If all of Antarctica's ice were to melt, this would result in a sea-level rise of some 60 m, to which the melting of Greenland's ice would add a further rise in the order of 7 m, give or take an uncertainty of a few metres. September is the month in which sea ice is at its yearly minimum and 1979 marks the first year that data of this kind became available in meaningful form. The lowest concentration of sea ice on record was in September 2002. While thick ice with a snow covering has an albedo of 90 percent, that of a thin layer of ice is 50 per cent, and that of an ice-free ocean is in the order of 6 per cent.