Assessing the Robustness of Arctic Sea Ice Bi-Stability in the Presence of Atmospheric Feedbacks

Arctic sea-ice loss is influenced by multiple positive feedbacks, sparking concerns of accelerated loss in the coming years or even a tipping point, where a sea-ice equilibrium disappears at a given CO2 value and sea ice rapidly evolves to a new steady state. Such a tipping point would imply a bi-stability of the Arctic climate-where multiple steady-state Arctic climates are possible at the same CO2 value. Previous works have sought to establish the existence of bi-stability using a range of models, from zero-dimensional sea ice thermodynamic models to fully coupled global climate models, with conflicting results. Here, we present a new model of the Arctic that includes both sea-ice thermodynamics and key atmospheric feedbacks in a simple framework. We exploit the model's simplicity to identify physical mechanisms that control the timing and extent of sea-ice bi-stability, and the abruptness of ice loss. We show that longwave radiation feedbacks can have a strong influence on Arctic surface climate from atmospheric temperature increases alone, even without major contributions from clear-sky moisture or convective clouds suggested previously. While winter sea-ice bi-stability is robust to changes in uncertain model parameters in this study, summer sea ice is more sensitive. Finally, our model indicates that positive feedbacks may modulate the CO2 threshold of sea-ice loss and the width of bi-stability much more strongly than the abruptness of loss. These results lead to a comprehensive understanding of the conditions that favor Arctic sea-ice bi-stability, particularly the role of atmospheric feedbacks, in both future and past climates.