Sea ice is frozen seawater floating on the surface of the ocean. Some sea ice is permanent, persisting from year to year, and some is seasonal, melting and refreezing from season to season. Because the extent of the sea ice is important both for the Arctic marine ecology and for the role it plays in the Earth's climate, understanding the variation of this extent during the year and from year-to-year is vital. The first step in understanding the behavior of the sea ice is to calculate the average behavior of the sea ice over a single year. This behavior, called the climatology, is calculated by averaging the sea ice concentration over each month of a long period, in this case from October 1978 through September 2002. This animation shows the 23-year average sea ice concentration in the northern hemisphere for each particular month of the year. Generally, the minimum extent of sea ice occurs in September, and the maximum occurs in March.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 18.104.22.168.0