The atmosphere is cooled by radiation which goes around greenhouse gases. The amount going around doesn't matter. A gate half open won't keep in half the sheep. The cooling occurs until equilibrium is established with the amount of energy coming in from the sun.
The amount of radiation going around greenhouse gases is said in Wikipedia to be 15-30%. Calculations by climatologists are based upon none going around. They calculate the amount of radiation getting to the top of the atmosphere using "radiative transfer equations." Those equations cannot account for equilibrium, which is an adjustment of every influence upon temperature.
If the amount going around were calculated without equilibrium, there would be a 100% error in the range (15-30%), while the product of the radiative transfer equations is said to have about 1% error. That product serves as the primary effect by carbon dioxide, which no one in science questions. Only secondary effects are argued.
But the gate is not 15-30% open. Each molecule in the atmosphere radiates energy, with 15-30% going directly into space. That which is absorbed by greenhouse gases is re-emitted with 15-30% going into space. It means the gate is about 99.99% open.
The atmosphere cools as fast as heat enters it leaving very low temperatures in equilibrium with the sun's energy. The equilibrium temperatures are very cold, because heat leaves in all directions during all hours, while it enters from one direction, half the time. The energy from the sun lands on the surface (mostly), while it leaves from the entire atmosphere at a depth of 12-15 kilometers.