  Based on new radiative transfer numerical evaluations, we reconsider an
argument presented by Schack in 1972 that says that saturation of the
absorption of infrared radiation by carbon dioxide in the atmosphere sets in as
soon as the relative concentration of carbon dioxide exceeds a lower limit of
approximately 300 ppm. We provide a concise brief and explicit representation
of the greenhouse effect of the earth's atmosphere. We find an equilibrium
climate sensitivity (temperature increase $\Delta T$ due to doubling of
atmospheric $CO_2$ concentration) of $\Delta T \simeq 0.5 ^0C$. We elaborate on
the consistency of these results on $\Delta T$ with results observationally
obtained by satellite-based measurements of short-time radiation-flux versus
surface-temperature changes.

Atmospheric and Oceanic Physics