Last week, the National Snow and Ice Data Center, in Boulder, Colorado, announced that the Arctic sea ice had reached a new low. The sea ice shrinks in the summer and grows again during winter’s long polar night. It usually reaches its minimum extent in mid-September. On September 16, 2012, the N.S.I.D.C. reported, the sea ice covered 1.3 million square miles. This was just half of its average extent during the nineteen-eighties and nineties, and nearly twenty per cent less than its extent in 2007, the previous record-low year.

It would be difficult to overstate the significance of this development.