What Is an Ice Cap?

An ice cap is a dome-shaped mass of glacier ice that covers the land beneath it and spreads outward under its own weight, with a total area of less than 50,000 square kilometers. The size threshold is the standard glaciological convention: a comparable body larger than 50,000 square kilometers is called an ice sheet, and only Greenland and Antarctica qualify. Ice caps sit between valley glaciers and ice sheets in scale, and they differ from both in shape. A valley glacier is steered by the terrain it flows through, while an ice cap is thick enough to bury the terrain and impose its own dome geometry, as the National Snow and Ice Data Center's overview of glaciers describes.

Well-known examples include Vatnajökull in Iceland, which covers roughly 7,700 square kilometers, Austfonna on the Svalbard archipelago, the Devon and Penny ice caps in the Canadian Arctic, and Quelccaya in the Peruvian Andes, the largest tropical ice cap. A smaller and more elongated body constrained by underlying ridges is usually called an ice field rather than an ice cap.

Formation and Flow

An ice cap forms where annual snowfall exceeds annual melt for long enough that buried snow compacts into firn and then into glacial ice, a transition that typically takes decades to centuries depending on temperature and accumulation rate. Once the ice is a few tens of meters thick, it deforms plastically and begins to flow. Flow radiates outward from a central dome toward the margins, driven by the surface slope rather than by the bed slope. Where the margin meets a valley, the ice cap discharges through fast-moving outlet glaciers that can move orders of magnitude faster than the interior. The dividing line between the accumulation zone and the ablation zone, called the equilibrium line, migrates up or down in elevation as climate changes, and its position is one of the most direct indicators of whether the ice cap is growing or shrinking.

Measurement and Monitoring

Ice caps are monitored with a combination of field and satellite techniques. Ablation stakes and snow pits give direct point measurements of surface mass balance, while ground-penetrating and airborne radar sound the ice thickness and map the bed. From orbit, laser altimetry from ICESat-2 tracks surface elevation change, interferometric synthetic aperture radar measures surface velocity, and satellite gravimetry from the GRACE and GRACE-FO missions resolves month-to-month changes in total ice mass by detecting the resulting variation in Earth's gravity field. Combining these gives both the volume change and the mechanism behind it, separating surface melt from increased discharge through outlet glaciers.

Ice Caps and the Water Cycle

Although ice caps hold far less water than the two ice sheets, they respond much faster to warming because they are thinner and occupy a narrower elevation range. Glaciers and ice caps outside Greenland and Antarctica together store enough water to raise global sea level by a few tenths of a meter, and they currently account for a substantial share of the observed rise, according to the glacier quick facts compiled by NSIDC. Regionally they matter more than that figure suggests. Meltwater from ice caps in Iceland, Norway, Patagonia, and the Andes feeds hydroelectric reservoirs and dry-season river flow, so changes in their mass balance carry direct consequences for water supply and power generation.

Applications

The study of ice caps has applications in a range of fields, including:

• Sea level projection and climate modeling
• Water resource and hydropower planning in glacierized basins
• Satellite remote sensing algorithm development and validation
• Paleoclimate reconstruction from ice cores
• Natural hazard assessment, including glacial lake outburst floods and subglacial volcanic eruptions
• Aviation and field logistics in polar regions