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Greenhouse gases absorb and emit thermal infrared radiation at wavelengths determined by their vibrational and rotational transitions. Different gases have distinct spectral signatures; for example, CO₂ absorbs strongly at 15 μm while methane and water vapor absorb at different frequencies. These molecular spectral properties, combined with atmospheric abundance, determine each gas's radiative forcing and contribution to the greenhouse effect.
From your study of spectroscopy and radiative transfer, you know that molecules absorb and emit electromagnetic radiation at specific wavelengths determined by their quantum energy levels. For greenhouse gases, the crucial wavelengths fall in the thermal infrared (roughly 4–100 μm), which is where Earth's surface and atmosphere emit most of their radiation. The greenhouse effect exists because certain atmospheric gases are transparent to incoming solar radiation (mostly visible light) but opaque to outgoing infrared radiation, trapping energy that would otherwise escape to space.
The reason only certain gases are greenhouse gases comes down to molecular structure. A molecule must have a dipole moment that changes during vibration to interact with infrared radiation. Symmetric diatomic molecules like N₂ and O₂ — which make up 99% of the atmosphere — have no permanent dipole and no dipole change during their symmetric stretch, making them infrared-inactive and invisible to thermal radiation. In contrast, molecules like CO₂, H₂O, CH₄, and N₂O have vibrational modes that produce oscillating dipole moments. CO₂, though symmetric overall, has an asymmetric stretch and a bending mode that create temporary dipoles, making it a potent infrared absorber despite having no permanent dipole moment. Water vapor, with its bent geometry, has a permanent dipole and multiple strong absorption bands.
Each greenhouse gas has a characteristic absorption spectrum — a fingerprint of wavelengths where it absorbs strongly. CO₂'s dominant absorption band is centered near 15 μm (the bending mode), which happens to coincide with the peak of Earth's outgoing infrared emission at typical surface temperatures. This spectral coincidence is why CO₂ is so climatically important despite its relatively low concentration. Methane absorbs near 3.3 μm and 7.7 μm, while water vapor absorbs broadly across much of the infrared, with key windows near 8–12 μm where the atmosphere is relatively transparent. The atmospheric window near 10 μm is critical because it is one of the few spectral regions where surface radiation can escape directly to space; any gas that absorbs in this window (like ozone near 9.6 μm) has an outsized climate effect.
The radiative impact of a greenhouse gas depends on both its absorption strength and its atmospheric concentration. A gas can be molecule-for-molecule a powerful absorber but climatically insignificant if present in trace amounts. Conversely, a weaker absorber at high concentration can dominate the greenhouse effect — water vapor is the single largest contributor precisely because it is abundant. For CO₂, doubling its concentration does not double its radiative effect because its core absorption band is already nearly saturated (the atmosphere is already opaque at 15 μm). Additional CO₂ matters because it widens the absorption band at its edges, where the atmosphere is still partially transparent, and because it absorbs in the upper atmosphere where the air is thinner and emission to space is more efficient. This logarithmic relationship between concentration and forcing — each doubling adds roughly the same increment of forcing — is fundamental to understanding why climate sensitivity is expressed per doubling of CO₂.