[ExI] Diminishing influence of increasing carbon dioxide on temperature

[Anders Sandberg]

spike <spike66 at att.net> , 10/8/2014 11:35 PM:


 

 I need to dig around in my green notebooks and see if I can find those calcs; I think it was from about 6 to 8 yrs ago.  At the time I looked at the negative feedback balancing the increased absorption: the radiation to space using the Stefan-Boltzmann equation.  I concluded that the reason CO2 has never caused a runaway greenhouse effect is that it cannot.  Even if CO2 retains *all* the energy in its absorption band, that T^4 term still overpowers everything else.

 
I think this is right; it is the water vapour feedback that is needed for a runaway greenhouse. 
Raymond T. Pierrehumbert's excellent "Principles of Planetary Climate" has a nice treatment of the Ingersoll-Kobayashi limit (basically, with enough water vapor the stable fixed point disappears). I think he discussed a "pure" CO2 greenhouse too, but in that case there is no sudden jump to overheating. 
In any case, the reason there has not been any overheating so far might have been that Earth in the past was closer to the outer range of the life zone than we previously believed. Although the combined geological-biological-climatological system can be rather unpredictable; see some of the papers at http://www.pik-potsdam.de/~bloh/evol/ 

Anders Sandberg, Future of Humanity Institute Philosophy Faculty of Oxford University
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