Note: A user let us know that the Kelly source fields appear to include other data sources that are temporally displaced by one month. For example, the August 1936 Kelly source appears to have included chart information from the July DMI charts or yearbooks. We suggest you delay downloading the V2 data until we are able to investigate this problem more throughly. When the issue is resolved we will remove this warning.
Data Set ID:

Gridded Monthly Sea Ice Extent and Concentration, 1850 Onward, Version 2

Observations from historical sources are the basis of this monthly gridded sea ice concentration product that begins in 1850. In 1979, these sources give way to a single source: concentration from satellite passive microwave data. The historical observations come in many forms: ship observations, compilations by naval oceanographers, analyses by national ice services, and others. Monthly sea ice concentration is given in a 1/4 degree latitude by 1/4 degree longitude grid. In addition to the concentration array, for each month a corresponding source array indicates where each of 16 possible sources is used. The file of concentration and source arrays conforms to NetCDF-4 standards.

This is the most recent version of these data.

Version Summary:

In general, the V2 extent and area values are slightly higher than those derived from V1 (Figure 24). To create V1.1, inconsistencies in the V1.0 land mask were addressed by applying an “if ever land, always land” rule to grid cells. This had the effect of slightly expanding the land mask, so that the area covered by ocean cells, and thus the area that could contain sea ice, was proportionally reduced. The improved V2 land mask leaves more ocean cells in which ice may occur. In total, about 450,000 sq km of coastal ocean are added. V2 ice extent and area differ from extent and area derived using V1, simply because the V1 and V2 land areas are not the same. This difference is greatest in the winter months, when ice reaches a greater expanse of coastline.

Most of the higher V2 extent and area values are the result of the changed land mask, with the change in input CDR source data version being an additional factor in the satellite era. There is a seasonal effect in the change attributable to the improved land mask

  • Sea Ice > Sea Ice Concentration > SEA ICE CONCENTRATION
Data Format(s):
  • NetCDF
Spatial Coverage:
N: 89.9, 
S: 30.1, 
E: 180, 
W: -180
Platform(s):DMSP, SHIPS
Spatial Resolution:
  • 1/4 degree x 1/4 degree
Temporal Coverage:
  • 1 January 1850 to 31 December 2017
Temporal Resolution1 monthMetadata XML:View Metadata Record
Data Contributor(s):John Walsh, William Chapman, Florence Fetterer

Geographic Coverage

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As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Walsh, J. E., W. L. Chapman, F. Fetterer, and J. S. Stewart. 2019. Gridded Monthly Sea Ice Extent and Concentration, 1850 Onward, Version 2. [Indicate subset used]. Boulder, Colorado USA. NSIDC: National Snow and Ice Data Center. doi: [Date Accessed].

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No technical references available for this data set.

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