In this talk, I will discuss the climate sensitivity, a frequently used measure of the severity of global warming. I will introduce this measure, discuss what determines it, and show how it relates to forecasts of global warming. I will present different methods of estimating it, and discuss the recent phenomenon of “lukewarming,” in which recent observational estimates give low values for the climate sensitivity. I will also discuss recent results, from myself and others, suggesting that both our estimates of the sensitivity and the sensitivity itself are likely to increase over time.