Grounding line

Marine ice sheet instability

Even if the ice sheet is grounded below the sea level, MISI cannot occur as long as there is a stable ice shelf in front of it. The boundary between the ice sheet and the ice shelf, known as the grounding line, is particularly stable if it is constrained in an embayment. In that case, the ice sheet may not be thinning at all, as the amount of ice flowing over the grounding line would be likely to match the annual accumulation of ice from snow upstream. Otherwise, ocean warming at the base of an ice shelf tends to thin it through basal melting. As the ice shelf becomes thinner, it exerts less of a buttressing effect on the ice sheet, the so-called back stress increases and the grounding line is pushed backwards. The ice sheet is likely to start losing more ice from the new location of the grounding line and so become lighter and less capable of displacing seawater. This eventually pushes the grounding line back even further, creating a self-reinforcing mechanism.