8.3 Greenhouse Effect and Photon Absorption

Solar radiation arriving at Earth is concentrated in visible and near-infrared wavelengths, where much of the atmosphere is comparatively transparent. Earth's surface, however, emits mainly thermal infrared due to its much lower temperature. That separation in spectral bands is the core reason atmospheric composition matters so strongly.

Greenhouse gases absorb portions of outgoing infrared radiation, then re-emit radiation in all directions. The upward component still escapes eventually, but the downward component returns energy toward lower atmosphere and surface. The surface therefore equilibrates at a higher temperature so that outgoing radiation to space can once again match absorbed solar input.

A compact model introduces an atmospheric absorption fraction $g$ for surface infrared. A fraction $(1-g)$ of surface radiation escapes directly through the atmospheric window, while the absorbed fraction is re-emitted upward and downward. Even this stripped model captures the direction of greenhouse warming and the role of top-of-atmosphere balance.

The model can include non-radiative upward transfer from surface to atmosphere (convection and latent heat). This redistributes energy between layers but does not change the requirement that net planetary input equals net planetary output at equilibrium. It does, however, change how warm each layer must become to satisfy both layer-level and top-level balances.

In practice, increasing $g$ while holding other parameters fixed reduces direct surface escape and increases atmospheric reradiation contribution. Surface temperature then rises until total outgoing longwave to space recovers the absorbed solar value.

At molecular level, greenhouse gases have discrete rotational and vibrational energy transitions whose spacings often correspond to infrared photon energies. When an IR photon matches an allowed transition, absorption occurs and molecular internal energy increases. Subsequent collisions and re-emission redistribute that energy across the atmosphere.

Re-emitted photons are not targeted 'back to the ground'; emission is approximately isotropic at local scales. The key is statistical directionality in aggregate: with emitting layers above the surface, a substantial downward IR component exists, increasing net surface energy input compared with a no-atmosphere case.

Main greenhouse gases discussed at this level are water vapor, carbon dioxide, methane, and nitrous oxide. Their climate influence depends on concentration, spectral absorption bands, vertical distribution, and interaction with other processes such as clouds and circulation.