What if all the sea ice melted?

Near the North Pole and around Antarctica, the sea freezes. The ice floats on top. That is sea ice.

What if all the sea ice melted? Would the sea rise up high?

Sea ice is frozen ocean water. It forms, grows and melts right in the sea, unlike icebergs and glaciers, which start out on land. At some time of year, sea ice covers about 34 million square kilometers of ocean, roughly three times the area of Canada.

Many people think that if all this ice melted, the sea would rise and flood the coasts. Is that right? In this lab you can melt floating ice in a model and measure the sea, then compare it with ice melting on land.

About 34 million km², roughly 9 percent of the world ocean, carries sea ice at some point in the year (NSIDC). Because it forms from and floats on seawater, sea ice differs fundamentally from land ice in how it affects sea level.

This lab tests a common idea, that melting sea ice would flood coastlines, by comparing the sea-level effect of equal volumes of floating and grounded ice. It then asks why scientists still watch sea ice so closely.

Sea ice grows in winter and shrinks in summer, every year.

Around Antarctica, almost all of it melts away each summer. Then it freezes again.

Sea ice grows and shrinks with the seasons. Around Antarctica, about 18.5 million square kilometers freeze each winter, but only about 2.5 million are left at the end of summer. So most of that ice melts every single year, and refreezes the next winter.

Arctic sea ice is thicker, usually 2 to 3 meters. Scientists estimate its volume with a computer model fed with observations: on average from 1979 to 2025, it held about 28,000 cubic kilometers of ice in April and about 11,500 in September.

NSIDC gives average late-winter extents of 15.5 million km² (Arctic) and 18.5 million km² (Antarctic), falling to about 6.5 and 2.5 million km² at summer's end; Arctic minima since 2007 have been 3.5 to 5 million km², about a 13 percent per decade decrease since 1979. Arctic ice is typically 2 to 3 m thick, Antarctic 1 to 2 m.

Volume is harder to observe. The University of Washington's PIOMAS reanalysis, which assimilates observations into an ice-ocean model, gives a 1979–2025 mean annual cycle from 28,000 km³ in April to 11,500 km³ in September.

In our pretend lab, Olly melts all the Arctic's sea ice at once.

The sea goes up by only about 2 millimeters. That is a tiny, tiny bit!

Set 'Floating ice that melts' to 28, for 28 thousand cubic kilometers: all the Arctic's sea ice in spring. The model's sea rises by only about 2 millimeters, and a gently sloping shore moves just 19 centimeters.

Now put the same 28 thousand cubic kilometers on land instead. The sea rises about 7 centimeters, more than 35 times as much. Even if you melt 1,000 thousand cubic kilometers of floating ice, the slider's top value and far more than all the sea ice there is, the model's sea rises only about 7 centimeters.

With shelfIce = 28 thousand km³, the model adds 1.93 mm (0.19 m of shoreline retreat at a steepness of 100). The same volume of grounded ice adds 71.0 mm, a ratio of about 37. At the slider's maximum, 10⁶ km³ of floating ice adds 69 mm, while 10⁶ km³ of land ice adds 2.53 m.

The floating result is only the density difference: ice displaces its own mass of seawater (1,028 kg/m³), but melts into fresh water (1,000 kg/m³), which takes up slightly more room. The lab's 28 covers only the Arctic, because the sources used here give no Antarctic sea-ice volume. NASA's point applies to all sea ice, Arctic or Antarctic: it is already floating.

Put ice cubes in a glass of water. Mark the water line. When the ice melts, the water line stays put!

Floating ice is already in the water. Ice on land is not.

A floating ice cube sinks into the water until it has pushed aside its own weight of water. When it melts, it turns into just about that same amount of water, so it fills the space it was already taking. That is why the level in a glass of ice water does not rise as the ice melts.

Ice on land is different. It is not in the sea yet, so all its meltwater is new water for the ocean. That is the big difference between this lab and melting glaciers.

Archimedes' principle: a floating body displaces a mass of fluid equal to its own. NASA puts it plainly: sea ice is already displacing its own weight, so its melt raises the sea no more than melting cubes overflow a glass of ice water.

The model's small residual, V·ρ_ice·(1/ρ_fresh − 1/ρ_sea), about 2.5 percent of the ice volume, assumes meltwater is fresh. NSIDC notes new sea ice holds salty brine, which drains out as it ages; the model leaves that saltiness out. It also ignores timing and uneven regional sea level.

Sea ice is shiny and white. It bounces sunlight away and keeps the poles cool.

Seals rest on it, walruses use it, and polar bears hunt on it.

Melting sea ice barely changes sea level, but it changes a lot else. Bright sea ice reflects 50 to 70 percent of the sunlight that hits it, and snow-covered ice up to 90 percent. Dark open ocean reflects only about 6 percent and soaks up the rest, which warms the water and melts still more ice.

Sea ice is also a home. Seals and walruses rest and have their babies on it, and polar bears hunt and breed there. Tiny algae grow on the ice. When it melts in spring, they help start a bloom of phytoplankton, tiny ocean plants at the base of the food web: fish eat them, seals eat the fish, and polar bears and orcas eat the seals.

NSIDC: the open ocean reflects about 6 percent of incoming solar energy, sea ice 50 to 70 percent and snow-covered sea ice up to 90 percent, so ice loss exposes darker water that absorbs more heat, an amplifying feedback that makes the poles especially sensitive. Sea ice is a resting and birthing platform for seals and walruses and a hunting and breeding ground for polar bears.

NASA's Earth Observatory adds an indirect link to sea level: when ice shelves fed by glaciers break up, those glaciers can speed up, adding new land ice to the ocean, as observed on the Antarctic Peninsula.