Climate Sensitivity: More Motled Thoughts
Well, I'm still trying to understand how and where Motl gets his non-SB emission curves. Now that I understand that he is computing emissions at the ground I need to throw in a quibble. After deriving the relation
dT/d(RF) = T/(4 RF), he proceeds to plug in T = 288 and RF = 235. RF and T are not independent - one determines the other, and RF(T=288K) is not equal to 235 W/m^2 - the radiant output of a 288 K black body would be 390 W/m^2 - an error by almost a factor of tw by Motl. The sea and land infrared emissivities average well over 95%, so the terrestial Earth is emitting around 370-380 W/m^2 at 288 K. The whole point of the greenhouse effect is that this number is not equal to the 235 W/m^2 emitted at the top of the atmosphere.
The difference between these numbers, 380-235 =145 W/m^2 is the energy returned to Earth due to absorption and re-emission by atmospheric gases.
UPDATE: OK, now I understand. Motl is effectively calculating the climate sensitivity for a so called grey body Earth. By contrast, I did the blackbody Earth. The reasons that the feedbacks have the signs they do, and add significantly to the sensitivity number are well explained here by another skeptic incidentally, but one whose logic seems a lot clearer to me. He believes in a relatively low sensitivity based on estimates from geological history data. It's worth noting that others who attempt to make historically based estimates have come up with different values.