AGW isn't a new idea, it's been known since 1896
Svante Arrhenius came up with it in 1896. The primary objections were that CO2 wouldn't increase because of ocean absorption (obviously wrong), that H2O absorbed the same wavelengths (not true, better instrumentation showed otherwise), and that aerosol pollution would outweigh greenhouse gas pollution.
During the 1970s, scientists started to realize that the objections were wrong (the mainstream media didn't though), and the modern consensus formed in the 1980s.