This isn't a conspiracy "theory" per se, but the big fossil fuel companies certainly conspired to totally discredit the good science showing how their emissions would lead to global warming. Judging by how things are going in the current day, including literacy or even basic acceptance of a warming planet among the general population, as well as the political inaction on the matter, I would say they largely succeeded.

https://en.wikipedia.org/wiki/Global_Climate_Coalition