Thursday, May 13, 2010
Unlike the Arctic—an ocean basin surrounded by land, where sea ice extends all the way to the pole—the Antarctic is a large continent surrounded by ocean. Because of this geography, sea ice has more room to expand in the winter, but it is also closer to the equator. The result is that Antarctica’s sea ice extent is larger than the Arctic’s in winter, but smaller in the summer. Total 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) for a time series beginning in September 1999 and ending in February 2010. 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.
September/February(maximum/minimum) September Average Extent (millions of square kilometers) February Average Extent (millions of square kilometers)
1979–2000 mean 18.7 2.9
1999/2000 19.0 2.8
2000/2001 19.1 3.7
2001/2002 18.4 2.9
2002/2003 18.2 3.8
2003/2004 18.6 3.6
2004/2005 19.1 2.9
2005/2006 19.1 2.6
2006/2007 19.4 2.9
2007/2008 19.2 3.7
2008/2009 18.5 2.9
2009/2010 19.2 3.2
The year-to-year and place-to-place variability is evident in these images from 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.
• 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.
• NSIDC. State of the Cryosphere. Accessed May 13, 2009.
• Scott, M. (2009, April 20). Sea Ice. Accessed May 13, 2009.