Sea ice is frozen seawater that floats on the ocean surface. It forms in both the Arctic and the Antarctic in each hemisphere’s winter; it retreats in the summer, but does not completely disappear. This floating ice has a profound influence on the polar environment, influencing ocean circulation, weather, and regional climate.
As ice crystals form at the ocean surface, they expel salt, which increases the salinity of the underlying waters. This cold, salty water is dense and can sink to the ocean floor, where it flows back toward the equator. The sea ice layer also restricts wind and wave action near coastlines, lessening coastal erosion and protecting ice shelves. Sea ice also creates an insulating cap across the ocean surface, which reduces evaporation and heat loss to the atmosphere. As a result, the weather over ice-covered areas tends to be colder and drier than it would be without ice.
Sea ice also plays a fundamental role in polar ecosystems. When the ice melts in the summer, it releases nutrients into the water, stimulating the growth of phytoplankton, the center of the marine food web. As the ice melts, it exposes ocean water to sunlight, spurring photosynthesis in phytoplankton. When ice freezes, the underlying water gets saltier and sinks, mixing the water column and bringing nutrients to the surface. The ice itself is habitat for animals such as seals, Arctic foxes, polar bears, and penguins.
The influence of sea ice on the Earth is not just regional; it’s global. The white surface reflects far more sunlight back to space than ocean water does. (In scientific terms, ice has a high albedo.) Once sea ice begins to melt, a self-reinforcing cycle often begins. As more ice melts and exposes more dark water, the water absorbs more sunlight. The sun-warmed water then melts more ice. Over several years, this positive feedback cycle (the ice-albedo feedback) can influence global climate.
Contrary to some public misconceptions, sea ice does not influence sea level. Because it is already floating in the ocean, sea ice is already displacing its own weight. Melting sea ice won’t raise ocean levels any more than melting ice cubes will cause a glass of ice water to overflow.
The Sea Ice Life Cycle
When seawater begins to freeze, it forms tiny crystals just millimeters wide called frazil. How the crystals coalesce into larger masses of ice depends on whether the seas are calm or rough. In calm seas, the crystals form thin sheets of ice, nilas, so smooth that they have an oily or greasy appearance. These wafer-thin sheets of ice slide over each other and form rafts of thicker ice. In rough seas, ice crystals converge into slushy pancakes. These pancakes slide over each other to form smooth rafts, or they collide into each other, creating ridges on the surface and keels on the bottom.
Some sea ice holds fast to a coastline or the sea floor—“fast ice”—while pack ice drifts with winds and currents. Because pack ice is dynamic, pieces can collide and form much thicker ice. Leads—narrow, linear openings ranging in size from meters to kilometers—continually form and disappear.
Larger and more persistent openings, polynyas, are sustained by upwelling currents of warm water or steady winds that blow the ice away from a spot as quickly as it forms. Polynyas often occur along coastlines when winds blow persistently offshore.
As water and air temperatures rise each summer near the Poles, some sea ice melts. Differences in geography and climate cause Antarctic sea ice to melt more completely in the summer than Arctic sea ice.
Ice that survives the summer melt season may last for years. For ice to thicken, the ocean must lose heat to the atmosphere. But the ice also insulates the ocean like a blanket. Eventually, the ice gets so thick that no more heat can escape. Once the ice reaches this thickness—3 to 4 meters (10 to 13 feet)—further thickening isn’t possible except through collisions and ridge-building.
Multiyear ice increasingly loses salt and hardens each year that it survives the summer melt. In contrast to multiyear ice, first-year ice is thinner, saltier, and more prone to melt in the subsequent summer. As of March 2015, multiyear ice accounted for 31 percent of the ice cover. The rest was first-year ice.