Just like the current global warming debate, climatologists noticed and extremely slight rise in average temperature (less than 1 degree C), and immediately started asking what WE were doing to cause it.
You are quite mistaken on cause and effect.
Actually, the theory of greenhouse gases was understood over 100 years ago, and scientists like Arrhenius made some early predictions that increased CO2 could increase the climate. This was long before we were able to measure a significant increase in global temperature.
If you have an explanation for why increased CO2 would not have a greenhouse effect, please submit your results to a reputable, peer-reviewed journal.