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From 10% to 25% depending on who you talk to. Small but not miniscule.

I've never heard anyone cite percentages that high, except the boobs (congressmen) in the congressional hearings over global warming who were all trying to do math in their head all based upon the the parts-per-million changes in CO2 concentration.

Look, there is one way to look at how small CO2 is in the atmosphere just by looking at the almost-universally accepted concentration level. It is widely accepted that the current concentration is around 380 ppmv (parts per million by volume). Now, this in an increase over pre-industrial levels, which were around 280 ppmv, for a change of about 100 ppmv. So based upon these numbers, one can without controversy calculate that CO2 makes up only about 38/1000ths of 1% of the earths atmosphere (simple math). And the increase over the past 150 years has been about 1/100th of 1% of the atmosphere. Why do AGW alarmists believe that a change in atmosphere content this trivial (1/100th of %1)could cause such a catastrophic increase in tropospheric temperature? Well, they say there is an amplifying effect, where such a small change (1/100th of 1%) causes water vapor to become an even more effective greenhouse gas, so that ultimately these small changes in CO2 "ultimately" causes the warm-up. I don't buy it. I think the only place they see this amplifying effect is in their GCM's (General Circulation Models) which are computer models that have this amplification embedded in their models. And in order to justify this amplifier in their model, they basically have to null-out most natural causes such as solar irradiance variability and albedo effects of GCR's (Galactic Cosmic Rays).

So, no matter what percentage of total greenhouse gases that the current levels of CO2 represents, one can't deny that is extremely minute, and probably cannot explain the apparent increase in temperature over the past few decades.

A lot of the reason people began to worry about CO2 on this planet was because of the true greenhouse effect that exists on the planet Venus. But Venus is dramatically different from Earth. Where CO2 in the Earth's atmosphere amounts to 0.038%, it comprises 97% of Venus' atmosphere. And even at those concentrations and with Venus' orbit much closer to the sun, scientists calculate that such a greenhouse effect would only amount to about 25C. Venus is much hotter than that, of course, but it is because of other gases.

With CO2 concentrations of 380ppm (granted its likely to increase steadily until we move away from fossil fuels, which will inevitably occur) I feel pretty safe that we won't be cooked alive anytime soon.

As Lindzen points out here http://www.heartland.org/Article.cfm?artId=19485 the increase from 280 to 380 should have produced more than the observed 1F increase over the last century. CO2 is a minor GH gas and even a large increase would cause a minor increase in temperature, but not "trivial". The water vapor feedback is real and probably also nontrivial although I am very skeptical of water vapor models that don't model weather properly (even leaving out precipitation altogether) As for solar variability, that is steadily increasing, but very slowly (http://www.ucar.edu/news/releases/2006/brightness.shtml) and only explains about 0.2C over 2000 years.

The cosmic ray flux is generally decreasing, but that decrease is dwarfed by the solar cycle. As the Svensmark charts show, the slight decrease in cosmic rays (and correlating decreases in low clouds) is very small compared to the 22 year solar cycle. If the climate effect was profound enough to be behind a nontrivial part of the recent warming, we would also see a profound 22 year climate cycle. We don't, so the effect is interesting but trivial for climate (unlike CO2).