Mechanisms which determine time-dependent changes of the marine ice margin in dynamic ice sheet models are important but poorly understood. Here we derive an empirical formulation for changes in the marine extent when modeling the Northern Hemisphere ice sheets over the last glacial cycle in a three-dimensional thermo-mechanically coupled ice sheet model. We assume that the strongest control on changes in marine extent is ice calving, and that the variable most crucial to calving is water depth. The empirical marine extent relationship is tuned so that the major marine retreat history of the North American and Eurasian ice sheets are modeled accurately in time and space. We find that this empirical treatment based on water depth is sufficient to reproduce the observations, and discuss the implications for the physics of marine margin changes and the dynamics of the Northern Hemisphere ice sheets since the Last Glacial Maximum.