An ice shelf is a thick slab of ice that floats on the ocean while remaining attached to a glacier or ice sheet on land. Think of it as the point where a land-based glacier flows out over the sea and begins to float. Ice shelves range from about 160 to 2,000 feet thick and can extend tens to hundreds of miles from the coastline. They exist primarily around Antarctica, where the largest ones rival the size of Texas or France, though a few smaller ones also form in the Arctic.
How Ice Shelves Form
Ice shelves begin as glaciers. Glaciers are massive rivers of compressed snow and ice that flow slowly across land under their own weight. When a glacier reaches the coast and pushes out over the ocean, the ice eventually becomes thin enough relative to its width that it starts to float. The zone where this transition happens is called the grounding zone, a belt a few kilometers wide where the ice alternates between resting on the seafloor and floating freely as tides raise and lower the water level. Beyond this zone, the ice is fully buoyant, and that floating portion is the ice shelf.
The grounding zone shifts back and forth with the tides, so it’s not a crisp boundary. But at the scale of an entire ice sheet, it functions like a line separating grounded ice from floating ice. Everything seaward of that line is the ice shelf. Everything landward is the glacier feeding it.
Why Ice Shelves Matter for Sea Level
Ice shelves themselves are already floating, so when they melt or break apart, they don’t directly raise sea levels, for the same reason a melting ice cube in a full glass doesn’t make it overflow. But they play a critical indirect role. An ice shelf acts like the neck of an hourglass: pinned against islands, peninsulas, or the coastline on its sides, it pushes back against the glaciers flowing into it, slowing their movement toward the ocean. Scientists call this the buttressing effect.
When an ice shelf weakens or disappears, that back-pressure vanishes. The glaciers behind it speed up dramatically, dumping land-based ice into the sea and raising water levels. This isn’t hypothetical. After the Larsen B Ice Shelf on the Antarctic Peninsula collapsed in 2002, the glaciers feeding it accelerated by about 300% on average. Their ice loss jumped from 2 to 4 billion metric tons per year before the collapse to between 22 and 40 billion metric tons per year by 2006.
The stakes are enormous. The Antarctic ice sheet holds enough ice to raise global sea levels by 58 meters (about 190 feet) if it all melted. Ice shelves are essentially the gatekeepers holding thousands of meters of land-based ice in place.
How Ice Shelves Lose Mass
Ice shelves shrink through two main processes: calving and basal melting. Calving is the dramatic one you’ve probably seen in videos. Chunks of ice crack off the front edge of the shelf and fall into the ocean as icebergs. Basal melting is less visible but, it turns out, more significant. It happens when relatively warm ocean water circulates beneath the shelf and melts it from underneath.
A NASA-led study found that between 2003 and 2008, basal melting accounted for 55% of all Antarctic ice shelf mass loss, roughly 1,325 trillion kilograms per year. Iceberg calving contributed the remaining 1,089 trillion kilograms per year. That finding challenged the long-held assumption that calving was the dominant driver of ice loss. It also means that warming ocean temperatures, not just warmer air, are a major factor in ice shelf stability.
Notable Collapses
The Larsen Ice Shelf Complex along the Antarctic Peninsula has provided the most dramatic examples of collapse. Larsen A, the northernmost section, lost about 1,500 square kilometers of ice in a sudden event in January 1995. Then in the summer of 2002, scientists watching daily satellite images saw nearly the entire Larsen B Ice Shelf splinter and collapse in just over a month. These events provided some of the clearest real-world evidence of what happens to glaciers when the ice shelf bracing them disappears.
In 2017, a trillion-ton iceberg broke off the Larsen C shelf further south, raising questions about its long-term stability. These collapses are not isolated curiosities. They offer a preview of what could happen to far larger ice shelves like the Ross Ice Shelf, Antarctica’s biggest, which spans nearly 372 miles and rises 50 to 160 feet above the waterline.
Life Beneath the Ice
Despite perpetual darkness and near-freezing temperatures, the ocean beneath ice shelves supports more life than scientists once expected. When icebergs calve and expose areas that had been hidden for centuries, researchers have found thriving ecosystems underneath, including large corals, sponges, icefish, giant sea spiders, and octopus. These discoveries suggest the sub-shelf ocean is not the barren environment it was long assumed to be.
How Scientists Monitor Ice Shelves
Satellites are the primary tool for tracking ice shelf thickness and stability across Antarctica. The European Space Agency’s CryoSat-2 mission, specifically designed to monitor polar regions, uses radar altimetry to measure the height of ice shelves above the waterline. Because the shelves float in a predictable balance with the ocean (the same physics that keeps a boat at a consistent waterline), scientists can calculate total ice thickness from that surface height measurement alone.
CryoSat-2 covers 92.3% of Antarctic ice shelves and performs especially well near grounding zones, where previous satellite missions struggled with the abrupt changes in surface slope. Combined with aircraft surveys and field measurements, this satellite data gives researchers a continent-wide picture of which ice shelves are thinning, how fast, and where the losses are concentrated.