Energy is the foundation of the human economy, but our energy system is now so large that its waste product, CO2, can alter global climate. We’ll discuss the scale of humanity’s activities, their physical effects, and possible energy technologies of the future.  One goal is to provide simple physical frameworks and back-of-the-envelope calculations that can lead to core insights (for example, that biofuels cannot be a solution, or that CO2 tends to make climate less rather than more variable).