Sea Ice Extent
According to the National Snow and Ice Data Center (NSIDC), the Northern Hemisphere sea ice extent — which is measured from passive microwave instruments onboard NOAA satellites — averaged for September 2012 was 3.61 million square km (1.39 million square miles), 48.7 percent below average. This was the smallest September sea ice extent in the 1979-2012 period of record. The previous smallest September Arctic sea ice extent occurred in 2007, at 4.3 million square km (1.66 million square miles). The six lowest September sea ice extents have occurred since 2007. This marks the 16th consecutive September and 136th consecutive month with below-average Arctic ice extent. Ice coverage was below average across all regions of the Arctic, except the East Greenland Sea, where the ice extent was near average. September Northern Hemisphere sea ice extent has decreased at an average rate of 13.0 percent per decade.
On September 16th, the Northern Hemisphere sea ice extent dropped to 3.41 million square kilometers (1.32 million square miles) reaching its annual minimum before beginning its annual growth cycle. The 2012 annual minimum extent was 760,000 square kilometers (293,000 square miles) below the previous record minimum, which occurred on September 18, 2007 and 49 percent below the 1979-2000 average. During the Arctic sea ice melt season, between March 20th, when the annual maximum extent occurred, and September 16th, 11.83 million square kilometers (4.57 million square miles) of ice was lost. This marks the largest seasonal Arctic sea ice loss in the satellite record, surpassing the 10.65 million square kilometers (4.11 million square miles) of ice loss during the 2008 melt season.
Animation of 2012 Arctic Ice Melt
Video provided by NOAA's Environmental Visualization Laboratory
Sea Ice Volume Anomlay
Source: UW's Polar Ice Center
When using Arctic sea ice extent to monitor the state of sea ice conditions across the Arctic, no information is available on the thickness of the ice. To compensate for this, the Polar Science Center at the University of Washington developed a modeled dataset to measure the volume of Arctic sea ice using the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS). Sea ice volume is an important climate indicator because it depends on both ice thickness and extent and therefore more directly tied to climate forcing than extent alone. According to this dataset, Arctic sea ice volume reached a monthly low value during September 2012, at 3,400 km3, the smallest monthly sea ice volume on record. The previous record small Arctic sea ice volume for September occurred in 2011 at 4,200 km3. The September 2012 value is 72 percent lower than the mean over 1979-2011 period, 80 percent lower than the maximum in 1979, and 2.0 standard deviations below the 1979-2011 trend.
The September 2012 Southern Hemisphere sea ice extent was 19.39 million square km (7.48 million square miles), 3.51 percent above average and the largest September sea ice extent in the 1979-2012 period of record. Antarctic sea ice extent during September has increased at an average rate of 0.9 percent per decade, with substantial interannual variability.
The Southern Hemisphere sea ice extent reached its annual maximum extent on September 26th at 19.44 million square km (7.51 million square miles). This marked the largest annual maximum extent of Antarctic sea ice extent on record and surpassed the previous record of 19.36 million square km (7.47 million square miles) which occurred on September 21, 2006.
Arctic summer sea ice is shrinking much more rapidly than the rate at which Antarctic winter sea ice is expanding. Over the 1979-2012 record, the Arctic has experienced significant ice loss, while the growth of Antarctic sea ice has been slight. The September 2012 record low Arctic sea ice extent was 6.2 standard deviations below its 1979-2000 average, while the record large Antarctic sea ice extent was 2.1 standard deviations above its 1979-2000 average. Differences in hemispheric weather patterns, ocean currents, and geography partially account for these differing sea ice trends. A more detailed description of these differences is available through the NSIDC.
For further information on the Northern and Southern Hemisphere snow and ice conditions, please visit the NSIDC News page.