Sea ice

Key Takeaways

Sea ice forms entirely from freezing ocean water, unlike icebergs, which break off from land-based glaciers and ice sheets.
Once sea ice reaches 3 to 4 meters (10 to 13 feet) thick, it effectively shuts down the exchange of heat between the ocean and the atmosphere.
As seawater freezes it expels salt, creating dense, salty water that sinks and helps drive the global ocean conveyor belt, the AMOC.

Sea ice is frozen seawater that floats on the ocean surface, forming and melting with the seasons, and beyond reflecting sunlight it helps drive global circulation by expelling salt as it freezes, creating dense water that sinks and feeds the deep-ocean conveyor.

What is sea ice?

Below-freezing temperatures in the Arctic and Antarctic cause the ocean’s surface to freeze into ice crystals. When the water is calm, those crystals form wafer-thin sheets of ultra-smooth ice that slide over each other and freeze together into thick rafts. In choppy waters, ice crystals form slushy pancake-like chunks that can merge to create rough ice. Ridges form where wave action causes the pancakes to collide. Wind, waves, and currents move loose pieces of ice, called drift ice, across the water, creating openings where new ice can form or pile ice pancakes together. Some freezes to the coastline or ocean floor where it remains in place. Such ice is called fast ice.

Sea ice grows over the winter and recedes in summer. The thicker the ice, the more likely it is to last through the summer melt, leading to still thicker layers of sea ice the following year. Such multiyear ice hardens each year, leading to overall gains in local ice cover. First-year ice, on the other hand, tends to be thin and is more likely to melt during the summer.

Sea ice creates a barrier between the ocean and atmosphere, reducing movement of heat between the two environments. As the ice layer grows thicker, less heat is able to move across it. Once the ice reaches 3 to 4 meters (10 to 13 feet) thick, it effectively shuts down interactions between the air and water.

Why is it important

As seawater freezes, ice crystals expel salt, which creates freshwater ice that floats on the ocean’s surface and releases salt into the water below. The increase in salinity creates dense water that sinks to the ocean floor, where it begins a slow migration toward the equator, contributing to the global ocean conveyor belt, the Atlantic Meridional Overturning Circulation (AMOC). This massive current redistributes energy throughout the world’s oceans and contributes to global climate.

The surface of the ice plays a critical role in reducing incoming energy from sunlight. Ice is highly reflective; most incoming sunlight bounces off the surface and returns to space. This keeps temperatures near the poles relatively cool, even during summer months. As ice begins to melt, however, it forms pools of dark water that absorb sunlight. This warms the water, further melting the ice and creating larger pools of water that absorb more sunlight. This feedback loop can contribute to rapid collapses of sea ice, making polar regions especially vulnerable to climate change.

Summer melt along the sea ice edge releases trapped nutrients into the water, which leads to increases in phytoplankton—tiny photosynthetic organisms that form the base of the marine food web. This attracts a variety of marine life, including higher-level predators, such as fish and whales, to areas along the ice rim. The ice itself provides essential habitat for marine mammals, including polar bears, seals, and Arctic foxes. These animals use the ice for birthing, hunting, and nursing their young. As sea ice declines, polar bears, in particular, are struggling to survive, and in their efforts to find food, they are encountering humans more frequently.

Because sea ice is created from seawater, it does not contribute to rising sea levels when it melts.

Group Operations Leader John Kemp uses a hook to recover a top float mooring in the Beaufort Sea of the Arctic Ocean to acquire vertical profiles of temperature salinity and velocity in the water column, measure ice draft, and obtain bottom pressure readings over the past year. (Photo by Rick Krishfield, © Woods Hole Oceanographic Institution)

How are scientists studying sea ice?

Since 1979, satellites have allowed for continuous measurements of sea ice extent—the total area covered by ice—as well as its overall depth. This has allowed researchers to track sea ice as it expands in winter and retreats in summer. These records show that sea ice is now covering less area in both summer and winter in most years, with the Arctic most strongly affected by changes in global climate. In the last two decades, summertime losses have exceeded those seen prior to the 1980s and 1990s, and the most extreme ice loss events have happened in the last 15 years. Models predict that by the year 2100, the Arctic will be completely free of ice in summer.

Scientists also rely on historical records, such as those kept by Vikings and European sailors that made note of Arctic ice at various locations and times. They also use records from Antarctic whalers, who recorded the location of every whale kill. Because whales tend to feed along the ice edge, these records can be used to estimate ice extent at the time of hunting. Scientists also examine ice cores for signs of phytoplankton, since their presence in a core similarly reflects the sea ice boundary.

Did you know?

Sea ice glossary

Mariners have adopted a number of different names for icebergs and pack ice. The following glossary of ice terms is from Bowditch's Glossary of Marine Navigation.

Learn more