Arctic sea ice cover this month fell to the lowest summer minimum extent since satellite records began in 1979, according to the National Snow and Ice Data Center (NSIDC). "We are now in uncharted territory," said NSIDC director Mark Serreze. "While we've long known that as the planet warms up, changes would be seen first and be most pronounced in the Arctic, few of us were prepared for how rapidly the changes would actually occur."

Arctic sea ice cover on Sept. 16 fell to the lowest summer minimum extent in the satellite record—3.41 million square kilometers (1.32 million square miles), as shown in an animation from the NASA Goddard Space Flight Center. The previous record minimum was reached in 2007—4.17 million square kilometers, an area about the size of the European Union minus Greece. The current 3.41 million is the equivalent of the European Union minus Greece, Portugal, Ireland, Britain and Germany.

Arctic sea ice cover grows each winter as the sun sets for several months, and shrinks each summer as the sun rises higher in the northern sky, reaching its minimum in September. Though the September minimum bounce back a little from 2007’s nadir, in every year since then the minimum has been lower than it was in every year before 2007—until now. 

Arctic sea ice extent—defined by the NSIDC as the total area covered by at least 15% of ice—varies from year to year because of changeable weather conditions. But while 2007 saw summer weather particularly inimical to the persistence of ice—with lots of warm southerly winds and clear skies—this year has seen far fewer such special circumstances. A powerful cyclone broke up a lot of ice in the East Siberian and Chukchi seas in early August—but the rate of ice loss outstripped that seen in 2007 both before the storm and after it. This year's minimum is nearly 50% lower than the 1979-2000 average—pointing to cumulative conditions reaching a breaking point.