Sea ice is frozen seawater floating on the surface of the ocean. Some sea ice is semi-permanent, persisting from year to year, and some is seasonal, melting and refreezing from season to season. The sea ice cover reaches its minimum extent at the end of each summer and the remaining ice is called the perennial ice cover. The 2007 Arctic summer sea ice has reached the lowest extent of perennial ice cover on record – nearly 25% less than the previous low set in 2005.
Wish they had more info on the time-frame/source of the satellite (?) imagery, but you get the idea.
via Gernot’s facebook page