All of us who are middle class or above in the U.S. and other industrialized nations spend money on many things we do not need. We could instead donate that money to organizations that will use it to make a huge difference in the lives of the world’s poorest people—people who struggle to survive each day on less than we spend on a bottle of water. For decades, that is what I’ve been advocating we should do. But this concern for the poor appears to be in tension with the need to protect our environment. How would the world cope if everyone were to become affluent and match our per capita rate of greenhouse gas emissions?