Arctic sea ice extent

Arctic sea ice extent measures how much of the northern ocean is covered by at least fifteen percent ice, counted as total area of those covered grid cells rather than the ice itself. It comes from satellite microwave sensors, which detect the sharp contrast between open water and frozen surfaces day and night, through cloud and polar darkness, and is compiled daily by the US National Snow and Ice Data Center. The figure swings hugely with the seasons, typically peaking near fifteen million square kilometres in late winter and falling to roughly four to six million by September, with a long-term downward trend that makes it one of the clearest fingerprints of a warming climate. One caveat: extent says nothing about thickness, so a broad but thin, fragmented ice cover can look deceptively healthy.

Record high

16.635 million km², 1 Mar 1979

Record low

3.340 million km², 16 Sep 2012

First measurement

26 Oct 1978

Measurement cadence

daily

Measurements in this record

15,864