How CO₂ slows Earth’s escape of heat
Sunlight passes through the atmosphere and warms Earth’s surface. The warm surface then releases energy upward as infrared radiation—the kind of radiation we feel as heat. Some of that energy travels straight out to space, but greenhouse gases absorb particular wavelengths along the way.
A CO₂ molecule can absorb infrared energy when it makes the molecule vibrate in certain ways. It does not hold that energy indefinitely: the molecule can emit infrared radiation again, in any direction. Some goes upward; some returns downward and is absorbed by the surface or lower atmosphere. In this sense, greenhouse gases make it harder for heat to escape directly to space—they do not form a literal lid that prevents heat from leaving.
Why the surface warms
Adding CO₂ increases the odds that infrared radiation will be absorbed and re-emitted before it reaches space. The atmosphere’s effective level of emission to space shifts higher, where the air is generally colder. Colder air emits less infrared energy, so at first Earth sends less energy to space than it receives from the Sun.
That imbalance prompts warming. The surface and lower atmosphere heat up, increasing the amount of infrared radiation emitted, until the planet again releases as much energy to space as it absorbs. The new balance is reached at a higher average surface temperature.
CO₂ matters even though it makes up only a small fraction of the air because its absorption overlaps wavelengths at which Earth gives off substantial energy. Other gases, especially water vapor, also absorb infrared radiation; warming can increase the amount of water vapor in the air, amplifying the initial effect. The key mechanism is not that CO₂ creates heat, but that it changes how efficiently Earth can shed the heat it receives.
