The Greenhouse Effect

Sunlight arrives mostly as visible light, passes through the atmosphere, and hits the ground, which absorbs it and heats up. The warm ground then radiates energy back out, but not as visible light. Because the ground is much cooler than the sun, it radiates in longer wavelengths: infrared. This matters because the atmosphere is transparent to incoming visible light but not to outgoing infrared.

Certain gases, CO2, water vapor, methane, nitrous oxide, have molecular bonds that happen to vibrate at frequencies matching infrared wavelengths. When an infrared photon hits a CO2 molecule, the molecule absorbs it and starts vibrating faster. A moment later it re-emits a photon of infrared, but in a random direction, not necessarily back toward space. Some of that re-emitted energy heads back down toward the surface. Multiply this by trillions of molecules and the net effect is that a lot of infrared energy gets delayed and recirculated near the surface instead of escaping straight to space. That recirculation is what keeps the surface warm.

Without any greenhouse gases, Earth would lose heat to space almost as fast as it absorbed sunlight, landing around 33 degrees C colder than today, too cold for liquid water across most of the planet. So the greenhouse effect itself is not the problem; it is the reason Earth is habitable. The trouble starts when the concentration of these gases rises well beyond natural levels. More CO2 molecules mean more absorption and re-emission events, which push the altitude where infrared finally escapes to space higher and colder, so Earth must warm further at the surface to balance energy coming in and going out. That shift from natural baseline to enhanced, human-driven warming is the pivot into the rest of the unit.