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Satellite passive-microwave data, available for most of the period since 1972, have enabled a global description of the sea ice cover impossible prior to the 1970s. In this paper we analyze sea ice extents in the northern hemisphere over the period 1973–1987, using data derived from the Nimbus 5 electrically scanning microwave radiometer (ESMR) and the Nimbus 7 scanning multichannel microwave radiometer (SMMR). The record shows an average seasonal cycle ranging from a summer minimum of 8.5×106 km2 in September to a winter maximum of 15×106 km2 in March, with considerable interannual variability found both regionally and hemispherically. When yearly averaged sea ice extents are examined for trends, some regions (for example, Baffin Bay/Davis Strait) exhibit upward trends, while other regions (for example, the Kara and Barents seas) exhibit downward trends. The northern hemisphere overall shows no significant trend over the 1973–1987 period, although over the ESMR years (1973–1976) there was an upward trend and over the 8 full SMMR years (1979–1986) there was a lesser downward trend. The record as a whole gives no definitive indication from the varying sea ice extents of any consistent warming or cooling of the north polar region.