Chapter 5 – Ice in the sea

Sea ice plays a key role for climate and is important as habitat and for human activities and economies. Observations show and models indicate that climate and sea-ice regimes are changing. Sea-ice extent in the Arctic decreased substantially during the last 30 years; Antarctic sea ice is decreasing in some areas, but overall it has shown a slight increase during this period. Climate models project further decreases in sea-ice extent in the Arctic during this century and comparable decreases in Antarctic sea-ice extent. There are uncertainties attached to the rate at which these changes will occur, and there is a risk of tipping points being crossed and abrupt reductions in sea ice occurring. To reduce these uncertainties, more large-scale continuous observations are needed, especially of ice and snow thickness. Changes to sea ice will have major impacts on both the physical and biological environment at all scales from global to regional. The reduction in albedo (reflection of solar radiation) resulting from less ice cover is a feedback mechanism that accelerates the rate that sea ice declines and also the rate at which Earth warms. Changes in sea ice contribute to altering the ocean thermohaline circulation, especially in the North Atlantic. Sea ice is a complex environment with a diversity of habitats and seasonal variation to which life in the polar seas is closely adapted. Many species are now being affected by changes in sea ice in the Arctic, and, if the changes continue, there is a strong risk of species extinctions. There is a range of direct consequences of changes in sea ice for economies and human well-being – including threats to indigenous cultures and opening of new sea routes and economic opportunities. Sea ice: ice found at sea which has originated from the freezing of sea water. Sea ice may be discontinuous pieces (ice floes) moved on the ocean surface by wind and currents, driven together into a single mass (pack ice), or a continuous sheet attached to the coast (land-fast ice).

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