Originally Posted by svguy

Volume is a red herring; the key is a combination of the (for lack of better term) "heat content" of what is there coupled with its longevity.

By "heat content", that is a function of the wavelengths that it absorbs and the % of longwave radiation from the ground that it "catches"; and the resulting reemission.

C02 is a fairly efficient heat catcher at the re-emitted longwave radiation spectra. C02 is now currently at around 390 ppmv (maybe a bit less) or about .03% of the atmosphere. Is that the small number you were looking for? If so, its a red herring per the discussion above.

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The level has risen from about 315 ppmv since 1960.

As per the amount due to anthropogenic sources, C14:C12 ratios, C13:C12 ratios, and C12:C14 ratios can be directly measured. Since anthropogenic carbon is from "isolated" carbon (i.e. locked away for 10s of millions of years, or more), its ratio profile is very different than that in the "short term" carbon cycle loop. These direct measurements have pointed a strong finger at the vast majority of the increase in C02 from the late 1950's as being "anthropogenic" in nature.

I do hope you are not going down the "its only .03% of the atmosphere" argument. By itself that number is completely useless. And, I do hope you are not going down the "we don't know where its coming from" argument, since the hard science pretty much tells us that.