We first understood the greenhouse effect back in 1859, with...

We first understood the greenhouse effect back in 1859, with John Tyndall's work.
https://en.wikipedia.org/wiki/John_Tyndall#Main_scientific_work

And by 1896 Arrhenius had calculated how human emissions could affect global temperatures.
https://en.wikipedia.org/wiki/Svante_Arrhenius#Greenhouse_effect

If you want to test it, try living on Mars with a very thin greenhouse atmosphere. It's cold.