Sea Ice Regime

Sea ice is an important climatological agent. A closed sea ice cover essentially diminishes exchanges of energy, matter, and momentum between the atmosphere and the underlying ocean. The high albedo of sea ice surfaces relative to open water strongly influences regional to global radiation budgets and thus climate. Sea ice also plays a less obvious role with respect to biogeochemical processes. Sea ice biota, that is, organisms particularly adapted to the harsh conditions in the sea ice pore space, utilize this medium as a habitat and are important seed species for the spring bloom and the growth of plankton in summer (Legendre et al., 1992; Thomas and Dieckmann, 1994). It is known that the global atmospheric carbon dioxide budget is strongly influenced by uptake at the near-surface layers by phytoplankton in polar waters. Without sea ice, which serves as a wintering-over habitat for planktonic organisms, these exchange processes would also be influenced, leading to increasing carbon dioxide concentrations and an enhanced greenhouse warming.

There have been numerous indications for changes in the Arctic sea ice cover over the last few decades. Evidence for these changes comes firstly from observed ice thickness changes in parts of the Arctic Ocean. Sea-ice draft data, which were acquired on submarine cruises between 1993 and 1997, have been compared with data obtained between 1958 and 1976 (Rothrock et al., 1999). The results obtained indicate a thinning of 1.3 m in most of the deep parts of the Arctic Ocean from 2.1 to 1.8 m during the observational period. While the extensive thinning of Arctic sea ice as proposed by the Rothrock et al. study has been challenged (Winsor, 2001), the main conclusion of a sea ice cover in transition has been confirmed (Holloway and Sou, 2001). The second evidence for a changing Arctic sea ice cover comes from satellite remote sensing. Passive microwave observations reveal a reduction in areal sea ice extent of approximately 3% per decade since 1978 (Johannessen et al., 1999). This change amounts to a reduction of 14% in area of the multiyear ice (i.e., ice that has survived at least one summer) between 1978 and 1998. These conclusions are supported by comparisons between observations and results obtained from global climate models (Vinnikov et al., 1999). Moreover, the climate model results reveal that the decrease in sea ice cover significantly exceeds rates that would be expected through natural climate variation. This leads to the question of to what extent presently observed changes in the climate system can be attributed to human influence or whether these changes are primarily an expression of natural variability. We will return to this question in the last section, after briefly reviewing the present trends in global surface temperatures.

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