[llamabeast]

The bottom line is that, regardless of the details, the amount of CO2 in the atmosphere is much, much higher than it was (a factor of 5 perhaps? something like that). And CO2 definitely has a considerable effect on the way radiation passes through the Earth's atmosphere.

That can't fail to cause changes of one kind or another to the climate. Mostly, change is bad, because locally both human society and flora and fauna have adapted to the status quo. Natural mechanisms (migration in particular, and also adaptation) can cope with slow changes, but not with fast ones. Imagine if all the local climates switch around, so that areas which were fertile become desert or swamps, and vice versa. Even if overall "warmer is better" (which I think is not true, but hey), it'd take us a very long time to adapt to that. Our cities and populations are all in the wrong place, not to mention the damage to nature. In the meantime there'll be a lot of suffering.