Sea ice is frozen seawater that floats on the surface of the ocean. It forms in the polar regions where there is little sunlight during the winter and it gets cold enough for the ocean to freeze. In some places, sea ice can survive for several years. It is measured by satellites.
Sea ice is an important component of the climate system because it regulates the transfer of heat and momentum between the atmosphere and the ocean. Sea ice is light and reflects much of the sunlight that reaches the surface. In contrast, open water is dark and absorbs light. Therefore, when sea ice melts revealing open water, more energy is absorbed at the surface. Sea ice also provides a barrier to the exchange of freshwater, gases, and tiny airborne solid and liquid particles known as aerosols.
Plants and animals at all levels of the food chain live in or around sea ice. Sea ice is a major component of polar ecosystems.
As sea ice is formed from frozen seawater and floats on the sea surface, melting of the ice does not contribute to sea level rise (unlike melting glaciers and ice sheets which are stores of frozen water on land and do add water to the ocean when they melt).
The behaviour of the Arctic and Antarctic sea ice are somewhat different owing, in part, to the different configuration of ocean and land near the two poles.
The Arctic is an ocean basin surrounded by land. Sea ice covers the high latitudes of the northern hemisphere throughout the year with a maximum in extent occurring in March and a minimum in September. Arctic sea-ice extent has declined in all months since regular satellite monitoring began in the late 1970s. Estimates of sea-ice volume produced using a model combined with available observations, suggests there has also been a long-term decrease in sea-ice volume. Year to year changes around the long-term trend are largely driven by short-term weather events.
The Antarctic is a land mass surrounded by ocean. Sea ice fringes the Antarctic continent, with a minimum extent occurring in February or March, when sea ice retreats almost to the coast, and a maximum extent occurring in September or October. Until 2015, there had been a slight long-term increase in Antarctic ice extent. In late 2016, however, Antarctic sea ice extent dropped suddenly and has stayed relatively low ever since. Year to year changes around the long-term trend are largely driven by short-term weather events.
Sea ice concentration can be estimated from microwave emissions detected by satellites. The open water and areas of sea ice have different microwave "signatures" that can be used to determine what the ice concentration is. This can sometimes be difficult because shallow pools of water can form on the surface of the ice, which look like open water from the satellite’s viewpoint.
Prior to satellite measurements, information was gathered from ship reports, reconnaissance flights and coastal observers. This information was put together by analysts to create sea-ice charts or atlases that were used for navigation. Combining these different kinds of information can be difficult. There is less information about Antarctic sea ice generally prior to the satellite era.
The IPCC AR5 WG1 concluded that “In the case of the Arctic we have high confidence in observations since 1979, from models and from physical understanding of the dominant processes; taking these three factors together it is very likely that anthropogenic forcing has contributed to the observed decrease in Arctic sea ice since 1979.”
For the Antarctic the IPCC said “Overall we conclude that there is low confidence in the scientific understanding of the observed increase in Antarctic sea ice extent since 1979, owing to the larger differences between sea ice simulations from CMIP5 models and to the incomplete and competing scientific explanations for the causes of change and low confidence in estimates of internal variability”. This statement preceded the rapid drop in Antarctic sea ice extent observed in 2016.