The temperature of the Earth's surface has not changed by more than 0.1 C since the year 2000, and it may even have cooled slightly. Most computer models predicted that the increase of CO2, from about 370 to 400 ppm during that period, should have caused a warming of around 0.3 C. There are many possible reasons for the failure of the models, but one may be insufficient careful attention to important, and often neglected details of how CO2 molecules really absorb and emit radiation in the Earth's atmosphere. Some of the physics in question is related to early work by Princeton's Robert Dicke on collisional line narrowing.