Greenhouse gases



Monatomic gases can translate. Diatomic gases can also rotate and vibrate. Molecules with 3 or more atoms can also bend. The first 2 are totally transparent to infrared radiation. Only molecules with 3 or more atoms like water and carbon dioxide emit and absorb infrared radiation. Also methane, nitrous oxide and Ozone.

Mass of Atmosphere
If Earth's atmosphere were only slightly more massive then the air would be warmer and the amount of water vapor in the air would be much greater and lightning would therefore be much more common. The lightning would break apart the air molecules into highly reactive radicals which would bond to dust in the air and get washed down into the sea where they would end up getting subducted into the Earth. In this way the Earth's average air pressure is maintained at its current level.

During most of it's history Earth only had one atmospheric cell that extended from the pole to the equator and as a result Earth was very much warmer.
 * 1) Hadley cell

During an ice_age the Earth only has two cells. Ice ages are probably caused by deforestation caused by megafauna.
 * 1) Polar cell
 * 2) Ferrel cell

The Earth's atmosphere currently has 3 cells.
 * 1) Polar_cell
 * 2) Ferrel_cell
 * 3) Hadley_cell