The ice cover in the Arctic sea shrank to its summer minimum—and sixth-lowest level on record—on September 17, according to data released on Monday by NASA-supported National Snow and Ice Data Center (NSIDC) at the University of Colorado, Boulder. Sea ice extent on September 17 was measured at 1.94 million square miles (5.02 million square kilometers), which is 463,000 square miles below average. September is the month when Arctic ice reaches its lowest “extent” of the year, at the end of the Northern Hemisphere summer. Sea ice is frozen ocean water that melts each summer and refreezes each winter. It acts as an air conditioner for the planet, reflecting energy from the sun.