In April 1895, when famed Norwegian explorer Fridjtof Nansen attempted to mush across the frozen Arctic Ocean to reach the North Pole, he was stopped by endless rows of sea ice ridges. “It was a veritable chaos of ice blocks, stretching as far as the horizon,” wrote Nansen in his expedition account, Farthest North. Dragging a sled over them was “enough to tire out giants.”

The gnarled ice-scape that thwarted Nansen is now largely a thing of the past, according to research reported in Nature on Wednesday. Arctic sea ice has undergone an abrupt, permanent, and consequential change in its structure—from thick and ridged to thin and flat. The shift occurred around 2007, when record-low summer ice cover triggered