At many moments in history, humanity’s propensity for wanton destruction has demanded legal and moral restraint. One of those times, seared into modern consciousness, came at the close of World War II, when Soviet and Allied forces liberated the Nazi concentration camps at Auschwitz and Dachau. Photographs and newsreels shocked the conscience of the world. Never had so many witnessed evidence of a crime so heinous, and so without precedent, that a new word—genocide—was needed to describe it, and in short order, a new framework of international justice was erected to outlaw it.

Another crime of similar magnitude is now at large in the world. It is not as conspicuous and repugnant as a death camp, but its power of mass destruction, if left unchecked, would strike the lives of hundreds of millions of people. A movement to outlaw it, too, is gaining momentum. That crime is called ecocide.