The Effective Radiating Level ERL
ERL: The lowest level in the atmosphere from which infra red radiation is able, on average, to escape upwards to outer space without being reabsorbed
As concentrations of absorbing gases such as CO2 are increased, the ERL rises, decreasing the total mass of air above and keeping the opacity of that air constant.
The intensity of the radiation emitted to space increases with the temperature at the ERL
Notes:
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 outer space without being reabsorbed on the way. This height (in practice around 8-10 km) is called the Effective Radiating Level. It is idealized as representative of a band of heights that all vary in a similar manner.