[MrB]

A fifth, eh?
A fifth of WHAT exactly,

seeing as how WATER VAPOR makes up 95% of the “greenhouse gases” in our atmosphere, followed by CO2 at 3.6%, followed by nitrous oxide at .95%, and finally methane at .36%....

http://www.geocraft.com/WVFossils/greenhouse_data.html

[lanternman]

C02 concentration in the atmosphere is approx. .032% not 3.5%