[en] The analysis of composite historical records and, more recently, of 30-years long in-situ as well as remote sensing data records revealed strong and significant (compared to natural variability) changes in the Arctic sea ice cover : shrinking of the ice extent and particularly of the perennial cover, thinning, negative mass balance and accelerated drift and deformation. These changes can hardly be explained by the known natural variabilities, i.e. are most likely the consequence of anthropogenic forcing, amplified by positive feedbacks involving the albedo of the arctic ocean. Projections suggest a shift from a perennial to a seasonal state, i.e. an almost ice-free Arctic, in summer before the end of the 21st century. Consequences will be at various levels, including an amplification of climate warming in the Arctic and possibly beyond, impacts on ecosystems and indigenous populations, or economical impacts through the opening of shipping routes or oil exploitation, and therefore on geopolitical issues.