A NASA satellite image from September 16, 2007 and released on September 21, 2007, shows Arctic summer sea ice. Arctic sea ice melted to its lowest level ever this week, shattering a record set in 2005 and continuing a trend spurred by human-caused global warming, scientists said on September 20, 2007. REUTERS/NASA/Handout. EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS.