Helpful when reading this thread to keep in mind Michael Mann's six stages of climate change denial: 1. CO2 is not actually increasing. 2. Even if it is, the increase has no impact on the climate since there is no convincing evidence of warming. 3. Even if there is warming, it is due to natural causes. 4. Even if the warming cannot be explained by natural causes, the human impact is small, and the impact of continued…
For example, having Al Gore as a prominent figure of the AGW theory movement for a number of years is enough to make them suspicious. They see him fly in private jets, own multiple humongous homes, make investments that will pay off if things like carbon credits become mainstream, release a movie 9 months after Katrina that promised more and more severe hurricanes that never materialized, etc, etc. He might not be a duck but he sure seems to quack a lot.
The other main denial seems to be things like, "It's cold today - in your face global warming".
I think Michael Mann is giving people far too much credit.