The floating ice shelves along the seaboard of the Antarctic ice sheet restrain the outflow of upstream grounded ice1, 2. Removal of these ice shelves, as shown by past ice-shelf recession and break-up, accelerates the outflow3, 4, which adds to sea-level rise. A key question in predicting future outflow is to quantify the extent of calving that might precondition other dynamic consequences and lead to loss of ice-shelf restraint.