Sea ice plays a number of important roles in the Earth system. It reflects heat from the sun, provides a habitat for marine life, affects ocean circulation, and helps moderate temperature and climate.
In both polar regions, sea ice cover reaches a minimum near the end of summer and a maximum near the end of winter. During the winter freeze in Antarctica the sea ice cover expands to an area roughly twice the size of Europe. Satellite images show that since the 1980s, the extent of Antarctic sea ice has increased slightly, while Arctic sea ice extent has been has been shrinking during the same period.
Although sea ice loss affects Earth's climate, the melting sea ice doesn't affect sea level because it is already floating on the ocean surface, and already displacing its own weight. Melting sea ice won't raise ocean level any more than melting ice cubes will cause a glass of iced tea to overflow.
Since 1979, satellites have provided a near-continuous record of Earth's sea ice. The most valuable data sets come from satellite sensors that observe microwaves emitted by the ice surface because, unlike visible light, the microwave energy radiated by the sea ice surface passes through clouds and can be measured even at night. The near-continuous sea ice record began with the Nimbus-7 Scanning Multichannel Microwave Radiometer (October 1978-August 1987) and continued with the Defense Meteorological Satellite Program Special Sensor Microwave Imager (1987 to present). The Advanced Microwave Scanning Radiometer for EOS (AMSER-E) on NASA's Aqua satellite has been observing sea ice since 2002.
The Advanced Microwave Scanning Radiometer for EOS (AMSR-E) on NASA's Aqua satellite has been observing sea ice since 2002. The AMSR-E instrument is shown here, with the large antenna, near the front and top of the satellite.
Arctic sea ice
Over the last few years, a lot of scientific attention has been devoted to sea ice in the Arctic, where ice cover has clearly shrunk and thinned since the beginning of the near-continuous satellite data record in 19781. Climate scientists are interested in Arctic sea ice decline because sea ice plays an important role in moderating the Earth's temperature by reflecting radiation from the summer sun.
As the white reflective polar ice decreases, an increase in the areas of dark blue open water absorbs more heat. More heating leads to faster melting, reinforcing the cycle. If this trend continues, the Arctic Ocean could become free of ice in the summer.
The graph above shows the average monthly Arctic sea ice extent in September from 1979 to 2010, derived from satellite observations. The September 2010 extent was the third lowest in the satellite record.
Antarctic sea ice
Antarctica's sea ice coverage 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-historically rising to an extent of roughly 18 million square kilometers (about 6.9 million square miles). Ice extent reaches its minimum in February, when it dips to roughly 3 million square kilometers (about 1.2 million square miles).
Since 1979, the total annual Antarctic sea ice extent has increased about 1 percent per decade, and there is some indication that this may be connected with the ozone hole delaying the impact of greenhouse gas increases on the climate of the continent.2
The graph above shows the average monthly Antarctic sea ice extent in February from 1979 to 2010, derived from satellite observations.
Portions of this article are adapted from "Sea Ice," by Michon Scott, NASA's Earth Observatory, April 20, 2009.
1 R. Kwok & D.A. Rothrock, "Decline in Arctic sea ice thickness from submarine and ICESat records: 1958 - 2008," Geophys. Res. Lett. 36, L15501 (2009)
Parkinson, C. L. and D. J. Cavalieri, "Arctic sea ice variability and trends, 1979-2006," Journal of Geophysical Research – Oceans, 113, C07003, doi:10.1029/2007JC004558 (2008).
Cavalieri, D. J. and C. L. Parkinson, C. L., "Antarctic sea ice variability and trends, 1979-2006," Journal of Geophysical Research – Oceans, 113, C07004, doi:10.1029/2007JC004564 (2008)