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 Recent analysis of the ice cover in the Arctic Ocean has established that significant changes have occurred in the latter part of the last century. Based on analysis of upward looking sonar observations from US nuclear submarines it has been shown that the average ice thickness has decreased by 1.3 m, from 3.1 m in the 1958-1976 period to 1,8 m in the 1990´s, in average 4 cm per year, or 40% of the total ice volume (Rothrock et al., 1999). Furthermore, analysis of microwave satellite observations has established that the total area has decreased by 6% over the last two decades (1978-1998) (Johannessen et al., 1995, Bjørgo et al., 1997, and Cavalieri et al., 1997), while the multi-year ice area has decreased 14% over the same period (Johannessen et al., 1999). Comparison of observations (in situ and satellites) since 1900 with trends seen in two coarse resolution global climate models, forced by observed greenhouse gases and tropospheric sulphate aerosols correlates very well. This is suggesting strongly (Vinnikov, et al., 1999) that the observed decrease in sea ice extent since 1950 is related to the antrophogenic global warming. Prediction by these two coarse resolution global climate models suggests furthermore a substantial decrease of the ice extent in this century (Vinnikov, et al., 1999). However, the elevated indices of the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) (the correlation between these two indices are very high suggesting that they are parts of the same system) has pumped warm air and water masses into the Arctic from the North Atlantic, causing ice melting as well as a mechanism for exporting multi-year ice through the Fram Strait ((Rothrock et al., 1999, Hurrell, 1995, and Thompson and Wallace, 1998). Even some effects on the variability of the sea ice from El Ninõ has been reported (Gloersen, 1995). Therefore it is very important to assess the natural variability of the ice cover in the last century in order to further investigate if an abrupt change will take place during this century. 

Extrapolating the ice thickness decrease of 4 cm/year indicates that the Arctic Ocean could be ice free 50 years from now, causing a dramatic change in the albedo, with significant effects on the global climate system. However, we should also be aware that Russian ice thickness estimates based on dispersion relationships between the damping of swell propagation into the Arctic Ocean measured from the North Pole Stations during the period 1972-1991 (Johannessen et al., 1999, Nagurnyi et al., 1994, and Nagurnyi et al., 1999) indicates an average of 0.5- 1.0 cm decrease per year. This is 4-8 times less than the results form the nuclear submarine data. This demonstrates the need to assess objectively all available ice thickness observations from the Arctic Ocean during the last century in order to estimate the natural variability and trends of the ice volume and mass. 

Furthermore, it is important to study the ice cover in the Baltic region in order to predict potential future abrupt changes, since this is a very important transport corridor for year around shipping operations (Vapio, 1991, and Sandven et al., 1999). 

The effects and impact of a decreasing ice cover are multiple: 

We hypothesise that an Arctic Ocean with decreasing ice cover with cold water, which has high capacity for CO2 absorption, could become a new region for an important sink of the atmospheric CO2, which will tend to mitigate global warming. 
Rough estimates based on observations of carbon fluxes in the Greenland Sea (Andersen et al., 2000) indicate that 0.3-0.6 Pg of carbon can be absorbed each year by an ice free Arctic Ocean. This is an increase of 15-30% from what the world oceans absorbs today and 5-10% of the 6-7 Pg antrophogenic carbon emissions or in the same order of magnitude as the agreed reductions in the Kyoto agreement. 
Other positive effects of a decreasing ice cover in the Arctic and the Baltic Seas are on marine transportation and easier and safer logistics for offshore oil activities in the Arctic region (Vapio, 1994, and Johannessen et al., 1997). 
Increased fisheries in new previously ice-covered regions will contribute positively to the global food supplies. 
A negative effect could be that the melting of the ice drastically will change the stratification of the upper layer in the Nordic Seas and North Atlantic Ocean, slowing down the deep water formation and furthermore the thermohaline circulation (conveyor belt) (Mauritzen and Hakkinen, 1999). A reduced transport of heat by the Gulf Stream/ North Atlantic current (Sarmiento et al., 1998) will cause a significant impact on the climate in Europe. 
Change in the marine ecosystem (e.g. less plankton in the North Atlantic caused by melting of sea ice (Reid et al., 1998)) will have a negative impact on the marine biodiversity (including polar bears, which totally are pending on the ice cover) for the Arctic and sub-Arctic regions. 
References
Rothrock, D.A., Y. Yu and G.A. Maykut (1999) Thinning of the Arctic sea-ice cover, Geophysical Resesearch Letteres, 26(23), pp. 3469-3472
Johannessen, O.M., M.W. Miles and E. Bjørgo (1995) The Arctic's shrinking sea ice, Nature,376, pp. 126-127
Bjørgo, E., O.M. Johannessen and M.W. Miles (1997) Analysis of merged SMMR-SSMI time series of Arctic and Antarctic sea ice, Geophysical Research Letters, 24, pp. 413-416
Cavalieri, D.J., P. Gloersen, C.L. Parkinson, H.J. Zwally and J.C. Comiso (1997) Observed hemispheric asymmetry in global sea ice changes, Science, 278, pp. 1104-1106 
Johannessen, O.M., E.V. Shalina and M.W. Miles (1999) Satellite evidence for an Arctic sea ice cover in transformation, Science, 286, pp. 1937-1939
Vinnikov, K.Y, A. Robock, R.J. Stouffer, J.E. Walsh, C.L. Parkinson, D.J. Cavalieri, J.F.B. Mitchell, D. Garrett and V.F.  Zakharov (1999) Global warming and the northern hemisphere sea ice extent, Science, 286, pp. 1934-1937
Hurrell, J. (1995) Decadal trends in the North Atlantic Oscillation, Science, 269, pp. 676-679
Thompson, D.W.J. and Wallace, J.M. (1998) The Arctic Oscillation signature in wintertime geopotential height and temperature fields, Geophysical Research Letters, 25, pp. 1297-1300
Gloersen, P. (1995) Modulation of hemispheric ice cover by ENSO events, Nature, 373, pp. 503-505
Nagurnyi, A.P., V. G. Korostelev and P. A. Abaza (1994) Bull. Russian Acad. Sci., Physics, Suppl. Phys. Vibrations, 58, pp. 168
Nagurnyi, A.P., V. G. Korostelev and V. V. Ivanov (1999) Meteorology and Hydrology, 3, pp. 72, in Russian.  English translation available from NERSC. 
Vapio, A. (ed) (1991) The Baltic Sea, Elsevier Oceanography series, 30, Amsterdam. 
Sandven, S., M. Mäkynen, M. Hallikainen, A. Seina, H. Grönvall, M. Similae, J. Karvonen, M. Nizovsky, A. Cavanie, R. Ezraty, R. Gill, H. H. Valeur, R. Tonboe, L. Toudal Pedersen, V. Alexandrov, A. Bogdanov, M. Lundhaug, Ø. Dalen and K. Kloster (1999) Integrated  use of new microwave satellite data for improved sea ice observation - IMSI, Final report, IMSI report no. 12, NERSC Technical report no. 170, 140 pp
Andersen, L.G., H. Drange, M. Chierici, A. Fransson, T. Johannessen, I. Skjelvan and F. Rey (2000) Annual fluxes in the upper Greenland Sea based on measurments and a box model approach, Tellus, 52B, pp. 1013-1024
Johannessen, O.M., A.M. Volkov, V.D. Grischenko, L.P. Bobylev, S. Sandven, K.Kloster, T. Hamre, V. Asmus, V.G. Smirnov, V.V. Melentyev and L. Zaitsev (1997)  ICEWATCH Ice SAR monitoring of the Northen Sea Route, In Operational Oceanography - the challenge for European co-operation, proceedings of the First International Conference on EuroGOOS, Editors J. Stel, H.W.A. Behrens, J.C. Borst, L.J. Droppert, and J.P. van der Meulen. Elsevier Sciences Ltd., ISBN 0 444 82892 3, pp. 224-233
Mauritzen, C. and S. Hakkinen (1999) On the relationship between dense water formation and the Meriodional Overturning Cell in the North Atlantic Ocean, Deep-Sea Research I,  46, pp. 877-894
Sarmiento, J.L., T.M.C. Hughes, R.J. Stouffer and S. Manabe (1998) Simulated response of the ocean carbon cylcle to anthropogenic climate warming, Nature, 393, pp. 245-249
Reid, P.C., M. Edwards, H.G. Hunt, A.J. Warner (1998) Plankton change in the North Atlantic, Nature, 391, pp. 546 

 

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