Back in the early 80s, I did an experiment in one of my science classes. Green house gases were under investigation for their possible role in a myriad of atmospheric problems at the time. Ozone depletion, acid rain, city smog and global warming.
My science project was to determine whether elevated levels of CO2 gas in an enclosed environment had any effect on heat retention.
The experiment involved a plastic enclosure, heating source, thermometer and stop watch.
We took the plastic enclosure filled with normal air and added heat to the enclosure until the thermometer (open to the air of the enclosure) read 180 degrees Fahrenheit and then removed the heat. We took temperature readings every minute until the air in the enclosed temperature returned to room temperature.
We then did the same to the enclosure with additional CO2 added. We added a piece of dry ice to the enclosure. We didn't have exact measurements of the CO2 levels, we just added a chunk of dry ice, cracked open the lid until the dry ice evaporated to keep the pressure the same and then sealed the enclosure.
We then heated the enclosure to 180 degrees, same as before, removed the heat source and clocked the time it took for the enclosed air to equilibrate to room temperature.
This very simple unsophisticated high school science experiment comparing the heat retention of two gaseous environments differing in CO2 levels showed that the temperature of the enclosure containing the increased gaseous CO2 took longer to equilibrate to room temperature than the enclosure filled with just air.
If I recall it was 4 seconds longer. Repeating the experiment gave consistent results.
I took part in this experiment back in 1982. Yet there are people today who will deny that such experiments that show it's possible that CO2 can retain heat even exist.