Well, a mistake in itself.
Denier is a strong word, used essentially to talk about the Shoah deniers, people believing nazis never killed all those people and that extermination camps don't exist.

So denier is not a good word for what we're talking about. Cause there is not such a huge and solid proof for climate change.

But I still think, from what I know, that most people being "deniers" are still a bit mindlessly obstinate ^^
They are people saying the impact of mankind on environment is not that big because "you don't know maybe it would have happened anyway!" 

(and most of them are American! xD )