Global Warming: Understanding the forecast is based on class lectures for undergraduate nonscience majors at the University of Chicago, developed by Ray Pierrehumbert and myself. The class serves as partial fulfillment of our general education or “core” science requirements. We teach the class, and I have written this textbook, in a mechanistic way. We aim to achieve an intuitive understanding of the ropes and pulleys of the natural world, a fundamental scientific foundation that will serve the student longer than would a straight presentation of the latest predictions. The text aims at a single problem – assessing the risk of anthropogenic climate change. The story ranges from science to economics to policy, through physics, chemistry, biology, geology, and of course atmospheric science. We see the distant past and the distant future. In my opinion, by looking at a problem from many angles, the student gets a pretty decent view of how a working scientist really thinks. This is as opposed to, say, taking a survey tour of some scientific discipline.