Hysteresis of the Antarctic ice sheet with a coupled climate-ice-sheet model

Anthropogenic greenhouse gas emissions and resulting global warming raise uncertainties in the future of currently existing ice sheets. The Antarctic ice sheet, which contains the equivalent of 58 meters of potential sea level rise, is expected to have a relatively small role on sea level rise in this century, but is expected to continue to lose mass afterwards and could become a major driver of sea level rise on longer timescales (Van Breedam et al., 2020; Winkelmann et al., 2015).

The Antarctic ice sheet interacts with the solid Earth, the ocean and the atmosphere, resulting in various positive and negative feedbacks, enhancing or limiting ice sheet growth (Fyke et al., 2018). Positive feedback mechanisms, such as the albedo-melt and elevation-temperature feedbacks, enhance the ice sheet's response to an initial change in forcing, potentially resulting in nonlinear changes, and it is thus crucial to model these feedbacks on long timescales, when significant changes of the ice sheet’s topography can occur. Nonlinear changes can lead to a hysteresis behaviour, with widely different equilibrium states for a given CO₂ level or temperature anomaly, depending on the initial condition (Pollard and de Conto, 2005; Garbe et al., 2020; Van Breedam et al., 2023).

In this study, we explore the hysteresis of the Antarctic ice sheet from the present-day configuration, using an intermediate complexity climate model, iLOVECLIM, representing the atmosphere, ocean and vegetation, coupled to an ice sheet model, GRISLI. Simulations start from either a pre-industrial ice sheet or an ice-free, isostatically rebounded geometry, and different CO₂ levels are applied.

Crucially, the albedo-melt feedback is accounted for in our coupled setting, which strengthens nonlinear behaviour and leads to critical CO₂ thresholds for the ice sheet melt or growth. This enhances the ice sheet hysteresis, with widely different equilibrium ice volumes at a given CO₂ level, depending on the initial ice sheet geometry. The CO₂ thresholds either trigger the complete Antarctic ice sheet loss or near-complete regrowth. The orbital configuration influences these CO₂ thresholds : a higher (lower) summer insolation in the Southern Hemisphere decreases (increases) the CO₂ threshold for Antarctic deglaciation (glaciation).

These findings highlight the importance of ice sheet-atmosphere interactions, notably the albedo-melt feedback, in projecting future long-term ice sheet behavior. Neglecting these feedbacks could lead to an overestimation of CO₂ thresholds for the Antarctic ice sheet destabilization, with implications for future long-term sea level rise under high emission scenarios.

This study has recently been accepted in Geophysical Research Letters.