Quote by vanesch
Mmm, maybe I misunderstood you. Maybe you are implying that the modest amount of CO2 is the reason why there is a certain amount of water vapor in the atmosphere. If we would remove all CO2, then all or a large fraction of the water vapor would also disappear (for an amplification factor close to 8). Well, the simple case is implemented in MODTRAN where relative humidity is used instead of constant water vapor.
For our tropical atmosphere, we go from 280 ppm and 289.2 W/m2 to 560 ppm and 286 W/m2 at 300 K (that's our famous 3.2 W/m2 forcing for a CO2 doubling), and we now have to increase the surface temperature not by 0.8K but by 1.5K (because heating up gives more water vapor in the atmosphere and hence more greenhouse effect).
So we get about a 87% extra feedback this way, true. That's not the 800%. It comes close to the 200%, if this relative humidity doesn't do anything else. But increasing the absolute water vapor pressure increases things like convection (it is the working principle of a cooling tower!). So it is not even said that this feedback is really there.
So using this feature, we get a cllimate sensitivity of not 0.8K but of 1.5K. And that's the highest modtran can do.

Would you please substract the energy, required for the enhanced evaporation rate to maintain the higher humidity, from the energy available to heat the surface?