with the caveat that I don't understand this particularly well, and just got a crash course from Hallucigenia recently, apparently the orbital version of this (mostly the impact of Jupiter and Saturn on the Earth's orbit) kinda-sorta explain the observed ice ages and whatnot, see
Milankovitch cycles - Wikipedia for a pretty good explanation. However, some astrophysicist seems to have come up with a solar physics cycles model that might turn out to be an alternative explanation:
New Ice Age Theory - Slashdot -- I'm not qualified to assess the relative merits, but he doesn't seem like a complete crank.
Anyway, though, it drives me completely nuts when issues get politicized enough that more people are pushing a political agenda with "spin science" (which may or may not be wrong) to the point where it's hard to have an intelligent discussion of a topic, because there's too much FUD around. As far as I've been able to tell from scientists I respect, though, the science-spin-FUD in Gore's "Inconvenient Truth" movie is a fairly accurate representation of the best guesses of most academics who make careers out of studying this stuff. Although anyone who says they understand the whole system is blowing smoke, most of the objections to the fundamental premise of increased greenhouse gas leading to climate change seem to be taking nit-picky details and using them as an excuse for a political position of "it's not 100% proven right, so we might as well assume it's wrong, since that's aligned with our politics."
A more concerning issue, for me, is that we don't have any real, tested theory about what will happen to the Earth as we shift a few parameters (particularly CO2 levels) into domains where we haven't seen them before, so we don't know what will happen, and when whatever does happen happens, we'll be stuck with it for many lifetimes. Since we pretty much evolved to be in the "sweet spot" that we like now, any change is likely to be for the worse, so I think there's a very strong rational argument for trying to head off a potential problem at the pass, since even if there were only a 10% chance that we could be at risk for making Earth a lousy place to live, I'm not really into Russian Roulette, and I've never heard a convincing argument that the "gloom and doom" climate scientists stand a 10% or less chance of being right.
I make a habit of trying to be very critical of overstated claims, because seeing science misrepresented for political spin bugs me no matter what the source, and despite this tendency I'm convinced that there is real reason for concern on this issue.