The areal extent of the sea ice cover in the Arctic Ocean has declined in the last 40 years with increased decadal variability. The trend is clearly influenced by the radiation balance over all seasons. A cloudiness increase in the fall, winter and spring contributes to a reduction in the absolute amount of net longwave radiation at the sea surface. In the summer, the reduced cloud cover has led to an increase in shortwave radiation, permitting more net outgoing radiation, and yielding a small increase in the total incoming radiation. All of these trends promote ice reduction, and may suggest the importance of clouds during a possible global warming in the near future. The effects of clouds and radiation are comparable with the albedo reduction associated with more open water, which absorbs more solar radiation in the summer. Analyses of the decadal variabilities reveal qualitatively the same effects as those of the radiation on the ice cover.