The window for action on climate change is closing rapidly. We are hurtling ever faster towards climate catastrophe—the destruction of a habitable world for many species, perhaps the near-extinction of our own. As anxieties about global temperatures soar, demands for urgent action grow louder. What can be done? Can this process be reversed? Once temperatures rise, is there any going back? Some are thinking about releasing aerosols into the stratosphere in order to reflect sunlight back into space and cool the earth. And this may be necessary, if it actually works. But it would only be the beginning; it’s what comes after that counts.