If you were teaching a graduate seminar in public policy and challenged your students to come up with the most difficult possible problem to solve, they’d come up with something very much like climate change. It’s slow-acting. It’s essentially invisible. It’s expensive to address. It has a huge number of very rich special interests arrayed against doing anything about it. It requires international action that pits rich countries against poor ones. And it has a lot of momentum: you have to take action now, before its effects are serious, because today’s greenhouse gases will cause climate change tomorrow no matter what we do in thirty years.