Understanding Fast Ice in Polar Ecosystems and Climate Dynamics

Caitlin Dempsey


Landfast sea ice, also known as fast ice, is sea ice that stays in place because it’s attached to the coast, icebergs, ice shelves, the ocean floor, or other features on the continental shelf.  The term gets its name from the fact that the sea ice is “fastened” to a land feature and doesn’t move with winds or ocean currents.

As frozen seawater attached to coastlines, fast ice regulates the seaward flow of land ice and provides a habitat for animals. Fast ice occurs in both the Arctic and Antarctic regions.

Fast ice forms in narrow bands, typically up to 200 km wide, and varies in thickness from a few centimeters to several meters. Fast usually forms in the fall and lasts through winter. Each summer, rising water and air temperatures near the Poles cause some fast ice to melt. Due to differences in geography and climate, Antarctic sea ice tends to melt more thoroughly in the summer compared to Arctic sea ice.

In some places the ice can persists throughout the summer and become multi-year ice. Multiyear ice becomes less salty and harder each year it survives the summer melt. In contrast, first-year ice is thinner, saltier, and more likely to melt during the following summer.

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A satellite image of a snowy landscape and ocean with swirls of ice near Greenland.
Fast ice, anchored to the shore or the shallow ocean bottom, is visible at the top of this satellite image and along various sections of the coast. Image: MODIS, NASA, public domain, June 4, 2024.

Types of fast ice

Fast ice can be classified into two types: level fast ice and rough fast ice. Level fast ice forms directly as the water cools. Rough fast ice forms when level ice breaks and moves around or when pack ice becomes stuck due to dynamic interactions. Both types can become thicker through various processes.

Fast ice versus pack ice

Fast ice is attached to the shore or the sea floor, while pack ice moves freely with the currents and winds. As pack ice drifts, gaps called leads frequently open and close between the ice floes. Some openings, known as polynyas, remain open for longer periods due to strong winds or ocean currents.

An annotated satellite image of a white icy area.
An annotated satellite image showing pack ice and fast ice. Image: MODIS satellite image, NASA, public domain.

The important of fast ice

Despite covering only a small portion of the total sea ice (about 4% to 13%), fast ice significantly influences various physical, biological, and chemical processes, impacting both local environments and the global system. Until recently, fast ice was often ignored in research because its distribution wasn’t well understood, earning it the label of a “missing piece of the Antarctic puzzle.”


Fraser, A. D., Wongpan, P., Langhorne, P. J., Klekociuk, A. R., Kusahara, K., Lannuzel, D., … & Wienecke, B. (2023). Antarctic landfast sea ice: A review of its physics, biogeochemistry and ecologyReviews of Geophysics61(2), e2022RG000770. DOI: 10.1029/2022RG000770

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.