It is the largest movement the world has ever seen, it may also be the most important – in terms of what’s at stake. Yet it’s not east being green. Environmentalists have been reviled as much as revered, for being killjoys and Cassandras. Every battle begins as a lost cause and even the victories have to be fought for again and again. Still, environmentalism is one of the great social innovations of the twentieth century, and one of the keys to the twenty-first. It has arisen at a key juncture in history, when humans have come to rival nature as a power determining the fate of the earth.