Environment: Trends

Sea ice typically covers 14 to 16 million square kilometers (5.4 to 6.1 million square miles) in late winter in the Arctic and 17 to 20 million square kilometers (6.6 to 7.7 million square miles) in the Antarctic's Southern Ocean. The amount of sea ice that remains at summer's end is much less in the Antarctic (3 to 4 million square kilometers, or 1.1 to 1.5 million square miles) than in the Arctic (7 to 9 million square kilometers, or 2.7 to 3.5 million square miles). Satellites have shown the fluctuation in sea ice from year to year since 1972. Data suggest that since then, Arctic ice has been decreasing at an average rate of about 3 percent per decade, while Antarctic ice has increased by about 0.8 percent per decade.

Satellite data have indicated an even more dramatic reduction in Arctic ice cover in recent years. In September 2002, Arctic sea ice reached a record minimum: 4 percent lower than any previous September since 1979, and 14 percent lower than the average ice extent from 1979 to 2000. In the past, a low ice year would be followed by a rebound to near-normal conditions, but 2002 was actually followed by two more low-ice years, both of which almost matched the 2002 record low. When the low ice years of 2002 to 2004 are taken into account, the average ice extent each September from 1979 to 2004 is decreasing at a rate of 7.7 percent per decade.

sea ice extent departures from monthly means for the Northern Hemisphere
Sea ice extent departures from monthly means for the Northern Hemisphere, from Sea Ice Trends and Climatologies from SMMR and SSM/I-SSMIS, NASA Team algorithm.
Credit: National Snow and Ice Data Center, University of Colorado, Boulder, CO.

Scientists theorize that the gradual reduction in sea ice results from a combination of shifting atmospheric winds that naturally break up ice cover, as well as higher temperatures, which may be linked to the greenhouse effect.

NSIDC provides a more in-depth analysis of sea ice trends for a scientific audience, called the Sea Ice Index. The Index provides information such as images and graphs of average ice extent trends and anomalies (the difference between the average ice edge and that of a given month).