Sea ice
Frozen seawater that shapes climate, navigation, and polar ecosystems.
Sea ice forms when seawater freezes, and because it is less dense than liquid water, it floats on the ocean’s surface. This ice covers roughly 7% of the Earth’s total surface and about 12% of the world’s oceans, with the vast majority found in the polar ice packs of the Arctic Ocean and the Southern Ocean surrounding Antarctica. These polar packs undergo a natural yearly cycle, reaching their maximum extent in winter and retreating during summer. Within the ice, salty brine channels create a habitat for microorganisms, which form the base of unique food webs. The presence or absence of sea ice also influences navigation routes, regional weather patterns, and global ocean circulation. Sea ice plays a crucial role in Earth’s climate: its bright white surface reflects solar energy back into space, a process called the albedo effect that helps keep the planet cool, while the ice also insulates the ocean below, limiting the exchange of heat, water vapor, and gases like carbon dioxide between the sea and the atmosphere. Satellite records have shown a marked decline in Arctic sea ice extent and thickness in recent decades, a trend linked to global climate change, while Antarctic sea ice shows more regional variability but has also recently experienced declines. Sea ice is highly dynamic, constantly deformed by winds, currents, and temperature fluctuations, leading to a wide variety of ice types and features. It differs from icebergs, which originate from ice shelves or glaciers that calve into the ocean, though sea ice may sometimes contain embedded icebergs depending on location.
- coverage
- 7% of Earth's surface, 12% of world's oceans
- thickness_range_first_year
- 0.3 m to 2 m
- thickness_range_old_ice
- 2 m to 4 m
- brine_volume_growth_period
- typically below 5%
- air_volume_growth_period
- typically 1–2%, up to 15% in summer
Lore & Background
Sea ice is classified by whether it is attached to the shoreline (fast ice) or free to drift (drift ice or pack ice). Drift ice consists of floes, individual pieces of sea ice 20 meters or more across, with sizes ranging from small (20–100 m) to giant (over 10 km). The marginal ice zone is the transition region between open ocean and consolidated pack ice, characterized by fragmented floes. Sea ice is also classified by age: new ice, nilas, young ice, first-year ice, and old ice (which includes second-year and multiyear ice). Multi-year ice is more common in the Arctic than in the Antarctic due to drift patterns.
Reader's Guide
Sea ice is significant for its role in Earth's climate system, reflecting solar energy via the albedo effect and insulating the ocean, which limits heat and gas exchange with the atmosphere. It provides habitat for microorganisms in brine channels, forming the base of unique food webs. Satellite records show a marked decline in Arctic sea ice extent and thickness in recent decades, linked to global climate change, while Antarctic sea ice shows regional variability with recent declines. Sea ice also shapes navigation routes, regional weather, and global ocean circulation. Its physical properties—density, thermal conductivity, and mechanical strength—depend on brine and air volumes, which vary with age and temperature. Leads and polynyas, areas of open water within ice expanses, are important for wildlife and navigation. Sea ice differs from icebergs, which originate from ice shelves or glaciers.
Did You Know?
- Sea ice covers about 7% of Earth's surface and about 12% of the world's oceans.
- Multi-year ice is much more common in the Arctic than in the Antarctic.
- Leads are narrow, linear open-water areas within sea ice, varying in width from meters to kilometers.
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