As meteorology and climatology are hobbies of mine, I'll give my $0.02 worth.
First, there's nothing new about the idea of CO2-induced global warming. Even back in the Fifties and Sixties, the consensus was that industrial CO2 probably would raise the Earth's temperature around 1 to 2 degrees Celsius by the middle of the 21st century.
But then the computer modelers and climate scientists decided that, hey, 1 to 2 degrees wasn't urgent enough. So they began assuming that a warming would have other effects, like melting permafrost and whatnot, and used these alleged secondary effects to trump up the extent of the warming. That's how 1 to 2 degrees turned into 2 to 5 degrees, and then the infamous 4 to 12 degrees Celsius that would indeed have catastrophic effects.
However, there are a couple of glaring issues with the doomsday scenarios.
First, there's no way to justify anything close to those numbers based on CO2 alone. Each doubling of CO2 has the same effect on climate. In other words, going from 0 to 100ppm has the same effect as going from 100 to 200, which has the same effect as 200 to 400, which is much the same as 400 to 800, and so on. In other words, as CO2 concentrations increase, the effect of each new ppm of CO2 decreases. This is the problem with the assertion that temperatures are all of a sudden going to increase on an accelerating curve--the properties of CO2 just don't work that way.
Second, the computer models assume feedback loops that, while seeming logical on the surface, don't really stand up to scrutiny. More and more evidence is being discovered that the Earth's climate has in fact shifted back and forth and all around on its own, rather frequently. If the Earth were that prone to vicious cycles of temperature, we'd be buried in ice or half the planet would be a burning desert by now. The fact that the Earth is still around with a temperate climate after hundreds of millions of years indicates that it is able to absorb changes in temperature without, to use an unscientific term, freaking out about it.
Third, a look at ice cores and other fossil records shows, astonishingly, that CO2 is often a lagging rather than a leading indicator of temperature change. In other words, temperature changes and then CO2 changes. The fossil record also shows that, not too long ago (in geologic time) we tipped into a moderately intense Ice Age at over three times the ambient level of atmospheric CO2 we have today, and in the absence of any discernible forcing (like a meteor impact or heavy volcanism). Inject that level of CO2 into the IPCC model, and we'd be growing banyan trees at McMurdo in Antarctica. Obviously a screw is loose somewhere.
Fourth, if you're looking for an external forcing mechanism to account for temperature change, you need look no further than Sol itself.