A 30-year satellite record of sea ice extents derived mostly from satellite microwave radiometer observations reveals that the Arctic sea ice extent decreased by 0.30 ¿ 0.03 ¿ 106 km2/10 yr from 1972 through 2002, but by 0.36 ¿ 0.05 ¿ 106km2/10yr from 1979 through 2002, indicating an acceleration of 20% in the rate of decrease. In contrast, the Antarctic sea ice extent decreased dramatically over the period 1973--1977, then gradually increased. Over the full 30-year period, the Antarctic ice extent decreased by 0.15 ¿ 0.08 ¿ 106 km2/10 yr. The trend reversal is attributed to a large positive anomaly in Antarctic sea ice extent in the early 1970's, an anomaly that apparently began in the late 1960's, as observed in early visible and infrared satellite images.