It
is possible that there are additional variables that we're not aware of, but it is clear at this point that any new variables will not be very powerful. The reason we know this is that the climate models are very accurate: if you run the computer model backwards against known temperature values, we find that it matches very closely.
Temperature reconstructions for the past 1000 years. The black line towards the end is the directly measured temperatures. You'll see a couple of the lines terminate partway back because those measurements were not taken prior to those dates or the method doesn't allow for it. (good luck finding enough 1500 year old trees, for instance) The reason I show this graph is to show that temperature reconstructions are done with a lot of methods, and they all pretty much match.
CO2 was not discovered by elimination. Its absorption spectrum has been known for more than a hundred years, and we fine-tuned that knowledge down to the exact wavelength in the 1950's thanks to the USAF. (they were working on heat-seeking missiles) The theory that this might have a greenhouse effect came more than a hundred years ago as well, but at the time our fossil fuel usage was very small, so it was little more than a curiosity. Still, scientists love those experiments. You can experiment easily on a small scale to prove whether CO2 makes the air absorb more heat. Once that is done, you look for historical evidence that global temperatures have correlated to global CO2 levels, and that has also been established.
None of this approaches the discussion of what ought to be done, if it is in fact true. I don't think anything should be done. I think the earth will adjust and compensate. We should not radically change our economy. We are not even the worst offender. It is a policy issue.
The earth will adjust, certainly. We're in no danger of harming the earth. We are, however, in danger of harming ourselves. We can adapt to changes, but if those changes are too rapid there will be a significant human cost. Crop yields will suffer, increasing hunger issues. Melting ice caps will increase sea levels, slowly taking away usable land. Extreme weather events will increase in frequency. Animal species can only adapt so quickly also. It's not going to kill us all or end human civilization, but it's going to hurt like hell.
Plus, we're going to run out of fossil fuels eventually anyway. We've already seen how much we depend on the price of oil, with some of the spikes in price over the last couple years. Gas prices in the long term will never go down. We need to move ourselves off this black gooey crap that we pull out of the ground, and while we're doing it we can take care of that carbon issue. Gradual changes are always easier, and the sooner you start the more gradual it can be.