Sea ice in the Arctic Ocean has melted to its lowest volume in recorded history, according to new measurements. At the end of last summer, the sea’s ice pack melted to the lowest coverage ever, following an exceptionally warm winter. But the winter of 2007-2008 was colder than the last few years have been, and even after this summer’s melt season there is still more acreage of ice covering the water than last year. The problem is, the ice may be thinner than ever.