Atmospheric Carbon
Atmospheric carbon most naturally occurs as carbon dioxide (CO2). While carbon dioxide is currently a relatively minuscule component of the atmosphere, at about 0.039%, Earth’s early atmosphere was rich in CO2.
CO2 is transparent to visible light, as are N2 and O2, the other notable atmospheric gases. But whereas nitrogen and oxygen are also insensitive in the infrared spectrum, carbon dioxide absorbs infrared light.
In trapping infrared, CO2 prevents some of that radiation from escaping into space. Hence atmospheric carbon dioxide acts as a greenhouse gas, helping keep the planet below warm.
Water vapor has a powerful greenhouse effect, absorbing light in a broader spectrum than carbon dioxide. H2O absorbs essentially everything at wavelengths longer than 500 cm–1. CO2 centers its absorption around 667 cm–1.
Absorption dynamics depend upon quantum mechanical properties specific to a molecule’s geometry. Radiation at specific frequency ranges jiggles the bond angles of a molecule, affecting molecular vibrational mode, allowing a molecule to hold on to extra energy.
