Early Venus is suggested to have been much colder – and thus habitable – due to higher concentrations of CO2…because CO2 drives cooling in most of the Venus atmosphere (stratosphere, mesosphere, thermosphere).
Scientists have for decades agreed it is “well recognized” that CO2 molecules radiatively cool the atmospheres of planets like Earth, Mars, and Venus (Sharma and Wintersteiner, 1990) in the 15 μm band starting from 12 km above the surface on up.
Image Source: Sharma and Wintersteiner, 1990
For rocky planets like Venus “adding CO2 obviously cools the whole free atmosphere” (Wang and Yang, 2022).
“As the CO2 concentration is increased, the radiative cooling effect becomes stronger.”
Image Source: Wang and Yang, 2022
Again, CO2 is said to significantly cool down most of the Venus atmosphere. So when CO2 concentrations were allegedly much higher than now Venus is suggested to have been a habitable planet (Krissansen-Totten et al., 2021).
“Habitable past scenarios are favored if CO2-rich atmospheres radiatively cool the mesosphere.”
Scientists surmise the stratosphere of Venus would be at least 44 K hotter if it had an Earth-like N2-O2-dominated atmosphere and much lower CO2 concentrations.
Image Source: Krissansen-Totten et al., 2021
The two predominant reasons why the Venus surface-troposphere is so much hotter than Earth’s surface-troposphere are (1) its closer proximity to the Sun and (2) its 92 times greater atmospheric mass. The higher atmospheric mass modulates the surface temperature as it “increases the heat capacity of the atmosphere” (Chemke and Kaspi, 2017).
“An increase (decrease) in atmospheric mass causes an increase (decrease) in near-surface temperatures” (Chemke et al., 2016).
Image Source: Chemke and Kaspi, 2017
Image Source: Chemke et al., 2016
Contradicting the current paradigm, the presence or absence of CO2 or other greenhouse gases in the Venus atmosphere is not a requirement for the planet’s surface-troposphere heat retention. Put another way, backradiation from “greenhouse gas heating…can be discounted” as a surface-troposphere temperature determinant for Venus (Mulholland et al., 2020).