Multistability and transient response of the Greenland ice sheet to anthropogenic CO2 emissions

Ongoing CO₂ emissions into the atmosphere and associated temperature rise have dramatic consequences for the ice sheets on our planet. In this presentation, we focus on the Greenland ice sheet, which holds so much ice that a complete melting would cause the global sea level to rise by seven meters. However, a prediction of future mass loss of the Greenland ice sheet is challenging because it is a strongly non-linear function of temperature and occurs over very long timescales. With the fully coupled Earth system model of intermediate complexity CLIMBER-X, we study the stability of the Greenland ice sheet and its transient response to CO₂ emissions over the next 20 kyr. We find two bifurcation points within a global mean surface air temperature anomaly of 1.5°C. Each of these bifurcation points corresponds to a critical ice volume. If the Greenland ice sheet volume decreases below these critical values, returning to a previous atmospheric CO₂ concentration would not cause the ice sheet to grow back to its previous state. We also find increased mass loss rates and increased sensitivity of mass loss to cumulative CO₂ emission in the vicinity of these critical ice volumes. Altogether, our results suggest that global warming near the lower 1.5°C limit of the Paris agreement would already cause the Greenland ice sheet to irreversibly melt, although a complete melting would take thousands of years.