Summary
Highlights
Gases absorb specific parts of the light spectrum. Atoms and molecules have distinct energy levels for electrons. To shift an electron to a higher energy level, a molecule needs a precise amount of energy, which it obtains from photons. The energy of a photon corresponds to its color; red light has lower energy, and purple light has higher energy. When a photon carries the exact energy needed, the molecule absorbs it, causing the photon to disappear and creating a gap in the rainbow spectrum. If the photon's energy doesn't match, it passes through, which is why glass is transparent to visible light.
Carbon dioxide doesn't absorb light directly from the sun but rather from the Earth itself. The Earth emits infrared light, which is part of the electromagnetic spectrum, invisible to human eyes. Infrared light is visible to certain animals, like snakes. All objects with a temperature above absolute zero emit thermal radiation; hotter objects emit higher frequency light. For example, a heated piece of iron will first emit invisible infrared light, then glow red hot, and eventually white as it gets hotter. This principle explains why incandescent light bulbs are inefficient, wasting 95% of their energy as invisible heat.
Earth's infrared radiation would escape into space if not for greenhouse gases like carbon dioxide. Like other gases that absorb specific photons, carbon dioxide molecules absorb infrared photons, providing the exact energy needed to elevate their energy level. After absorbing an infrared photon, a carbon dioxide molecule quickly returns to its previous energy state, re-emitting the photon in a random direction. Some of this re-emitted energy returns to Earth's surface, causing warming. An increase in atmospheric carbon dioxide makes it more likely for these infrared photons to return to Earth, leading to climate change.