The greenhouse effect and planetary energy balance

Every object that absorbs energy and does not otherwise heat up indefinitely must radiate that energy away again, and for a planet warmed by its star, this radiative balance sets its temperature. A planet with no atmosphere absorbs sunlight, warms up, and radiates energy back to space as electromagnetic radiation; because the planet is far cooler than its star, this outgoing radiation is concentrated at much longer wavelengths — mostly infrared rather than visible light — following the same blackbody radiation principles that describe emission from any warm object. Equilibrium is reached when the absorbed power exactly equals the radiated power, and this balance alone, with no atmosphere involved, sets a "bare planet" equilibrium temperature that can be calculated purely from the planet's distance from its star and how much sunlight it reflects (its albedo).

An atmosphere containing greenhouse gases — carbon dioxide, water vapour, methane, among others — changes this picture through a wavelength-dependent effect. These gases are largely transparent to incoming visible sunlight, so they do not significantly block sunlight from reaching and warming the surface. But they strongly absorb the longer-wavelength infrared radiation that the warmed surface itself emits, and having absorbed that energy, they re-radiate it in all directions, including back down toward the surface. This does not violate energy conservation — the planet as a whole still radiates to space exactly as much power as it absorbs from the Sun once overall equilibrium is reached — but the presence of the atmosphere means the surface itself now receives additional downward radiation beyond direct sunlight, so a higher surface temperature is needed before the system as a whole reaches the same outgoing radiative balance. The atmosphere is not acting like a physical blanket or barrier that blocks outgoing energy entirely; it is acting as a partially absorbing, partially re-emitting layer that shifts where in the atmosphere the effective radiating surface sits, raising the surface temperature needed to maintain the planet's overall energy balance.

This mechanism explains why planets with thick, greenhouse-gas-rich atmospheres, such as Venus, have surface temperatures dramatically higher than their bare-planet equilibrium value would predict, while planets with thin or absent atmospheres, such as Mars or the Moon, have surface temperatures much closer to their bare-planet prediction — and it is the same underlying physics used to model how changes in atmospheric greenhouse gas concentration on Earth shift the planet's own equilibrium surface temperature.