The change in the amount of Arctic sea ice from 1980 (top) to 2012 (bottom). The oldest and thickest Arctic sea ice is disappearing at a faster rate than the younger and thinner ice at the edges of the ice cap. The rapid disappearance of older ice makes the Arctic Ocean's sea ice cap more vulnerable to further decline. Arctic multi-year ice "extent" -- which includes all areas where at least 15 percent of the ocean surface is covered by multi-year ice -- has been vanishing at a rate of -15.1 percent per decade. Over the same period, the "area" covered by multi-year ice -- which discards open water among ice floes and focuses exclusively on regions that are completely covered -- has been shrinking by -17.2 percent per decade. The image shows sea ice coverage in 1980 and 2012, as observed by passive microwave sensors on NASA's Nimbus-7 satellite and by the Special Sensor Microwave Imager/Sounder (SSMIS) from the Defense Meteorological Satellite Program (DMSP). Multi-year ice is shown in bright white, while average sea ice cover is shown in light blue to milky white. The data shows the ice cover for the period of November 1 through January 31 in their respective years.