OLogy Cards > sea ice

sea ice

Around the North Pole, temperatures are so cold that parts of the ocean turn into solid ice called "sea ice." It's also happening near the South Pole, where oceans surrounding Antarctica freeze and thaw with the seasons. The forming and melting of sea ice at both poles plays an important role in Earth's climate. In turn, Earth's changing climate has an impact on sea ice. Long-term studies show that Arctic Ocean sea ice is slowly shrinking.

Stats

Definition: ice that forms when sea water freezes
Location: polar oceans (Arctic Ocean and surrounding Antarctica)
Significance: Affects global climate and ocean circulation patterns, and provides habitat for variety of wildlife.
Properties: floats; forms and melts with the seasons
Cool fact: Polar bears and Arctic peoples such as the Inuit depend on sea ice as a hunting ground.

Story

A seal leaning on a piece of ice, pictured from the neck up.

Slowly Shrinking
As temperatures drop in winter months, sea ice forms across the polar oceans. By the end of the following summer, more than half of this ice has melted. Year after year, sea ice follows this natural cycle of freezing and thawing.

Since 1979, scientists have observed this cycle in the Arctic using satellite images. By comparing past and present data for a given month, they have found that the sea ice in the arctic has decreased 25% since 1979. Many scientists believe this reduction in sea ice is linked to a gradual rise in global temperatures.

Why does it matter? First, sea ice plays an important role in Earth's climate, cooling the Earth by affecting the ocean circulation and reflecting solar energy.

Second, sea ice at both poles is an important habitat for a variety of animals, from microscopic bacteria and algae to large mammals. Warm-blooded animals such as birds, seals, whales, and polar bears use the ice for migration routes, hunting grounds, and protection for raising their young.

Stumper

Various animals have adapted to life around sea ice. In both the Arctic and Antarctic, the sea ice is home to sea birds, whales, and:

penguins

polar bears

seals

Correct!

Penguins live only in Antarctica, and polar bears live only in Arctic regions. But seals can be found in both places. They have a layer of blubber to keep them warm. Under sea ice, seals hold their breath until they find cracks or holes in the ice to breathe.

Stumper

Fresh water freezes at 0 degrees Celsius (32 degrees Fahrenheit). The freezing temperature of salt water is:

lower

highter

the same

Correct!

Salt water freezes at a lower temperature than fresh water does. And the saltier the water, the lower the freezing temperature. Seawater, which is 3.5 percent salt, freezes at about -2.2 degrees C (28 degrees F).

Fact or Fiction

The only difference between sea ice and a glacier is the shape of the ice.

Fact

Fiction

Sea ice forms from salt water—but the salt is left behind. Glaciers are made of compacted snow, or fresh water.

Fact or Fiction

When water freezes, it becomes more dense.

Fact

Fiction

Most liquids do become more dense when they freeze. But not water. When water freezes, its molecules form a crystal structure with empty spaces. That's why ice floats.

Image credits: main image, National Science Foundation.

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