海冰
As the climate warms, snow cover and sea ice extent decrease. Large-scale measurements of sea-ice have only been possible since the satellite era, but through looking at a number of different satellite estimates, it has been determined that September Arctic sea ice has decreased between 1973 and 2007 at a rate of about -10% +/- 0.3% per decade. Sea ice extent for September for 2012 was by far the lowest on record at 3.29 million square kilometers, eclipsing the previous record low sea ice extent of 2007 by 18%. The age of the sea ice is also an important feature of the state of the sea ice cover, and for the month of March 2012, older ice (4 years and older) has decreased from 26% of the ice cover in 1988 to 7% in 2012. Sea ice in the Antarctic has shown very little trend over the same period, or even a slight increase since 1979. Though extending the Antarctic sea-ice record back in time is more difficult due to the lack of direct observations in this part of the world.