There are major regional differences for the Arctic sea ice, with the strongest decline in ice extent observed for the Greenland Sea (10.6 per cent per decade). The smallest decreases of annual mean sea ice extent were found in the Arctic Ocean, the Canadian Archipelago and the Gulf of St. Lawrence. In the marginal Arctic seas off Siberia (the Kara, Laptev, East Siberian and Chukchi Seas) a slight negative, but not significant, trend in ice extent was observed between 1900 and 20008. In contrast to the Arctic, there are signs of a slight increase in the extent of annual mean sea ice over the period 1979–2005 (+1.2 per cent per decade) based on the NASA Team retrieval algorithm. The IPCC concluded that this overall increase was not significant and that there are no consistent trends during the period of satellite observations. There are, however, indications that sea ice may be increasing more at the period of minimum coverage (March) than at the period of maximum sea ice extent in September. There is also regional variation (Figure 5.8) with an increase, for example, in the Ross Sea (+4.8 per cent per decade) and a loss in the Bellingshausen Sea (–5.3 per cent per decade). There are far fewer observations of sea ice thickness for the Antarctic than for the Arctic because of the lack of submarine measurements. It is therefore not possible to detect any trends in Antarctic sea ice thickness over recent decades. The reasons for the very different trends in Arctic and Antarctic sea ice extent over recent decades are not known at present and resolving this important question is a high research priority. Researchers are examining changes in the atmospheric circulation of the two polar regions as well as changes in ocean circulation.
From collection: Global Outlook for Ice and Snow
Hugo Ahlenius, UNEP/GRID-Arendal