changes in sea ice conditions are best demonstrated by the rate of melting over time. a composite record of arctic ice demonstrates that the floes' retreat began around 1900, experiencing more rapid melting beginning within the past 50 years. satellite study of sea ice began in 1979, and became a much more reliable measure of long-term changes in sea ice. in comparison to the extended record, the sea-ice extent in the polar region by september 2007 was only half the recorded mass that had been estimated to exist within the 1950–1970 period.