Sea Ice

Sea ice is a layer of floating ice that forms on the surface of the ocean as the seawater temperature decreases below -1.9°C (28.6°F). Sea ice that forms in the Arctic is able to last multiple years and may be as thick as 3 to 4 meters (9.8 to 13.12 feet), whereas sea ice near Antarctica melts each year during the summer.

The volume of Arctic sea ice is decreasing due to global warming. Even though sea ice in the Arctic polar sea ice is melting, it does not contribute directly to sea level rise. This is due to one unique property of water–water in the solid form (ice) is less dense than water. When ice melts, the volume of the water that was displaced is taken up by the melted ice. This is known as Archimedes Principle. The change in volume exactly counterbalances the extra volume of the ice that was up above the water's surface.

However, melting of the polar sea ice acts as a climate feedback. Arctic Sea ice and snow reflect more sunlight (have a higher albedo) than water. When this ice and snow melts, the darker, ocean water absorbs more heat, causing the Arctic Ocean to heat up even more. This is example of a positive feedback. In other words, it amplifies an already warming trend.

Greenland Sea Ice
Broken Sea Ice North of Greenland. Photo Credit: Courtesy Andy Mahoney, NSIDC

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