The ice was never supposed to melt this quickly. Although climate scientists have known for some time that global warming was shrinking the percentage of the Arctic Ocean that was frozen over, few predicted so fast a thaw. In 2007, the Inter- governmental Panel on Climate Change estimated that Arctic summers would become ice free beginning in 2070. Yet more recent satellite observations have moved that date to somewhere around 2035, and even more sophisticated simulations in 2012 moved the date up to 2020. Sure enough, by the end of last summer, the portion of the Arctic Ocean covered by ice had been reduced to its smallest size since record keeping began in 1979, shrinking by 350,000 square miles (an area equal to the size of Venezuela) since the previous summer. All told, in just the past three decades, Arctic sea ice has lost half its area and three quarters of its volume.