The monthly sea ice extent data produced by the National Snow and Ice Data Center (NSIDC) show monthly averages of sea ice extent in the Arctic and Antarctic. Sea Ice Index products are derived from 2 data sets: the Near-Real-Time DMSP SSMIS Daily Polar Gridded Sea Ice Concentrations and the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data. These satellite passive microwave-derived data sets are used to generate the monthly images and numbers that comprise the Sea Ice Index record of sea ice extent and concentration from November 1978 to present.
Median extents are calculated using a 30-year reference period of 1981 through 2010. The data are produced in a consistent way that makes the monthly index time-series appropriate for use when looking at long-term trends in sea ice cover. For more information, please see https://nsidc.org/data/docs/noaa/g02135_seaice_index/#text_summary. Resource Watch shows only a subset of the dataset. For access to the full dataset and additional information, see the Learn More link.
Sea Ice Index, Version 3
On Resource Watch, monthly data are shown although both monthly and daily products are available. Monthly products are better to use for long-term trend analysis because errors in the daily product tend to be averaged out in the monthly product and because day-to-day variations are often the result of short-term weather. Additionally, there are no data for the area not imaged by the sensor at the North Pole, named the Arctic pole hole, therefore it is assumed to be entirely covered by grid cells showing more than 15 percent concentration for that month, so the area is considered to be ice-covered.
Fetterer, F., K. Knowles, W. Meier, and M. Savoie. 2016, updated daily. "Sea Ice Index, Version 3." Boulder, CO: NSIDC. Retrieved from http://dx.doi.org/10.7265/N5736NV7. Accessed through Resource Watch, (date). www.resourcewatch.org.