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|The Discovery of Global Warming February 2011|
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. This 2007 minimum was unusually low, shocking observers. The decline in area has persisted, especially in summer but also year-round. Still more striking, and significant, has been a severe decline in the average thickness of the ice pack, and thus of its volume (graph).