The Physics of CO2 being a Greenhouse Gas

We review the physics of CO2 being a greenhouse gas in terms of well-known concepts: blackbody radiation which tells that Earth’s radiation because of the Sun’s heating lies in the infrared portion of the electromagnetic spectrum, the CO2 absorption spectrum which indicates the ability of CO2 molecules to absorb infrared photons sent out from Earth’s surface at particular frequencies (or wavenumbers), and the the photon’s mean free path which is the average distance over which a moving photon travels before being absorbed by the CO2 molecule. The size of the mean free path is strongly frequency (or wavenumber) dependent and inversely proportional to the CO2 concentration. This observation is the key to appreciate that an increase in CO2 concentration in the atmosphere has the effect of increasing Earth’s temperature because more photons (radiation) radiated by Earth have the likelihood of returning to the Earth’s surface, thereby heating it.