Sea Ice Explained: How Frozen Ocean Water Shapes Polar Climate

What Is Sea Ice?

Sea ice is frozen ocean water that forms naturally when seawater cools below its freezing point. Unlike
icebergs, which originate from glaciers on land, sea ice develops directly on the ocean surface.

Every winter, sea ice expands across large areas of the Arctic Ocean and around Antarctica. During warmer months, much of it melts or breaks apart, creating one of Earth’s most dynamic seasonal cycles.

How Does Sea Ice Form?

As air temperatures fall, the ocean surface cools until seawater begins to freeze. Tiny ice crystals first appear, gradually joining together into larger sheets that thicken throughout winter.

• Cooling: seawater loses heat to the atmosphere.
• Initial freezing: tiny ice crystals called frazil ice form.
• Young ice: thin sheets begin covering the ocean surface.
• Thickening: continued freezing creates larger, stronger ice cover.
• Seasonal evolution: winds and currents reshape the ice throughout the year.

Unlike freshwater ice, sea ice contains pockets of concentrated saltwater called brine. As the ice ages, much of this salt drains away, making older sea ice less salty and stronger.

Types of Sea Ice

Scientists classify sea ice according to its age, thickness and movement.

• Frazil ice: tiny floating crystals that form first.
• New ice: thin surface ice less than about 10 centimeters thick.
• First-year ice: ice that survives one winter.
• Multi-year ice: thicker ice that survives several summers.
• Pack ice: drifting sea ice pushed by winds and currents.
• Fast ice: sea ice attached to the coastline or seabed.

Sea Ice vs. Icebergs

Sea ice and icebergs are often confused, but they have completely different origins.

Why Sea Ice Is Important for Earth’s Climate

Sea ice acts like a giant mirror. Its bright white surface reflects much of the Sun’s energy back into space, a property known as albedo. When sea ice shrinks, darker ocean water absorbs more heat, accelerating regional warming.

Sea ice also helps regulate heat exchange between the ocean and atmosphere, influences weather patterns and contributes to the formation of dense polar waters that help drive global
thermohaline circulation.

Sea Ice and Ocean Circulation

As sea ice forms, much of the salt is expelled into the surrounding seawater, making it colder and denser. This dense water sinks, helping power deep ocean circulation.

Sea ice therefore connects directly to
ocean currents,
AMOC,
ocean temperature and Earth’s climate system.

Sea Ice and Polar Wildlife

Sea ice supports some of the world’s most unique ecosystems. It provides hunting platforms, breeding areas and feeding grounds for many Arctic and Antarctic species.

• Polar bears hunt seals from sea ice.
• Seals raise pups on stable ice platforms.
• Penguins depend on seasonal sea ice in Antarctica.
• Algae growing beneath sea ice support marine food webs.
• Whales and seabirds often follow seasonal ice edges where food is abundant.

Sea Ice and Climate Change

Arctic sea ice has declined significantly over recent decades, particularly during late summer. Scientists monitor sea ice extent, thickness and age because they provide important indicators of climate change.

Antarctic sea ice behaves differently and varies considerably from year to year, but recent years have also seen record-low sea ice extent in several seasons.

Common Sea Ice Terms

• Sea ice extent: total ocean area covered by at least 15% ice.
• Sea ice concentration: percentage of ice covering a specific area.
• Ice edge: boundary between open water and sea ice.
• Polynya: persistent area of open water surrounded by sea ice.
• Lead: long crack or opening within sea ice.

FAQ: Sea Ice Explained