What does marine ice sheet (in)stability mean in the context of climate overshoot?

In the classical marine ice sheet instability, a grounding line of an unbuttressed ice sheet on a retrograde bedslope (that is, upwards sloping in the direction of ice flow) is theoretically unstable. However, these theories assume that the forcing on the ice sheet changes slowly, compared to the timescale on which the ice sheet responds to climate perturbations. In a world where climate forcing is ramping up very quickly, this assumption probably doesn’t hold. This is particularly pertinent in the context of climate overshoots — temporary detours above the 1.5C Paris Agreement target — which look increasingly likely if we are to ultimately limit warming to 1.5C. For how long, and how far, can we overshoot 1.5C, while avoiding passing a tipping point, even if that tipping point is around 1.5C of warming? We probe these questions using a simple model of grounding line dynamics, in conjunction with few, more detailed simulations of the retreat of the Pine Island Glacier over the 20th century following its passing a tipping point. We demonstrate that temporary overshoots above tipping points are possible, provided that climate forcing is ramped down sufficiently quickly. However, the likelihood of extreme ice loss is very sensitive to how high the overshoot goes, demonstrating the need to limit overshoots to prevent significant, long timescale ice loss from the Antarctic Ice Sheet.