In 1941 the US government explained why increasing levels of CO2 would not have a significant impact on the climate.
“IT WILL be news to many people that man, during his geo- logically brief existence on earth, has never known a “normal” climate. We are now at the tail end of an ice age and living in a period of crustal and climatic violence as great as any the earth has known. This is why we have to think so much about the weather. Such periods of revolution have occurred briefly several times in the history of the earth.”
“Much has been written about varying amounts of carbon dioxide in the atmosphere as a possible cause of glacial periods. The theory received a fatal blow when it was realized that carbon dioxide is very selective as to the wave lengths of radiant energy it will absorb, filtering nut only such waves as even very minute quantities of water vapor dispose of anyway. No probable Increase in atmospheric carbon dioxide could materially affect either the amount of insolation reaching the surface or the amount of terrestrial radiation lost to space.“
Climate and Man: Part One – United States Department of Agriculture, US Department of Agriculture – Google Books