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.
The current focus of the sea ice work over the next two years is to assess the risk of rapid loss of sea ice in the Arctic. Specific themes are:
The objectives for the land ice work is to develop a basic capacity to represent glaciers and ice sheets within the climate model, including:
Simulations of the Arctic sea ice cover, generated by the HadGEM1 coupled climate model, are able to capture the observed long term decline in mean September ice extent. HadGEM1 is also capable of producing an episode of low September ice extent of similar magnitude to the anomalously low extent observed in 2007. The episode of low summer ice extent is largely driven by the synoptic conditions over the summer moving the ice across and out of the Arctic basin, and also due to pre-conditioning of the snow and ice which is thinner than usual in the Eastern Arctic at the start of the melt season.
Published in: Keen, A. B., H. T. Hewitt and J. K. Ridley, A case study of a modelled episode of low Arctic sea ice, Climate Dynamics, 41, 1229-1244, 2013
The HadGEM1 climate model has projected Arctic ice loss with reasonable accuracy up to the present day. The model projects that sea ice decline will continue over the 21st Century; however it also shows the possibility of a period of up to 20 years when the ice loss is temporarily slowed. This period is caused by reductions in ice export from north of Greenland, and by a weakening of the North Atlantic overturning circulation (which results in a reduction of warm water transported into the Arctic).
Published in: West, A. E., A. B. Keen, and H. T. Hewitt, Mechanisms causing reduced Arctic sea ice loss in a coupled climate model, The Cryosphere, 7, 555-567, 2013
Last updated: 28 January 2014