After months of record temperatures, scientists say Greenland's ice sheet experienced its biggest melt of the summer on Thursday, losing 11 billion tons of surface ice to the ocean -- equivalent to 4.4 million Olympic swimming pools. Greenland's ice sheet usually melts during the summer, but the melt season typically begins around the end of May; this year it began at the start. It has been melting "persistently" over the past four months, which have recorded all time temperature highs, according to Ruth Mottram, a climate scientist with Danish Meteorological Institute.