The new evidence that Greenland lived up to its verdant name in the not-so-distant past may represent an exciting scientific breakthrough, but it also heralds ominous possibilities for the future of humanity. Present-day atmospheric carbon dioxide levels are higher than they’ve been in millions of years; evidence of an ice-free Greenland in the more recent past means that it could take even less warming than once expected to deplete the continent’s all-important ice sheet. The frozen stronghold that covers Greenland contains enough fresh water to raise sea levels by 23 feet — a staggering volume that would reshape coastlines around the world.