[ExI] Diminishing influence of increasing carbon dioxide on temperature

[Anders Sandberg]

Dan <dan_ust at yahoo.com> , 13/8/2014 6:50 AM:
And is there any way Venus could have been much more Earth-like on its present orbit
Certainly. At a distance of 0.72 AU Venus planetary blackbody equilibrium temperature would just have been 17% larger than Earth (it scales as 1/sqrt(d)). Given the basic blackbody temperature of 256 K for Earth, this would be 299 K - nice and cosy. This is of course without taking the 31 K greenhouse factor into account. But albedo can fix this fairly well: the blackbody temperature scales as (1-a)^(1/4), so with an albedo of 0.46 (Earth is around 0.32) and identical greenhouse level it would have had the same temperature.
In practice, things are messier since temperature affects albedo. And different layers of the atmosphere have different temperatures. Very soon you will just want to run a simulation and tweak the parameters... and before long you realize that circulation really matters in this case due to the slow rotation. There are many papers on atmospheric models for terrestrial worlds and how they predict different life zones: I think the best conclusion right now is that the issue is somewhat in flux. 
Venus would likely have resisted hydrogen loss if it (1) had a magnetic field, (2) had a functioning stratospheric cold trap keeping photo-dissociation of water from releasing hydrogen in the exosphere. As far as I can tell at a glance, the Jeans escape formula is not super-sensitive to temperature - and Earth's exosphere is already 1000 K. 
Overall, I think the problem with Venus was that higher tidal forces (2.7 times what Earth has) slowed its rotation, and once it got slow the magnetic field dropped and the atmosphere started eroding, removing water at a high rate. As it dried up and overheated plate tectonics broke down, and we got periodic crustal melts that really turned it into a hellhole. 

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

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