The most common metric for tracking changes in Arctic sea ice over time is sea ice extent. Extent approximates the sea ice you would see from a bird's-eye view, which the US National Oceanic and Atmospheric Administration finds is declining at a rate of more than 14% per decade since satellite measurements began in 1979. Sea ice is not only shrinking across the surface, it is also thinning.
Measuring sea ice thickness is complicated, but a useful proxy for ice thickness is age. The winter ice pack in the Arctic was once dominated by multi-year, thick ice. Today, very little old ice remains – just 7%. Thick or thin, sea ice moves with ocean currents. Surrounding landmasses corral some ice in the Arctic Ocean, while the rest flows through the Fram Strait east of Greenland. Normally, the continual exit of ice has been balanced by growth in the Beaufort Gyre. In recent years, however, less ice has managed to survive its trip through the gyre's relatively warm southern arm.
Read more in the Sea Ice chapter of the Arctic Report Card: Update for 2013.
- Amber Harmon