Mechanism of the greenhouse effect

Solar radiation reaches the Earth mainly as visible and near-infrared short waves, which pass through the atmosphere almost unabsorbed. About 30% is reflected back to space by clouds, dust and bright surfaces (the albedo) and about 70% is absorbed by the surface and warms it. The warmed body then radiates according to its own temperature — at about 288 K the peak is near 10 µm, in the long-wave infrared.

• Greenhouse molecules absorb particular infrared bands because their vibrations change the dipole moment of the molecule; they then re-emit the energy in random directions, so part of it returns to the surface.
• The result is that the surface and the lower atmosphere are warmer than they would be in radiative balance with space: about 33 °C warmer than a bare blackbody Earth.
• The enhanced greenhouse effect is the additional warming produced when human activity raises the concentration of these gases; between 1750 and today CO₂ has gone from 280 to more than 420 ppm and the mean surface temperature has risen about 1.1 °C.
• Windows of transparency: the atmosphere lets visible light in and lets some infrared out, but absorption bands of CO₂, H₂O and CH₄ close part of the infrared window — that closure is the physical cause of the effect.