Marine-terminating glaciers may be less vulnerable to rapid and irreversible collapse than previously suggested, according to a new study, which finds that ice cliff collapse is limited by upstream thinning of the ice sheet and how quickly calved icebergs and sea-ice float away. The glaciers of Greenland and Antarctica slowly flow to the sea, terminating in massive vertical ice cliffs. Occasionally, these partially submerged margins can collapse under their own weight and trigger rapid disintegration of ice sheets. It's thought that this process, called marine ice cliff instability (MICI), could lead to the catastrophic retreat of some of the planet's largest ice sheets, substantially contributing to global sea level rise. However, current models of MICI remain controversial. While there is evidence supporting MICI in the paleo-record, it has yet to be observed in modern-day glaciers. Understanding the processes that govern the stability of marine-terminating glaciers is crucial to forecasting potential future sea level rise. Jeremy Bassis and colleagues developed an improved dynamic ice model that captures the complex behavior of marine-terminating ice cliffs. Unlike previous rheologic models, Bassis et al.'s is capable of simulating glacier flow and ice fracture simultaneously. According to the study, upstream ice thickness plays a key role in determining ice cliff collapse - dynamic thinning of the glacier can slow or stabilize cliff retreat. What's more, the resistive forces at the ice front - the packed sea-ice and icebergs that choke narrow fjords - can further slow, even halt, the retreat of an ice cliff. However, when there is significant upstream ice thickening, a transition to catastrophic ice sheet collapse can occur. Nicholas Golledge and Daniel Lowry discuss the study and its implications further in a related Perspective.