Cliff collapse Tall ice cliffs at the edges of ice sheets can collapse under their own weight in spectacular fashion, a process that can considerably hasten ice sheet mass loss. Bassis et al. used a dynamic ice model to demonstrate that this kind of collapse can be slowed either by upstream thinning of the ice sheet or by the resistive forces from sea ice and calved debris (see the Perspective by Golledge and Lowry). Conversely, when there is upstream ice thickening, a transition to catastrophic collapse can occur. Science , abf6271, this issue p. 1342 ; see also abj3266, p. 1266