Instabilities, thresholds and tipping points in the ice sheet system: from the marine ice sheet to the marine ice cliff instability*

Observations and theory are increasingly pointing to the possibility that ice sheets can rapidly collapse.  For example, over the past two decades we have witnessed the explosive disintegration of ice shelves on the Antarctic Peninsula, collapse of portions of the Pine Island and Thwaites glaciers in West Antarctica and sustained retreat of marine terminating glaciers surrounding the Greenland Ice Sheet.  Few of these events were predicted by models.  Part of the difficulty in projecting ice sheet changes stems from uncertainty related to the presence of "tipping points" and thresholds in the ice sheet system. Here, we review past and present evidence that suggests our ice sheets could be edging ever closer to tipping points. We show that, despite ongoing retreat, much of the retreat we have observed so far has been driven by atmospheric and oceanic forcing and we have yet to breach the tipping point.  However, continued warming will likely push portions of the ice sheets into an unstable regime where the ice sheet is subject to the "marine ice cliff instability".  The marine ice cliff instability is based on the idea that the finite strength of ice places a limit on the maximum ice cliff height possible at the ice sheet calving cliff. We show that sustained collapse is possible when retreating ice shelves expose a thick calving cliff. However, viscous thinning of the ice combined with small bed rock protrusions—called pinning points— provide a stabilizing forcing that reduces retreat rates.  Overall, advances in modeling the flow and fracture of ice sheets is allowing us to begin to simulate punctuated ice sheet decay providing the potential for actionable sea level projections.