Ice Shelves

Ice shelves form when glaciers or ice streams flow from land across the coastline and begin to float on the ocean. The transition from grounded ice to floating ice is called the grounding line. Snow accumulating on the shelf and ice arriving from upstream add mass; basal melting, surface processes and the calving of icebergs remove it. This makes an ice shelf distinct from sea ice, which forms when ocean water freezes. Ice shelves are especially extensive around Antarctica.

A floating shelf can resist the movement of the grounded ice feeding it, particularly where it presses against islands, peninsulas or submerged features. This restraining effect is known as buttressing. If a shelf thins or disintegrates, some of that resistance may be lost, allowing tributary glaciers to flow more quickly toward the ocean. The amount of buttressing varies among shelves and depends on their geometry and contact with surrounding features.

Warm ocean water can melt an ice shelf from below, while cracks can develop and release icebergs at the seaward edge. Calving is a normal part of shelf evolution and does not by itself mean a shelf is collapsing. Because floating ice already displaces seawater, its breakup has little direct effect on sea level. The more important potential effect is indirect: faster discharge of grounded glaciers into the ocean adds land ice to the sea and can raise sea level.

Satellite observations, field measurements and ice-core or ocean studies help researchers track shelf thickness, flow, grounding-line position and mass change. These observations clarify how ocean heat and atmospheric conditions affect shelves, how much freshwater enters Antarctic coastal waters, and how changes in buttressing influence the grounded ice upstream. Since shelves differ in their local conditions, the response of one cannot automatically be applied to all others.