Seriously... Al Gore has personally done more damage to the AGW cause then anyone else in the world.
THIS. He massively increased the politicization of the issue and almost single-handedly created the incentive to turn climate denialism in the US from a fringe conspiracy theory to a mainstream belief. I don't think things would be massively different today if he had stayed out of climate issues, but the brakes wouldn't be dragging nearly so hard on the science train. I don't think climate policy would be more contentious than any other environmental policy issue.
I'd say the biggest catastrophes in climate policy are:
1. Al Gore, for contributing massively to the politicization of climate science
2. Disinformation campaigns funded by fossil fuel companies and conservative think-tanks
3. Michael Chrichton's State of Fear, the Book of Revelation for climate denialists.