New Normals: Sea Ice Index to Change to a 30-year Base Period in July

NOAA@NSIDC plans to change the baseline climatological period for the Sea Ice Index and the Arctic Sea Ice News and Analysis (ASINA) from 22 years (1979-2000, inclusive) to 30 years (1981-2010, inclusive) this July. A 30-year period typically defines a climatology or climate normal and is the standard used by organizations like the World Meteorological Organization (WMO) and NOAA. Thirty years is deemed sufficiently long enough to average out most interannual variability but short enough so that longer-term climate trends are not obscured.

These maxims about climate normals come from the world of weather and climate. Sea ice responds to climate forcing differently, and the assumptions behind the use of 30-year normals for meteorology may not hold true for sea ice, particularly in light of the rapid decrease and repeated record low minimum extents in the Arctic during the past decade. However, NOAA@NSIDC thinks that it is important to move to a longer baseline and to match the 1981-2010 period used by NOAA data centers.

Monthly and daily ice extent and area data values will not change, but data and image products that are based on the mean or median will change. For example, the trend plots will have a different scale, and the value of the slope, expressed as change in percent per decade, will change, because this value is relative to the mean period.

The monthly and daily extent images will have the same extent although the position of the median extent lines will change. The concentration anomaly and trend images will also change.

In July, we will provide more information to help users put these changes into the larger context of changing climate and changing ice.

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