Sea Ice: More information

The maps and bar graphs show how the Arctic Ocean sea ice cover for different years and months compares to averages from 1979 to 2015. The maps show spatial patterns of the differences (anomalies) of sea ice concentration for each year and month. Sea ice concentration is the fraction of the ocean covered by sea ice and is expressed as a percentage. The bar graphs show anomalies of sea ice extent for the Northern Hemisphere as a whole. Sea ice extent represents all areas with at least a 15% ice concentration. The bar graphs of sea ice extent and the maps of ice concentration anomalies are based on the same data as used in the NSIDC Sea Ice Index.

sample sea ice imageThis sample image shows sea ice concentration anomalies for October 2012.

On the maps, areas with greater than average ice concentration are indicated in blues (positive anomalies), and areas with less than average ice concentration are indicated in reds (negative anomalies). The grey circle centered over the North Pole indicates where the central Arctic is not visible to the satellite instruments used to generate these maps. This region of the Arctic is visible to other satellite platforms and instruments. The maps of anomalies help show where changes in sea ice concentration are especially large.

Changes in sea ice extent and sea ice concentration are useful indicators of the Arctic’s changing climate. Sea ice cover in the Arctic Ocean also plays an important role in the Earth’s climate system. In summer, sea ice can reflect as much as 70% of incoming solar radiation back to space. When sea ice disappears, the dark ocean surface absorbs much of the incoming radiation, reflecting less than 10%. This extra energy increases the ocean temperature and temperature of the overlying atmosphere.

The bar graphs show decreases in sea ice extent for all months, but the strongest decreases occur in the summer and early autumn months, especially in September, when the annual minimum ice extent occurs. A strong downward trend in September sea ice extent starts to emerge in the late 1990s. After 2002, ice extent anomalies are all negative.  The record low extent over the period of satellite observations  occurred in September 2012. The second lowest extent occurred in September 2007.

The maps show little change in winter ice concentration  over the central Arctic Ocean. Most of the changes in winter ice cover occur in the North Atlantic and North Pacific as a result of fluctuations in the ice edge in these regions. The largest reductions in concentration, seen in August and especially September, have occurred over the Laptev and Barents Seas north of the coast of Northern Eurasia, and in the Beaufort and Chukchi Seas north of North America and eastern Siberia.  In the extreme years of 2007 and 2012, regions of strong negative anomalies extend well into the central Arctic Ocean.

Sea Ice Index products are derived from two data sets: the Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product, NSIDC-0081) and the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product, NSIDC-0051). These satellite passive microwave-derived data sets are used to generate the daily and monthly images and numbers that comprise the Sea Ice Index record of sea ice extent and concentration from November 1978 to present. Information on the accuracy and precision of passive microwave-derived sea ice concentration products can be found in documentation for the Sea Ice Index. Daily concentrations are averaged to generate monthly mean concentrations for each grid cell.


Fetterer, F., K. Knowles, W. Meier, and M. Savoie. 2002, updated daily. Sea Ice Index. Boulder, Colorado USA: National Snow and Ice Data Center.

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