Natural climate fluctuation is pretty much indisputable, even with human historical periods (medieval warm period, Little Ice Age, etc). Likewise, the current warming trend is also indisputable, and it's fairly certain that even if it's NOT human caused, it's probably at least human exacerbated.
The US didn't ratify the Kyoto treaty because, if I recall correctly, China and India among others were exempt. The US would have taken an economic hit as a result of the treaty while China, which has only gotten bigger and bigger as a major industrial country in the years since Kyoto, would not have been saddled with the same regulations. This is a legitimate economic issue, but the political argument shifted away from the arena of economics, where perhaps it might have been a bit easier to arrive at an agreement or way forward. The political argument shifted instead to one about the scientific validity of the research. Skeptics deny the science as a way of trying to preempt the political conversation that necessarily follows. I think this is a disingenuous approach. If someone (or some organization) has an issue with the proposed political remedies -- as I sometimes, perhaps often, do -- then they should make THAT that their argument, not the underlying science.