There is heated discussion over whether or not this year's seasonal sea-ice minimum in the Arctic constitutes a record (see http://go.nature.com/bmydx7 and, for example, http://go.nature.com/286mdp). But the debate misses the crucial point: this year's minimum is evidence that the unprecedented seasonal minimum of 2007 was not a one-off.
Continuous global satellite sea-ice observations began in 1972. A value for sea-ice coverage is retrieved from the surface microwave emission, using the different emission properties of water and ice. The sea-ice extent is conventionally defined as the sum of all data elements that have more than 15% ice cover.
Several remote-sensing groups work on this topic, using various methods and data from at least two different satellite instruments: the Advanced Microwave Scanning Radiometer — Earth Observing System (AMSR-E) on NASA's Aqua satellite and the Special Sensor Microwave Imager/Sounder (SSMIS).
All of these groups agree that the seasonal 2011 minimum is very close to the 2007 minimum. That value was about 25% less than the previous low in 2005, and almost 40% less than the climatological mean for the seasonal minimum (1979–2007). In autumn 2007, this value could be considered an outlier, caused by unusually warm weather over large parts of the Arctic Ocean.
However, the five seasonal minima since 2007 are the lowest on record. Although it is too early to speak of a trend, other observations, such as the thinning of Arctic sea ice over the past two decades, also suggest that the 2007 and 2011 minima are not single outliers.
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Melsheimer, C., Heygster, G. & Notholt, J. Sea-ice minimum is not a one-off. Nature 478, 188 (2011). https://doi.org/10.1038/478188a