What is our Focus Group About? 

Focus Group Statement

Contributions of the Antarctic Ice Sheet to global sea level strongly depend on the health of its flow-constraining ice shelves and the nature in which ice breaks off and calves from the ice sheet and ice shelves (Fürst et al., 2016; Reese et al., 2017; DeConto et al., 2021). Many factors influence the structural integrity of ice shelves, with conditions at their atmospheric (Wille et al., 2022) and oceanic (Walker and Gardner, 2017) boundaries critical to their evolution and stability. The drivers of calving are likewise diverse, including changes in oceanic heat flux and melting that can undermine ice shelves (Bassis and Ma, 2015), and the mechanical stress from ice flow dynamics that can weaken ice leading to its breakup (Bassis and Walker, 2012). Calving and ice shelf stability are highly coupled processes, particularly in high-end calving scenarios such as the Marine Ice Cliff Instability (MICI), in which abrupt ice shelf loss may initiate rapid collapse of exposed ice cliffs (Pollard et al., 2015).

In ISMIP6, a very idealized representation of ice shelf collapse was developed, based on the presence of liquid water at the ice surface simulated by climate models: if more than 72.5 cm of liquid water was present for at least 10 years on any location of the ice surface, the ice shelf would be considered unstable and therefore to collapse (Nowicki et al., 2020; Seroussi et al., 2020). This condition was derived based on observations of previous ice shelf collapse events, such as the Larsen B, Wilkins, and George VI ice shelf collapse (Trusel et al., 2015). Recent research has shown the limitation of such a parameterization, including the role of snow, firn, and ponded water (Jourdain et al., 2024; Dell et al., 2024). Acknowledging these limitations, we propose to develop a new framework to consider calving and ice shelf collapse in a consistent but flexible manner across ISMIP7 Antarctic simulations.

For the ice-shelf collapse protocol, we propose to focus on assessing ice-shelf surface boundary conditions, including snow and firn air depletion, as well as ice shelf stress state preceding previous collapse events, including the Wordie, Prince Gustav, Larsen A, Larsen B, Wilkins, Larsen C, or George VI ice shelves (Andreasen et al., 2023). Using these conditions, we will propose a new parameterization to estimate ice shelf collapse based on climate conditions and ice shelf stress state; this parameterization will be discussed with ice sheet modelers to ensure a wide adoption and will be based on conditions simulated by ice models to capture a wider range of evolution. Finally, we will assess climate models biases and propose bias corrections to correctly capture local forcings used for ice sheet models.

We will coordinate with the Antarctic SMB (Task E4) and ocean melt (Task E3) focus groups in order to ensure our proposed parameterizations are consistent with their recommendations, and climate models selected provide accurate information for the climate inputs needed.