“The bottom line is that the world, confronted with two dire threats to the earth’s fragile atmosphere, found two planetary responses with positive outcomes. The ozone layer is healing. That’s worth remembering as we struggle, often despairingly, to find common ground in the battle against climate change. Compared with the manifold complexities of global warming, dealing with ozone depletion was, in fact, relatively simple. But the key point is that it happened, and it’s worth asking why the world has not responded with similar resolve in dealing with the main global warming gases like carbon dioxide, about which we have known a lot for a long time. . . .