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. Each year, the minimum sea ice extent in the northern hemisphere occurs at the end of summer, in September. By comparing the extent of the sea ice in September over many successive years, long term trends in the polar climate can be assessed. This animation shows the minimum sea ice concentration in the northern hemisphere in September between 1979 and 2004. Since 1999, this minimum has shown an ice extent that is consistently 10% to 15% smaller than the average extent over the past 20 years.
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