What Is an Ice Shelf?

An ice shelf is a thick, floating plate of glacial ice attached to a coastline and extending over the ocean surface. Ice shelves form where continental ice sheets or glaciers flow downslope to the sea, reach the water's edge, and begin to float as the ice thickness exceeds the depth at which buoyancy takes effect. They occupy the interface between land-based cryosphere and the open ocean, and their stability has direct consequences for global sea level and ocean circulation patterns.

Ice shelves are found primarily in Antarctica, where they fringe roughly 44 percent of the coastline, and in Greenland and the Canadian Arctic. Antarctic shelves include some of the largest floating ice bodies on Earth: the Ross Ice Shelf covers an area comparable to France, and the Filchner-Ronne Ice Shelf is nearly as large. Thickness ranges from approximately 50 to 600 meters, with the base often far below the ocean surface and the exposed freeboard reaching tens of meters above it. According to the National Snow and Ice Data Center, under stable climate conditions, ice shelves can persist for thousands of years, supported by coastal rock formations such as peninsulas and offshore islands that provide lateral buttressing.

Formation and Mass Balance

Ice shelves grow as outlet glaciers and ice streams deliver meteoric ice from the interior of an ice sheet to the coastal margin. Snowfall accumulates on the upper surface and compacts into ice over time, adding to shelf thickness. Marine ice can also accrete to the base when cold, dense seawater partially freezes on contact with the shelf's underside. The seaward edge of an ice shelf periodically sheds large tabular icebergs in a process called calving, which is the primary mechanism by which mass is lost. On a dynamically stable shelf, calving is a near-cyclical process that roughly balances the input from inland glaciers.

Buttressing and Grounding Line Dynamics

The critical mechanical function of an ice shelf is buttressing: by exerting a resistive back-pressure on the glaciers feeding it, the shelf slows the rate at which ice drains from the continent into the ocean. When this resistance disappears through collapse, inland glaciers accelerate. Following the 2002 disintegration of the Larsen B Ice Shelf on the Antarctic Peninsula, research published in the Journal of Glaciology documented that the glaciers feeding that shelf accelerated between three and eight times their pre-collapse speeds, dramatically increasing the discharge of ice into the Southern Ocean. The grounding line, the boundary where grounded ice transitions to floating shelf, is a critical threshold: its retreat inland indicates that the ice sheet is losing mass and contributing to sea-level rise.

Remote Sensing and Monitoring

Sustained observation of ice shelves depends on satellite remote sensing, because the environments involved are too remote and hostile for comprehensive ground-based surveys. Radar altimeters, synthetic aperture radar (SAR), optical imagers, and gravity sensors such as those on the GRACE and GRACE-FO missions collectively track shelf elevation change, surface velocity, calving-front position, and mass loss over time. Ice sheet dynamics research at NOAA's Geophysical Fluid Dynamics Laboratory uses coupled ocean-ice models to project how warming ocean temperatures and changing atmospheric circulation will affect shelf stability and grounding-line migration over decadal timescales.

Applications

Ice shelf research has applications in a range of fields, including:

• Sea-level rise projections and coastal inundation planning
• Ocean circulation and deep-water formation modeling
• Climate sensitivity assessment and paleoclimate reconstruction
• Calibration and validation of satellite altimetry and gravimetry instruments
• Glaciological hazard assessment for polar operations and shipping routes