Sea Ice and IcebergsSea Ice Climatology
![Minimum and maximum sea ice cover for the Arctic and Antarctic](media/graphics/seaice_maxmin.jpg)
Sea ice forms when seawater freezes. As one might expect, Arctic sea ice forms in winter, reaching a maximum in March, and melts in summer, reaching a minimum in September.
![Annual Minimum Sea Ice Coverage in the Northern Hemisphere Relative to 1979-2000 Mean Sea ice extent trend for the Northern Hemisphere](media/graphics/seaice_trend.jpg)
In recent years, the thickness and extent of sea ice has been decreasing. This has opened up sea routes across the Arctic. Long-term climate models indicate entirely ice-free summers by the end of this century.