Closing the loop on elevation change at Summit, Greenland.

Surface elevation on a large ice sheet changes due to multiple physical processes, some of which imply mass change of the ice sheet, and some not. Accumulation of new snow, in absence of other processes, will increase surface elevation as new mass is added to the ice sheet. The density of this new snow affects the amount of mass added. Compaction of snow and firn, both new and old, decreases surface elevation with no corresponding change in mass. As ice flows out to the sides on an ice sheet, conservation of mass dictates that the surface elevation will decrease, corresponding to mass loss. In response to long-term changes in mass, the continental crust on which the ice rests seeks isostatic balance, resulting (since the last glacial maximum) in an increase in elevation, with no associated mass change. The summation of these processes results in net elevation change.