Sea ice is frozen seawater which floats on the surface of the ocean, pushed by the wind and by ocean currents. Sea ice grows during the winter months and melts during the summer.
Sea Ice is a Mushy Layer
In the Arctic, the maximum sea ice extent of around 15 million km2 (60 times the size of the UK) is reached in March and the minimum of 3-7 million km2 in September. Sea ice also forms in the Southern Ocean surrounding Antarctica with a maximum extent in October (19 million km2) and minimum extent in March (3-5 million km2).
Over open water up to 20cm of ice can grow in one day. The thicker the ice gets the slower it grows, with typical growth rates of about 1 cm per day for 2 metres of thick ice. Maximum late winter ice can reach thicknesses of about 3 metres but multi-year ice, which has survived melting away over one or more summer seasons, can be more than 10 metres thick if ocean currents force it up against other ice masses.
Unlike freshwater ice, sea ice is not just frozen water, but contains sea salt, mostly in pockets of concentrated brine. Sea ice is really a mixture of solid and liquid, and water and salt known as a “mushy layer”. The amount of salt in the sea ice affects how porous sea ice is, its mechanical strength and its thermal properties.
Sea Ice keeps the Earth’s Poles cold
While the open ocean absorbs around 94% of the sunlight that falls on it, sea ice reflects 60-90% of it. The presence of sea ice stops the polar regions from absorbing a lot of the sun’s energy. During winter, sea ice stops the air from warming up in other ways too. The Arctic’s atmosphere in winter is very cold with near surface temperatures around –40oC whilst the ocean is much warmer, close to its freezing temperature at -1.8oC. The sea ice cover separates the two, insulating the atmosphere and preventing heat in the ocean from warming the overlying air.
The atmosphere and ocean act as “heat engines,” always trying to restore a temperature balance by transporting the heat it collects at the equator toward the poles. Otherwise the earth would just keep on heating up. Our everyday weather is a manifestation of this phenomenon. Low-pressure systems, such as storms, which can be especially strong in winter, are one of nature’s best ways of transporting heat to the poles by atmospheric circulation. The oceans tend to transport heat in a slower and less violent fashion. Changes in the amount of sea ice alter how cold the poles are and affect the circulation of the atmosphere and the oceans.
Sea ice also affects the seawater density differences that drive the circulation of the ocean. When sea ice forms most of the salt is pushed out of the seawater that is freezing at the surface. The ocean under the forming sea ice becomes saltier and denser. This cold, dense, polar water sinks and moves along the ocean bottom toward the equator, while warm water nearer the surface at the equator travels towards the poles. Changes in the amount of sea ice can alter how much dense water sinks at the poles, which disrupts ocean circulation and how heat usually moves around the Earth.
Melting Sea Ice accelerates Climate Change
The Earth’s climate is warming. An important mechanism that enhances the decline in sea ice in a warming climate is called the ‘ice-albedo feedback’: Warmer air temperatures cause increased melting of sea ice. The reduction of sea ice results in the ocean absorbing more solar energy. The increased solar heating causes more ice to melt which, in turn, exposes more of the darker ocean surface, and increases solar absorption still further. Even a small increase in temperature can lead to this chain reaction resulting in ever greater warming over time, making the polar regions the most sensitive areas to climate change on Earth.
As sea ice is formed from frozen seawater and floats on the sea surface, melting of sea ice does not contribute to sea level rise – unlike melting glaciers and ice sheets which are stores of frozen water on land that add new water to the ocean when they melt. But changes to the sea ice cover will have potential implications for the Arctic region and beyond by allowing polar temperatures to increase faster and disrupting atmosphere and ocean circulation patterns.