Satellite observations help to reveal critical polar sea ice changes. Passive microwave measurements have been used to estimate sea ice extent, the length of the sea ice season, sea ice velocities, and to determine the timing of the seasonal onset of melt in spring and freeze in autumn. Satellite data show that over the past 39 years Arctic sea ice cover has been declining by 4.5 percent per decade on the average, by 2.9 percent her decade in March, when the sea ice reaches its maximum extent, and by 11.3 percent per decade in September, the month that marks the end of the summer melt season. Revealed distribution of ice of different ages demonstrates the extensive loss in recent years of the older ice types. The proportion of sea ice five years or older has declined dramatically over the satellite time period, from more than 16 percent of the March ice in the 1980s to 1.2 percent in 2016. A growing percentage of Arctic sea ice is only one or two years old. Less old multiyear ice and increased area of first-year ice implies that the ice cover is thinning, which makes it more vulnerable to further melting. Since the beginning of satellite passive microwave data record, the length of the melt season for Arctic sea ice has grown, sea ice starts melting earlier by 5 days per decade and it starts refreezing later by 6-11 days per decade, on average.