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Place 05 · The Southern Ocean · Chapter 7 of 7
The Ice That Decides the Coastline
Antarctica holds enough ice to raise the world’s oceans by about 58 m, and what matters most is whether that ice is floating or sitting on rock.
About 15 minutes · 1 video · 1 3D model · 2 quick activities
Start here
Floating ice versus ice on land
Drop ice cubes into a glass of water, let them melt, and the water level barely moves, because floating ice already pushes aside its own weight in water. Sea ice and floating ice shelves work the same way. Ice sitting on land is different: when it melts or slides into the sea, it adds water that was not there before, and the sea rises.
Watch this
See it move
Vox — Why scientists are so worried about this glacier. Five minutes on Thwaites, the West Antarctic glacier with the cheerful nickname “Doomsday Glacier.” It is from 2020, so a few numbers have been updated since.
While you watch, look for
- Why warm ocean water reaching the underside of the glacier is the problem
- What the glacier is holding back behind it
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The three ices
Sea ice, shelf, sheet
Sea ice is frozen seawater, a meter or two thick. An ice sheet is a continent-sized pile of ice on land; in Antarctica it is up to about 4.8 km thick. Where the ice sheet flows off the coast and starts to float, it becomes an ice shelf, 50 to 600 m thick and still attached to the land ice behind it.
The brake
Ice shelves hold back glaciers
An ice shelf breaking up does not directly raise sea level, because it is already floating. But ice shelves act as brakes for the glaciers feeding them. When the Larsen B Ice Shelf on the Antarctic Peninsula fell apart in 2002, losing about 2,700 km² in just over a month, the glaciers behind it sped up to two to six times their earlier speed, pushing more land ice into the sea.
The glacier
Thwaites and the rising sea
Global average sea level has risen about 21 to 24 cm since 1880, and in 2024 alone it rose 0.59 cm. Part of that is meltwater, and part is thermal expansion: warmer water takes up more space. Thwaites Glacier in West Antarctica, about 120 km wide, holds enough ice to raise the sea by 65 cm and already accounts for about 4% of global sea-level rise.
Photos: ArcticDesire.com Polarreisen, Yuanpang Wa, Ray Bilcliff / Pexels
Play with it
Dive to where the ice sheet sits on the seafloor
This 3D model comes from a 2021 study in which a remotely operated vehicle explored an ice cliff on the Weddell Sea coast, where the Antarctic ice sheet meets the ocean. This is grounded ice, resting almost on the seafloor itself, the kind that raises sea level when it melts or breaks away. Spin it and look for the thin gap between the bottom of the ice and the seafloor.
3D model: “Weddell Sea Tidewater Ice Cliff” by clkruse on Sketchfab, from Owsianowski and Richter (2021), Communications Earth & Environment.
Two questions to answer while you are in there
- Find the gap between the bottom of the ice and the seafloor. If warmer water worked its way into that gap and melted the ice from below, what would happen to the ice cliff, and to sea level?
- Ice cliffs like this one line about 38% of Antarctica’s coastline. Is this ice floating like an ice shelf or resting on the bottom, and why does that difference matter for sea level?
Try it yourself
Check yourself
Try it
Raises Sea Level or Barely Changes It?
Eight things happen to the ice and the ocean. Decide which ones raise global sea level.
Try it · interactive
Sea-Level Sorter
Melt sea ice, then a floating ice shelf, then grounded land ice, and watch a sea-level gauge after each; warm the ocean to add thermal expansion, and see where the coastline would sit if West Antarctica, or all of Antarctica, lost its ice.
Open the Sea-Level Sorter in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Ice shelf
Floating ice still attached to land ice
Ice sheet
Continent-sized mass of ice on land
Thermal expansion
Water taking up more space as it warms
Sea-level rise
Long-term increase in average ocean height
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: The Ice That Decides the Coastline
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 14.3 — Ice and Climate Change — LibreTexts · 8 minutes
- Our World Ocean 14.10 — What Causes Sea Level Change — LibreTexts · 8 minutes
- NSIDC — What happened to the Larsen Ice Shelf? — National Snow and Ice Data Center · 6 minutes
Photo credits (Pexels): Francesco Ungaro, Srattha Nualsate, ArcticDesire.com Polarreisen, Yuanpang Wa, Ray Bilcliff.