September 2011 monthly sea-ice extent (white) and 1979-2000 September average ice-edge location (blue line)

Arctic Ocean sea ice is dynamic, expanding in the winter and contracting in the summer. The ice cover can be as much as three times larger in winter than in summer. The extent of the sea ice remaining in the Arctic Ocean at the end of summer (in September) has been shrinking for the past 30 years. This change has accelerated since 2000 - observations show that the decline has been faster in the last ten years than in the previous twenty years. The extent of the ice cover in winter has also declined, although less rapidly than during summer. The March sea-ice extent has declined by about 10% over the past 32 years. Sea ice has started to melt earlier in spring. It is estimated that the melt began an average of 13 days earlier in the 2000s than it did in the 1980s. Across the entire Arctic, the length of the sea-ice melt season has increased by around 20 days since the 1980s.