Sea ice on the Arctic Ocean has been shrinking since the first satellite observations in 1979. Shown here are summer minimums; note the opening of the Northwest Passage across northern Canada in 2007. The 2007 minimum was unusually low, shocking observers. The decline in area has persisted (see graph). Still more striking, and significant, has been a severe decline in the thickness of the ice pack, so areas that still survive in summer are increasingly fragile.

Maps courtesy NASA. For updated information see the U.S. National Snow & Ice Data Center