The National Snow and Ice Data Center reports that the Arctic sea ice extent averaged for July 2011 reached the lowest level for the month in the 1979 to 2011 satellite record, even though the pace of ice loss slowed substantially during the last two weeks of July. Shipping routes in the Arctic have less ice than usual for this time of year, and new data indicate that more of the Arctic's store of its oldest ice disappeared.

Researchers look at ice age as a way to estimate ice thickness. Ice thickness matters to the overall stability of the ice cover, because older ice grows thicker over multiple seasons, while newly formed ice tends to be thin and vulnerable to melt. While the amount of older sea ice has increased somewhat since September 2007, an updated analysis of satellite-derived sea ice age recently published by James Maslanik and co-authors show the oldest ice (ice older than five years), has continued to decline. Until recently, the central Arctic Ocean and Canadian Archipelago served as refuges for some of the oldest, thickest ice. However, the new data show that ice age is now declining in these areas. Multi-year ice is more resistant to melting completely in summer, so it is not yet clear how much more ice will melt.

For more information visit the NSIDC homepage at www.nsidc.org