NASA: National Aeronautics and Space AdministrationEarth Observatory

Arctic Sea Ice

September 1999 & March 2000
Image Location
Map showing image location

Layers of frozen seawater, known simply as sea ice, cap the Arctic Ocean. Ice grows dramatically each winter, usually reaching its maximum in March. The ice 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 2009.

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. This series of record lows, combined with poor wintertime recoveries starting in the winter of 2004-2005, marked a sharpening in the rate of decline in Arctic sea ice. Sea ice did not return to anything approaching the long-term average (1979-2000) after 2002.

September–March September Average Extent (millions of square kilometers) March Average Extent (millions of square kilometers)
1979–2000 mean 7.0 15.7
1999–2000 6.2 15.3
2000–2001 6.3 15.6
2001–2002 6.8 15.4
2002–2003 6.0 15.5
2003–2004 6.1 15.1
2004–2005 6.0 14.7
2005–2006 5.6 14.4
2006–2007 5.9 14.6
2007–2008 4.3 15.2
2008–2009 4.7 15.2

After September 2002, the next record low occurred in September 2005. In the Northern Hemisphere summer of 2007, Arctic sea ice actually set a record in early August—more than a month before the end of the melt season. That September, the “preferred” navigation route through the Northwest Passage opened. One potential factor driving the sharp decline in 2007 sea ice was unusually sunny weather in the East Siberian Sea in late June.

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.

last updated: August 25, 2009