Sea Ice: Arctic and Antarctic Extent

Sea ice extent is the area of ocean covered by at least 15 percent ice. Satellites measure the microwave radiation that the surface sends out, and ice and open water differ in that signal. NSIDC turns it into an ice concentration for each 25 km grid cell, counts every cell with a concentration of 15 percent or more as ice in full, and adds up the cell areas. Extent is therefore larger than the area that the ice itself covers, and it changes with the cut-off: a 20 or 30 percent cut-off gives different numbers but, NSIDC reports, similar trends. Values here are in million square kilometres, with million square miles in brackets in the tables and facts. The Arctic figure includes the small area around the North Pole that the satellites cannot see, which NSIDC treats as ice covered.

The record starts on 26 October 1978. Until 20 August 1987 the sensor gave a value every other day, and there are no values from 3 December 1987 to 13 January 1988. The extent for 14 September 1984 is wrong according to NSIDC, and this page leaves it out. Since 1 January 2025 NSIDC has used a different instrument, AMSR2, as the input for the Sea Ice Index and fills missing grid cells in its own processing; this page does not adjust for the change. The newest days are near real time, and NSIDC revises them when it fills gaps on the following days.

This site computes the 1981-2010 daily average and its standard deviation for each calendar date from the daily values, filling a single missing day with the mean of the day before and the day after, as NSIDC does; for 1 January to 28 February it matches NSIDC's published climatology to 0.001 million sq km. The yearly maximum and minimum are the highest and lowest single daily value of a calendar year. NSIDC's news pages quote five day averages for the annual minimum, so their numbers can differ slightly, and in the years of every-other-day sampling the true extreme may fall on a skipped day. A yearly value is ranked only when the data run past the end of its season, which is why the current Antarctic maximum is not ranked yet. Monthly means need at least 14 observed days with no gap longer than 5 days. Trends are straight line fits to the monthly means with the 95 percent interval used elsewhere on this site. They describe the change in extent and say nothing about its causes.

The 1981-2010 average for 10 November is 10.33 million sq km (3.99 million sq mi), with a standard deviation of 0.54 million sq km. The Arctic yearly maximum is usually reached in March and the minimum in September.

The 1981-2010 average for 10 November is 16.65 million sq km (6.43 million sq mi), with a standard deviation of 0.32 million sq km. The Antarctic yearly maximum is usually reached in September and the minimum in February.

Where the interval includes zero (Antarctic February mean, Antarctic September mean), the data do not show a change in one direction over these years. NSIDC advises using monthly means rather than daily values for trends, because errors in single days average out.