On the other hand, the next phase of our history could see us melting a lot of ice rather than making it. If all the ice sheets melted, sea levels would rise by two hundred feet—the height of a twenty-story building—and every coastal city in the world would be inundated. More likely, at least in the short term, is the collapse of the West Antarctic ice sheet. In the past fifty years the waters around it have warmed by 2.5 degrees centigrade, and collapses have increased dramatically. Because of the underlying geology of the area, a large-scale collapse is all the more possible. If so, sea levels globally would rise—and pretty quickly—by between fifteen and twenty feet on average.

In the long run, incidentally, ice ages are by no means bad news for the planet. They grind up rocks and leave behind new soils of sumptuous richness, and gouge out fresh water lakes that provide abundant nutritive possibilities for hundreds of species of being. They act as a spur to migration and keep the planet dynamic. As Tim Flannery has remarked: "There is only one question you need ask of a continent in order to determine the fate of its people: ‘Did you have a good ice age?' " And with that in mind, it's time to look at a species of ape that truly did.