The summer minimum and the winter maximum are the two pivotal milestones in the annual cycle of Arctic sea ice, and are key indicators of Arctic climate. Over the past two-and-a-half decades, the extent of sea ice at the end of summer (mid-September) has declined significantly. The winter maximum (end of February or mid-March) ice covering is much younger and thinner than it was in the past. This pair of maps shows the median age of February sea ice from 1981-2009 (left) compared to February 2009 (right). Ice more than two years old is dark blue, ice that is one to two years old is medium blue, and ice that is less than one year old is light blue. Compared to the median conditions at the end of winter, the ice pack of February 2009 contains much less old ice (dark blue)

Photograph: MODIS/NASA