Harold, frequent contributor to Panda's Thumb, had an interesting observation of the argument that global warming is cyclical and caused by powerful forces on earth, and therefore we humans can't have had anything to do with it:
Indeed, just because multiple factors contribute to climate changes, does not mean that human activity cannot be an important factor.
In fact, I find this kind of logic exactly backwards.
If the climate had been exceptionally stable and unchanging for the past millions or billions of years, then it might make sense to deny that anything we puny humans could do would impact it.
And indeed, its a pretty safe bet that were that the case, it would be the first argument the deniers would trot out.