Rising levels of CO2 in the atmosphere had been well documented by 1950. What was the effect?
“Reduced to its simplest terms…whereas CO2 is almost completely transparent to solar radiation, it is partially opaque to the heat which is radiated back to space from the earth [mainly due to the expansion of industry]. In this way it acts as a heat trap, allowing the temperature near the earth’s surface to rise above the level it would attain if there were no CO2 in the air… It may be said that the climates of the world are behaving in a manner which suggests that slightly more solar heat is being retained in the atmosphere. This could be due to its increasing opacity to terrestrial heat as a result of the additions of CO2”
[Guy Callendar, “Can CO2 Influence Climate? ” Weather 4 (1949), pp. 310-314].