Resource: Antarctica: Sea Ice

Print Background Essay

Three percent of Earth's water exists as ice — most of it locked up in glacial formations. This ice, together with the seasonal sea ice that forms in the polar regions where temperatures remain below freezing year-round, helps to regulate temperature, moisture, and ocean salinity worldwide.

On average, seawater contains 35% salt, so it freezes at a slightly lower temperature (-2°C or 28°F) than pure water does. When sea ice forms, the frozen water floats above the denser ocean water, and the salt falls out of solution into the sea below. As the water begins to freeze, chunks of frazil ice form and collide with one another. Frazil ice is composed of small, spiky ice crystals and plates suspended in the water. The surface gradually takes on a greasy appearance before forming a solid ice cover.

In the Arctic Ocean, extremely thick ice forms because land surrounds most of the Arctic Ocean and creates a relatively calm setting for ice formation. In contrast, sea ice surrounding the Antarctic continent typically grows no more than a few meters thick. High winds and strong currents produce restless seas around the large landmass, and the ice tends to break up before it can thicken. Antarctic sea ice is known as seasonal ice because it forms and melts as the polar seasons change from winter to summer. In the Arctic, the seasonal fluctuation of sea ice is much less than in Antarctica. Sea ice that remains year-round is called perennial ice.

Data generated from satellite monitoring of polar ice formation indicate that sea ice coverage and thickness have decreased in both the Arctic and Antarctic over the past three decades. Although Antarctic sea ice has rebounded since a dramatic reduction in volume in the 1970s, the recent loss rate of Arctic sea ice — 8 percent per decade — is such that it could completely disappear by the end of this century. Even small changes in ice volume could have a significant impact on global climate, ocean circulation patterns, and polar animal habitats; such a dramatic change in ice volume would likely have an even more dramatic impact on the planet.

To explain the decline in sea ice, scientists have proposed several potential causes, including global warming accelerated by fossil fuel consumption; the effects of pollution, specifically the increased absorption of sunlight due to the accumulation of black carbon soot; and natural climatic variability.

To learn more about the important roles that ice plays in moderating global temperature and ocean circulation, check out Earth System: Ice and Global Warming.

To learn more about how water is circulated in the world's oceans, check out Great Ocean Conveyor Belt: Part I and Great Ocean Conveyor Belt: Part II.

To learn more about how glacial melting might affect global sea level, check out Mountain of Ice: If the Ice Melts.

Print Questions for Discussion

• Under what conditions does sea ice form?
• Describe the seasonal changes in the ice sheet around Antarctica.
• Why did it take people so long to reach the continent of Antarctica?
• How do the animals on Antarctica survive such harsh conditions?
• Why did the explorers have to abandon the ship Endurance?