Layers of frozen seawater, known simply as sea ice, cap the Arctic Ocean. Ice grows dramatically each winter, usually reaching its maximum in March. It melts just as dramatically each summer, generally reaching its minimum in September. These image pairs show Arctic sea ice concentration for the month of September (left) and the following March (right) for a time series beginning in September 1999 and ending in March 2013.
The yellow outline on each image shows the median sea ice extent observed by satellite sensors in September and March from 1979 through 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 1978, satellites have monitored sea ice growth and retreat, and they have detected an overall decline in Arctic sea ice. The rate of decline steepened after the turn of the twenty-first century. In September 2002, the summer minimum ice extent was the lowest it had been since 1979. Although the September 2002 low was only slightly below previous lows (from the 1990s), it was the beginning of a series of record or near-record lows in the Arctic.
The new lows, combined with poor wintertime recoveries from 2004 to 2007, heralded a sharpening in the rate of decline in Arctic sea ice. Since 2002, ice extent at the summer minimum has not returned to anything approaching the long-term average (1979-2000). Though winter ice extent has fluctuated, satellite and in situ observations have shown that there is less multiyear ice and more annual ice.
|September/March (minimum/maximum)||September Average Extent (millions of square kilometers)||March Average Extent (millions of square kilometers)|
In the summer of 2007, Arctic sea ice extent set a record low in early August—more than a month before the end of the melt season. That September, the preferred northern navigation route through the Northwest Passage opened. In the following years, summer sea ice had some relatively higher extents, though nothing approaching normal conditions. Then in 2012, a new record low was set—more than 700,000 square kilometers below the 2007 minimum. The driving factor in 2012 was the large amount of thin ice, which is more susceptible to melting from warming temperatures and to break-up by winds and waves. An unusually strong storm in August 2012 brought both of these conditions, resulting in a rapid disintegration and ice loss in the Beaufort, Chukchi, and East Siberian seas.
Cycles of natural variability such as the Arctic Oscillation are known to play a role in Arctic sea ice extent, but the sharp decline seen in this decade cannot be explained by natural variability alone. Natural variability and greenhouse gas emissions (and the resulting rise in global temperatures) likely worked together to melt greater amounts of Arctic sea ice. Some models forecast an ice-free Arctic for at least part of the year before the end of the twenty-first century.
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.
Some areas in the images, such as places along the Greenland coast or in Hudson Bay, may appear partially ice-covered when they actually were not. Over the years, satellite sensor capabilities have steadily improved, but some limitations remain, often due to weather and the mixing of land (coast) and water in the satellite sensor’s field of view. The gray circle at the center of each image is the “pole hole,” north of which satellite sensors have historically been unable to collect data. The sea ice estimates from the National Snow and Ice Data Center, NASA’s archive for sea ice data, assume that this hole is ice-filled.
- NSIDC. (2005, March 18). Arctic Ice Decline in Summer and Winter. Accessed May 13, 2009.
- NSIDC. (2007, September 25). Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I. Accessed May 15, 2009.
- NSIDC. (2008, October 2). Arctic Sea Ice Down to Second-Lowest Extent; Likely Record-Low Volume. Accessed May 13, 2009.
- NSIDC. Frequently Asked Questions about Sea Ice. Accessed May 13, 2009.
- NSIDC. Sea Ice Index. Accessed May 13, 2009.
- Earth Observatory. (2005, September 29). Continued Sea Ice Decline in 2005. Accessed May 13, 2009.
- Earth Observatory. (2007, September 27). Record Arctic Sea Ice Loss in 2007. Accessed May 13, 2009.
- Earth Observatory (2009, April 10). Amount of Old Ice in Arctic Hits Record Low in February 2009. Accessed May 13, 2009.
Arctic Sea Ice
September 1999 & March 2000
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