Climate, cryosphere and oceans

Changes in sea ice and land ice have important climate feedbacks, through albedo and ocean circulation. The melt of land ice results in sea level rise.

Sea ice cover seasonally insulates the atmosphere from the ocean preventing the exchange of heat and gases. The formation of sea ice during winter allows atmospheric temperatures to fall, due to a lack of sunlight, to colder than -30 °C. In addition, its formation extracts fresh water from the ocean, producing the cold saline bottom water which influences ocean circulation. As sunlight returns to the poles in summer, the ice melts back allowing the atmosphere to be warmed by the ocean and releasing fresh water to stratify the regional oceans. Sea ice, in both the Arctic and Antarctic, is expected to decline with global warming, the Arctic becoming ice-free in summer in the latter half of this century. Transport of heat to the Arctic from mid-latitudes, by ocean currents and winds is variable year by year resulting in different degrees of ice melt each year. Some climate models, including that of the Met Office Hadley Centre, successfully depict the observed variability and decline of the Arctic sea ice.

Sea ice decline does not significantly influence sea level as the ice is already floating. However, the melting of land-based ice, ice sheets and mountain glaciers, has an important contribution to global sea level rise. The melt from mountain glaciers also contributes to river flow and impacts the availability of summer water for agriculture. The Greenland ice sheet loses ice through surface melting in summer and through drainage of ice from the interior by large glaciers. Many glaciers, at the sea-ward fringes of both Greenland and Antarctica, have been speeding up. It is suggested that the speed-up, and the resulting increased ice flow to the ocean and associated sea level rise, are caused by a warming of the adjacent oceans.