In October 2012 Arctic sea ice doubled in extent.
As of October 20, 2017, NOAA@NSIDC Releases Sea Ice Index, Version 3.0
Text below from SII webpage explains the changes in versions.
NOAA@NSIDC is pleased to announce the release of Sea Ice Index Version 3 (V3). V3 changes the way the monthly average area and extent data values are calculated. The way monthly average area and extent images are created remains the same as in V2. All daily data remain the same as in V2.
The V3 method simply averages daily extent values, while the V2 method derived monthly average values from the gridded monthly-average concentration field. The change is in response to questions about what seemed to be an inconsistency between daily and monthly values. When users summed daily values, and then divided by number of days in a month to get a monthly average value, that number was different…
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