilibrium response to constant climates at different global warming levels. We analyze model results for differences in tipping behavior obtained by using various different representations of hydrology in the basal environment and ranging values for associated parameters. Here, we present the initial results of these experiments and discuss the relative importance of subglacial hydrology in determining if and when critical tipping points are exceeded for the Antarctic Ice Sheet.