There is a popular fantasy that the earth is 33C warmer with an atmosphere than without due to the radiative greenhouse effect, RGHE.
Let’s start at the very beginning, a very good place to start – so I hear.
The 33C difference is between an alleged average surface temperature of 288K/15C and 255K/-18C, the alleged surface temperature without an atmosphere. Let’s take a closer look.
Per IPCC AR5 glossary the average land surface temperature is measured 1.5 meters above the ground, but 80% of the land doesn’t even have reliable weather instrumentation or data. The average sea surface temperature is a combination of buckets and thermometers, engine cooling intakes, buoys, satellites, etc. This “global” surface average temperature, one number to rule them all, must represent: both lit side and dark sides, both poles, oceans, desert, jungles and a wide range of both land and sea surfaces. The uncertainty band must be YUGE!
The 255K is a theoretical calculation using the S-B ideal BB temperature associated with the 240 W/m^2 radiative balance at the top of the – wait for it – atmosphere, i.e. 100 km.
So, the 33C difference is between a) an average surface temperature composed of hundreds of WAGs that must be +/- entire degrees and b) a theoretical temperature calculation 100 km away that cannot even be measured and c) all with an intact and fully functioning atmosphere.
The surface of the earth is warm because the atmosphere provides an insulating blanket, a thermal resistance, no different from the insulation in the ceiling and walls of a house with the temperature differential determined per the equation Q = U * A * dT, simple to verify and demonstrate.
A voltage difference is needed for current to flow through an electrical resistance.
A pressure difference is needed for fluid to flow through a physical resistance.
A temperature difference is needed for energy to flow, i.e. heat, through a thermal resistance.
RGHE upwelling/downwelling/”back” radiation is a fictional anti-thermodynamic non-explanation for the “33C without an atmosphere” phenomenon that doesn’t exist.