"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."