Solar Radiation Budget
Notes:
The information in this diagram is obtained mostly from measurements made from Earth orbiting satellites, corroborated by spot measurements at the surface of the earth. It represents a multi-year average over the Earth as a whole.
The reflected solar radiation is mostly from clouds, with the rest from the Earth surface, desert and snow cover. The remainder of the incoming radiation is absorbed, mostly at the surface, but some in the atmosphere at ultraviolet and near infrared wavelengths. In this simple model, we assume the solar absorption all takes place at the surface (i.e none in the atmosphere) and is held fixed as the atmospheric carbon dioxide increases.
In the long run the solar energy absorbed at the earth’s surface must be compensated by emission to space of infra red radiation. Emission from the surface alone cannot do this, because the atmosphere as a whole is largely opaque in the infra red, implying that such radiation would be absorbed at higher levels. As one moves upward, the amount of matter absorbing infrared radiation between oneself and outer space decreases rapidly, both because the mass of air above is less and also because the concentration of water vapor in that air also decreases. At some height most radiation emitted upwards makes it to out without being reabsorbed on the way. This height (in practice around 5-10 km) is called the Effective Radiating Level. It will be idealized into a single level representative of a band of heights that all vary in a similar manner.