“Why don’t you mention the problem of overpopulation?” Whenever I give a talk about climate change and what to do about it, or about human progress during the past two centuries (or about any other topic, really), this is the one question that never fails to be asked during the Q&A. It is almost always meant to be a rhetorical one. The root cause of all our worries about climate and environment, or so the questioner believes, can be summarised in three words: too many people. Without addressing that root problem there can’t be any hope of a bright future. People beget more people, and before long you get an exponential growth curve that ends in disaster. A ticking time bomb. An explosion.