Simple model that gets you in spitting distance of a reasonable estimate of Equilibrium Climate Sensitivity
Background ECS - Equilibrium Climate Sensitivity - is the temperature change on Earth as a result of a doubling of atmospheric CO2. That means going from 280 ppm (the level prior to the industrial revolution) past 420 (where we are now) and on to 560 ppm. Most people probably understand why this would increase temperatures by now, but I'm going to repeat the argument briefly anyway, for completeness