You might have noticed that I'm not doing so many posts on the Greenhouse Effect anymore, that's because I have finally worked it out and there are few questions left unanswered.
I stumbled across a good article by Clive Best in which he arrives at the same conclusion as I have:
... the radiating level on Venus is set at 260 [degrees K at] 50 km up in the atmosphere because [the clouds at that altitude are] directly heated by the sun. So Venus' surface is heated through convection from above and not directly by CO2 greenhouse effects.
Just lose the word "directly" and we're good. Note: Clive still believes in the GHG myth despite all evidence the contrary which he presents so eloquently. And I'm not sure it really is 'convection', it is something slightly different which I refer to as the 'gravito-thermal effect', but hey.
So the bottom line question is that if you were to replace 90% of the CO2 molecules on Venus with an equal mass of N2 molecules would the surface temperature change ? I don’t think so – the surface temperature would remain exactly the same.
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On a related note, I had a spat in the comments (on YouTube, no less) with a devout Believer who came up with crap like "heat rises" (he doesn't know what 'heat' is) and thought he could score a point by saying "although the sun warms the clouds, they are still below freezing" - I had clearly stated that the cloud cover on Earth is, very approximately, 255K, which as any fule kno is -18 C. FFS.