Unlike the Arctic—an ocean basin surrounded by land—the Antarctic is a large continent surrounded by ocean. Because of this geography, sea ice has more room to expand in the winter. But the ice also stretches closer to warmer, lower latitudes, leading to more melting in summer. Antarctic sea ice peaks in September (the end of Southern Hemisphere winter) and retreats to a minimum in February.

These image pairs show Antarctic sea ice during the September maximum (left) and the following February minimum (right) from September 1999 to February 2011. Land is dark gray, and ice shelves—thick slabs of glacial ice grounded along the coast—are light gray. The yellow outline shows the median sea ice extent in September and February from 1979 (when routine satellite observations began) to 2000. Extent is the total area in which ice concentration is at least 15 percent. The median is the middle value. Half of the extents over the time period were larger than the line, and half were smaller.

Since the start of the satellite record, total Antarctic sea ice has increased by about 1 percent per decade. Whether the small overall increase in sea ice extent is a sign of meaningful change in the Antarctic is uncertain because ice extents in the Southern Hemisphere vary considerably from year to year and from place to place around the continent. Considered individually, only the Ross Sea sector had a significant positive trend, while sea ice extent has actually decreased in the Bellingshausen and Amundsen Seas. In short, Antarctic sea ice shows a small positive trend, but large scale variations make the trend very noisy.

The year-to-year and place-to-place variability is evident in the past decade. The winter maximum in the Weddell Sea, for example, is above the median in some years and below it others. In any given year, sea ice concentration may be below the median in one sector, but above the median in another; in September 2000, for example, ice concentrations in the Ross Sea were above the median extent, while those in the Pacific were below it.

At summer minimums, sea ice concentrations appear even more variable. In the Ross Sea, sea ice virtually disappears in some summers (2000, 2005, 2006, and 2009), but not all. The long-term decline in the sea ice in the Bellingshausen and Amundsen Seas is detectable in the past decade’s summer minimums: concentrations were below the median in all years.

This time series is made from a combination of observations from the Special Sensor Microwave/Imagers (SSM/Is) flown on a series of Defense Meteorological Satellite Program missions and the Advanced Microwave Scanning Radiometer for EOS (AMSR-E), a Japanese-built sensor that flies on NASA’s Aqua satellite. These sensors measure microwave energy radiated from the Earth’s surface (sea ice and open water emit microwaves differently). Scientists use the observations to map sea ice concentrations.

• References

• Cavalieri, D. J., and C. L. Parkinson (2008). Antarctic sea ice variability and trends, 1979–2006, Journal of Geophysical Research Oceans. 113, C07004.
• NSIDC. (2007, September 25). Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I. Accessed May 15, 2009.
• NSIDC. Frequently Asked Questions about Sea Ice. Accessed May 20, 2009.
• NSIDC. Sea Ice Index. Accessed May 13, 2009.
• Raphael, M.N. (2007). The influence of atmospheric zonal wave three on Antarctic sea ice variability. Journal of Geophysical Research. 112, D12112.
• Steig, E.J., Schneider, D.P., Rutherford, S.D., Mann, M.E., Comiso, J.C., Shindell, D.T. (2009). Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year. Nature. 457, 459-463.

• Links

• NSIDC. State of the Cryosphere. Accessed May 13, 2009.
• Scott, M. (2009, April 20). Sea Ice. Accessed May 13, 2009.