One of the largest icebergs ever recorded, twice the size of Luxembourg, broke off the Larsen C ice shelf in Antarctica earlier this week. Whether or not global warming was the direct cause (scientists are still debating), the event offers a harbinger of how a warmer planet could reconfigure the landscape. Fortunately, the floe was holding back relatively little land ice and will thus not contribute significantly to sea level rise, but next time we may not be so lucky. Higher temperatures from human-induced global warming will fracture even bigger ice shelves, fulfilling an almost forty-year-old prophecy from glaciologist John Mercer about the collapse of the West Antarctic ice sheet and the catastrophic effects that could follow. In the years to come, the world will face a momentous choice: either resistance to bold climate policy will thaw, or the ice sheets will.