How the greenhouse effect warms the Earth
Chemistry of the atmosphere • Carbon dioxide and methane
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Key concepts
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Shortwave and longwave radiation
Shortwave radiation refers to visible and near-visible light from the Sun with relatively high energy per photon. The atmosphere is largely transparent to these wavelengths, so much solar energy reaches and warms the surface. Longwave radiation refers to infrared emissions from the warmed surface. Infrared has lower energy per photon and interacts strongly with certain gas molecules. Certain materials and gases behave differently with different wavelengths: transparency to shortwave allows heating at the surface; absorption of longwave by gases traps heat in the atmosphere.
Absorption of infrared by greenhouse gases
Greenhouse gases such as carbon dioxide and methane have molecular vibrations and rotations with energy transitions that match infrared wavelengths emitted by the warmed surface. When infrared photons encounter these molecules, the molecules absorb energy and move to higher vibrational or rotational states. Absorption removes outgoing longwave radiation from the direct path to space. After absorption, molecules re-emit infrared radiation in random directions or transfer energy to other air molecules through collisions, increasing kinetic energy and raising air temperature. The net effect is a reduction in outgoing longwave radiation and warming of the atmosphere and surface.
Transparency to incoming solar radiation
The atmosphere’s relative transparency to shortwave solar radiation causes most incoming energy to reach the ground. The surface absorbs shortwave radiation and converts it to heat. The warmed surface then emits longwave infrared radiation. If the atmosphere absorbed shortwave radiation strongly, less energy would reach the surface and the greenhouse effect would be weaker. The contrast in atmospheric behaviour for shortwave versus longwave radiation causes surface warming.
Radiative balance and the role of greenhouse gases
Planetary temperature depends on a balance between incoming shortwave solar energy and outgoing longwave infrared energy. Greenhouse gases change this balance by absorbing outgoing longwave radiation, so less energy leaves the system at the same surface temperature. To restore balance when greenhouse gas concentrations increase, the surface and lower atmosphere warm until outgoing longwave energy increases enough to match incoming solar energy again. That warming is the observable effect of increased greenhouse trapping.
Limiting factors and feedbacks
The strength of the greenhouse effect depends on gas concentrations, the spectral overlap between emitted infrared and gas absorption bands, atmospheric temperature profile, and the presence of clouds and aerosols. Water vapour acts mainly as a feedback: warmer air holds more water vapour, which absorbs infrared and amplifies warming. Clouds can both reflect incoming shortwave (cooling) and absorb/emit longwave (warming), so their net effect depends on cloud type and altitude. Molecular lifetime and sources of gases matter: some gases such as methane are more efficient infrared absorbers per molecule but have shorter atmospheric lifetimes than carbon dioxide, which persists longer and accumulates, affecting long-term radiative forcing.
Key notes
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