I'm no expert, but I find it reasonable to believe that man's pollution could impact climate globally. However, I don't believe that capping emissions in the USA at the rates frequently discussed are worth doing. Just as Bush's pre-emptive war struck me as a huge, out-of-proportion response to a low probability possible future catastrophic outcome, the usual cap solutions I see strike me as parallels.
Also, I've long been skeptical of the environmentalist Chicken Little approach. When the Al Gore's of the world had been shutting down skeptical inquiry, declaring the debate was over, it began to become religion and not science so far as I could see. Those climategate emails did nothing to improve that view.
I say, have a nice meeting in Copenhagen, and the US should do nothing more than China or India do.